• Back to Encyclopedia Arctica homepage

    Ecology and Physiology

    Encyclopedia Arctica 5: Plant Sciences (General)

    Ecology and Physiology

    Autecology and phenology of Arctic Plants

    001      |      Vol_V-0166                                                                                                                  
    EA: Plant Sciences

    (Thorvald Sørensen)


            Conditions of plant life in the Arctic are justly considred unfavor–

    able. The rapid decrease in number of species toward the extreme North

    can probably be taken as a proof of a corresponding deterioration of the

    conditions. The high-arctic vegetation, poor as it is, represents one

    of the outposts of plant life on earth. Ecologically, therefore, it holds

    a position of its own. In the following paragraphs, the main causes of

    the poverty of vegetation and some general trends in the adaptation of

    plants to the unfavorable conditions will be discussed. Observations

    made by the author in northeast Greenland between 70° and 77° N. latitude

    form the main basis of the present paper.

            Plant distribution within an area is largely determined by the two

    leading ecological factors: temperature and access to water. In the true

    Arctic these factors have to be looked upon in a manner somewhat different

    from that to which we are accustomed in the milder southern latitudes.

    The constantly frozen subsoil acts as an unbroken bedrock. Water communi–

    cation is restricted to comparatively thin layers which are determined by

    the depth to which the soil thaws in summer. The immature soils are strongly

    affected by solifluction, which tends to be inimical to plant life. A

    closed plant cover is found only in localities with stable conditions in

    002      |      Vol_V-0167                                                                                                                  
    EA-PS. Sørensen L : Autecology and Phenology

    the upper soil strata [ ?] - for example, on mature moist or wet soils

    which have attained a state of equilibrium through the sorting by frost

    of the coarse material into a framework of stone rings, and the subse–

    quent removal of the fine-grained material by the action of silting water.

    Apart from this, the climax vegetation of the high-Arctic is the “speck

    tundra” where plant life is hampered by solifluction.

            Apparently, plant species of a chamephytic life from (that is, having

    the perennating buds at the soil surface, the aerial parts dying down in

    autumn) are best suited for keeping pace with the movements of the soil

    and preventing the shoot spices from being buried in the mud. Moreover,

    the cooling influence of the convection currents within the mud polygons

    should not beneglected. Only plants with very modest temperature require–

    ments can thrive in wet soil that is constantly cooled by the water

    ascending from the melting ice beneath. On theother hand, sloping terrain

    supplied with flowing or percolating water from distant snowbanks can

    contribute to an improvement of the temperature conditions and harbor

    relatively luxuriant vegetation composed of more exacting species.

            The North Greenland climate is [ ?] characterized by: ( 1 )

    extremely low winter temperatures, which sometimes fall to about −50°C.,

    ( 2 ) cool summer temperatures, the mean during the warmest month scarcely

    rising above 5°C. and ( 3 ) a very short season of only 2 to 3 months

    when the temperature normally remains above freezing point throughout

    the 24 hours.

            From field observations on the occurrence and distribution of the

    individual species, it must be inferred that the winter cold does not

    seriously hamper plant distribution in the Arctic. Snowfall in northern

    003      |      Vol_V-0168                                                                                                                  
    EA-PS. Sørensen: Autecology and Phenology

    Greenland often does not set in until the soil surface has been exposed

    to very low temperatures; thus, all plant species of the region, including

    the plants of the snow patches, can endure extreme cold without injury.

    On the other hand, it is evident that temperature conditions of the

    growing season, and their local variations with exposure, water supply,

    and geographical location, play an important part in plant distribution.

    The duration of the season with sufficiently warm temperatures for develop–

    ment and growth, flowering and fruiting, seems to be the ecological factor

    which, for the majority of plant species, comes closest of the critical

    minimum. The survival of the individual plant is not as important a

    problem as the maintenance of the stock, i.e., reproduction.

            The climate is indeed harsh. However, the meteorological data avail–

    able from these northern regions provide a somewhat exaggerated impression

    of severity because they do not give a reliable picture of the conditions

    under which the plants actually live. This is illustrated by the following

    information from latitudes 72° to 74° N. in northeast Greenland. During

    the favorable season the temperature of the surface soil (i.e., within

    the uppermost 5 centimeters) is 5° to 7°C. above the meteorological mean

    air temperature. In the spring, the mean temperature of the surface soil

    passes the 0°C. line 3 to 4 weeks earlier than the meteorological mean.

    In the autumn, however, the mean temperature of the air and the surface

    soil will intersect the 0°C. line at about the same time. In places

    with a rather slight southern exposure, the soil is commonly thawed to a

    depth of about half a meter by the time the mean air temperature in the

    spring reaches 0°C.

            These data show to what degree the real plant climate is influence s d

    004      |      Vol_V-0169                                                                                                                  
    EA-PS. Sørensen: Autecology and Phenology

    by insolation. North of the Arctic Circle, the unbroken daylight during

    the [ ?] summer causes a relatively high summer temperature. Thus for

    example, in East Greenland, the meteorological summer temperature of

    Denmark Harbor (about lat. 77° N.) does not differ substantially from

    that of scoresby Sound (about lat. 70° N.). Nevertheless, a surprising

    decline in the flora can be noted. About one-fourth to one-third of the

    flowering plants of Scoresby Sound have disappeared at Denmark Harbor.

    It has been shown that during midsummer the irradiation at ground level,

    coupled with the high transmission coefficient which is found in northern

    Greenland, does not decrease toward the north. In higher latitudes,

    however, an increasing fraction of the irradiation falls upon the north–

    facing slopes. In other words, the temperature conditions of the favored

    southern hill slopes rapidly become less favorable toward the north as com–

    pared with the meteorological air temperature. While southern exposure

    has an extremely favorable influence on plant climate in the eastern

    coastal part of central Greenland, it is of relatively minor importance

    in the northernmost parts.

            A few phonological observations may serve to throw some light upon

    the problems of plant climate (see Table I).

    Table I. Calendar Time in Relation to Phenological Data
    Location Year of

    Mean 24-hour temp–

    erature around 0°C.
    Breaking of

    Saxifraga oppositifolia

    in flower
    Denmark Harbor 1907 June 3 to 10 June 20 June 4
    (lat. 77° N.) 1908 June 8 to 16 June 18 June 7
    Clavering Island 1935 June 11 to 18 June 3 June 11
    (lat. 74° N.)
    Scoresby Sound 1892 June 6 to 13 May 15 May 23
    0 ( lat. 70° N.)

    005      |      Vol_V-0170                                                                                                                  
    EA-PS. Sørensen: Autecology and Phenology

            The mean air temperature at the two main observation stations,

    Denmark Harbor and Scoresby Sound, passes the 0°C. line at almost the

    same time. The breaking up of the rivers occurs about 35 days apart.

    At Denmark Harbor the break-up happens one or two weeks after, at

    Scoresby Sound about one month before, the 0°C. mean has been reached.

    It should be noted that the breaking up of the rivers indicates a large–

    scale melting of the snow masses and also the disappearance of the slight

    snow cover from the favorably exposed slopes. These phenological data

    undoubtedly give a clue to the understanding of the abrupt decrease in

    the number of species between 70° and 77° N. latitude.

            In East Greenland at 72° to 74° N. latitude, i.e., immediately south

    of the region most pronounced floral decline, the following succession of

    aspects may be noted. The vernal aspect, from the beginning of June to

    about June 25, is characterized by the flowering of Saxifraga oppositifolia,

    Cerastium P alpinum species of Draba , Melandryum (Lychnis) , and Potentilla,

    growing upon rock ledges and hills that are bare of snow in winter.

    This aspect is also characterized by the coming into leaf of the deciduous

    dwarf shrubs, mainly Betula nana and Vaccinium uliginosum.

            The aestival aestival aspect, from the end of June to the middle of

    July, or somewhat longer, is characterized by the flowering of Cassiope

    tetragona and Dryas , and of the plants which are associated with them in

    heaths, etc.

            The serotinous aspect, from the latter half of July to about August 10,

    is characterized by the flourishing of the most luxuriant grass and herb

    slopes, mainly harboring the southern species of the region. During this

    season also the Salix arctica heaths, which may only recently have become

    006      |      Vol_V-0171                                                                                                                  
    EA-PS. Sørensen: Autecology and Phenology

    free from snow, are adorned with innumerable candle-shaped catkins. In

    depressions and revines the snow-patch vegetation flourishes step by


            The autumnal aspect, in the second half of August, is characterized

    by intense coloring of the foliage of the deciduous dwarf bushes.

            The vernal plants, Saxifraga oppositifolia and its companions, are

    so hardy that one could visualize them holding on to the North Pole if

    only as much as a bird’s resting place of bare ground could be found:

    As early as the middle of the short summer, they are often subjected to an

    extreme drought, causing an interruption of their life functions long before

    the autumnal frost sets in. They represent the xerophytes among the high–

    arctic plants. Conversely, the plants contributing by their flowering to

    the serotinous and autumnal aspects are mainly hygrophytes. From an

    ecological viewpoint, two main types of hygrophytes can be roughly dis–

    tinguished: ( 1 ) the inhabitants of watered slopes which thaw moderately

    early and those of the real swamps, Gramineae and Cyperaceae predominating;

    ( 2 ) the constituents of the real snow-patch vegetation, dicotyledcnous

    plants predominating but the families Saxifragaceae, Cruciferae, Caryophyl–

    laceae, and Ranonculaceae being abundantly represented. Only the last type

    proceeds far to the north.

            From the phenological data it appears that the flowering of Saxifraga

    oppositifolia at latitude 77° N. is delayed by a fortnight as compared

    with latitude 70° N. In other words, the vernal aspect is delayed by a

    fortnight, i.e., a much smaller retardation over seven degrees of latitude

    than is observed in temperate regions. The rapid advance of the phenologic

    springtime in the North is not followed up by the next season. As a

    007      |      Vol_V-0172                                                                                                                  
    EA-PS. Sorensen: Autecology and Phenology

    consequence of the retarded snow-melt toward the north in areas subjected

    to a moderate snow cover, at least as compared with most areas of lower

    latitude, and especially their southern slopes, the summer aspect is

    subjected to a greater calendar retardation with increasingly high latitude.

    Strictly speaking, in the extreme North no real summer aspect as defined

    above is found, in fact, the vernal and the autumnal aspects meet.

            The vegetation types associated in East Greenland with a moderate

    snow-covering and a rather early thawing, such as some Cassiope tetragona

    heaths, are characteristic of the middle Greenland landscape. To the north,

    these vegetation types withdraw to narrow belts on the southern slopes

    skirting the hilltops; or they may disappear entirely, giving way to a

    vegetation found farther south in the depression having a prolonged snow–

    covering. Accordingly, the plant communities which by their flowering

    contribute to the summer aspect, are markedly diminished in area. The

    communities whose culmination characterizes the serotinous aspect to the

    south, i.e., the luxuriant grass and herb slopes, have here disappeared.

    The constituents of these exacting communities represent the abrupt floral

    decline which has been mentioned.

            The increasing retardation of the summer aspect in a south-to-north

    direction as compared with the vernal aspect, is paralleled by a similar

    retardation in a west-to-east direction, i.e., from the heads of the

    fjords toward the coast land in East Greenland. The last-mentioned re–

    tardation is caused by frequent fogs in the coastal regions, especially

    after the breaking up of the Arctic Sea drift ice in early summer. Thus,

    in the spring there is no essential phenological difference between inland

    and coast, while in the summer the flowering time of one and the same

    008      |      Vol_V-0173                                                                                                                  
    EA-PS. Sørensen: Autecology and Phenology

    species may differ by a fortnight. This general phonological retardation,

    which mainly [ ?] influences the late-flowering species, probably accounts

    for the well-known fact that in the coast lands the northern plant communi–

    ties proceed far to the south.

            From the phenological typification of the northern Greenland vegetation

    briefly outlined above, it may be inferred that plant distribution and

    differentiation of the vegetation are dependent mainly upon the duration

    of the growth season. The faster a plant can complete its yearly life cycle,

    the farther to the north and the deeper into the snow-covering sequence will

    it be able to thrive. However, it can be demonstrated that, generally the

    growth rate of the northern species is rather slow as compared with more

    southern species. This characteristic might impede a rapid flowering,

    although the peculiar developmental morphol g o gy of the high-arctic plants

    compensates for this drawback.

            The following peculiarities of arctic plants should be emphasized:

    ( 1 ) an advanced development of the floral organs during the year (or years)

    preceding flowering; ( 2 ) proleptic or anticipating development of innova–

    tion shoots which causes a distribution of the yearly leaf production

    over several few-leaved shoots within one and the same sympodium. The

    result s is a simultaneous expansion of the foliage and a possibility

    for the sympodium to develop flowers yearly in spite of the inconsiderable

    growth of the individual shoot apices; ( 3 ) irregular or at least a

    periodic development: a rather large number of species, particularly

    those inhabiting the late-thawing snow patches, show quite a periodic

    and almost continuous initiation of shoots and floral organs. Such plants

    have no real resting period. They winter in a torpid state, with floral

    009      |      Vol_V-0174                                                                                                                  
    EA-PS. Sorensen: Autecology and Phenology

    organs in all or at least in strongly varying developmental stages which

    range from scarcely observable initials to flowering and fruiting states.

    Generally, only flower buds which have not passed a cer g t ain developmental

    stage are able to continue their growth in the following spring. Never–

    the le a ss this development secures an effective utilization of even the

    shortest favorable period for the purpose of producing reproductive organs.

    Even if the flowers are apt to be destroyed year after year, some favorable

    summer may yield sufficient time for the flower buds that have wintered in

    a proper stage to complete their fruiting. It should be pointed out that

    only one aperiodic plant, Braya humilis , was found to winter uninjured,

    even on ground bare of snow, in all stages, and to continue its growth and

    development during the following summer. Many northern species, especially

    the aperiodical ones, are evergreen and have no bud protection at all.

    Hence, the shoot apices look rather alike all the year round. ( 4 ) Vivipary:

    a number of species, most of them belonging to the families Saxifragaceae ,

    Caryophyllaceae, Gramineae, propagate by means of vegetative “diaspores”

    such as bulbils and detached shoot apices. The vegetative diasporas seem to

    endure the winter frosts in every [ ?] stage of development. However, no

    marked correlation between vivipary and speriodicity could be demonstrated.

            In contrast to the decidedly northern species just mentioned, the

    species of a more southern distribution, in East Greenland reaching

    their northern limit slightly north of the 70th parallel, are usually distinguished

    by the following characteristics: no [ ?] proleptic shoots; concentration of

    the yearly leaf production to a single shoot generation, in some cases to

    the flowering shoot proper, and, consequently, successive development of

    leaves during the summer of flowering. Because the flower buds winter in

    010      |      Vol_V-0175                                                                                                                  
    EA-PS. Sørensen: Autecology and Phenology

    a young state, if initiated at all before wintering, plants of this

    developmental type require a relatively long prefloration period. Aperiodi–

    cal development, as described above, is scarecely met with among the southern

    species. Evergreen herbs are rare. Bud protection by scale leaves is


            The two developmental types pointed out above, the one attached to a

    northern, the other to a southern distribution, cannot be distinguished

    sharply Typification may serve to make clear some general trend of adaptive–

    ness to the unfavorable conditions which the northernmost plants on earth have

    to endure. The pronounced tendency to a proleptic formation of innovations

    connected with an inconsiderable yearly increment of the individual shoots,

    as demonstrated for the one type, results in the well-known cushion shape of

    many high-arctic plants. Such a growth form, it may be inferred in this

    instance, represents a secondary adaptation to an extremely short summer.

            Slow growth and small size make the high-arctic plants unable to com–

    pete for space with faster-growing and bigger southern plants. Evidently

    the northern species will suffer the fate of being exterminated by compe–

    tition, if for climatic reasons the southern species alone are able to

    live and propagate. Thus, the southern species have the opportunity of

    reaching their definite climatic northern limit. The high-arctic plants,

    on the other hand, will not be found so far to the south as their climatic

    tolerance would allow. They are the outsiders of the plant world. Their

    acquired adaptiveness to environment is mainly one-sided, as differentiation

    of conditions on inhabitable land is lowered toward the extreme north.

    Size is unessential because high-growing exacting competitors are absent

    from most of the domain of high-arctic plants. Their chief virtue is

    011      |      Vol_V-0176                                                                                                                  
    EA-PS. Sørensen: Autecology and Phenology

    contentment. Their demands are small in every respect. Evidently they

    live in a state of constant hunger if we judge from their altering size

    and habit as a response to manuring round lemmings’ holes and foxes’

    earths. High-arctic plants act on the defensive, so to speak. Their

    principal goal is to shorten their yearly life cycle at the expense of

    the vegetative equipment. Their developmental peculiarities, their habit

    and size, and their devious geographical area all have a phenological

    background. Their kingdom is characterized as being the land where time

    is all and where lack of time is constantly threatening continuation of


    012      |      Vol_V-0177                                                                                                                  
    EA-PS. Sørensen: Autecology and Phenology


    1. Kjellman, F.R. “Ur polarväxternas lif.” Nordenskiőod: Studier och

    Forskningar . Stockholm, 1883.

    2. Resvoll, Thekla. “Om planter som passer til kort og kold sommer.”

    Archiv. Fűr Mat. Nat . vol.35, 1917.

    3. Sørensen, Thorvald. “Temperature relations and phenology of the north–

    east Greenland flowering plants,” Medd. Grønland , vol. 125,

    no. 9, 1941.


    Thorvald Sørensen

    The Polar Tree Line

    001      |      Vol_V-0178                                                                                                                  
    EA-PS. Ilmari Hustich


            The approximate course of the polar tree line is shown in Figure 1.

    The boundary on the map attempts to include to its south the scattered trees,

    sometimes occurring as bushes only, which lie outside the forest region proper.

    In general, coniferous tree species go farther north than deciduous species.

    In continental areas the tree line tends to [ ?] lie further north than in mari–

    time [ ?] areas. This is the reason for the eccentric course of the tree line

    around the North Pole.

            The general factors which influence the climate also, of course, influence

    the course of the tree line; the northern situation of the tree line in Scandi–

    navia, for instance, is due to the ocean current in the Atlantic. The course of

    the tree line coincides fairly well with the course of the 10°C. July isotherm,

    and the marked northwest-southeast direction of the tree line in North America

    coincides completely with the general direction of the isotherms.

            The appearance of the tree species at their polar limit shows a general

    similarity all over the polar [ ?] area, in spite of the fact that different species

    form the tree line in different parts of the continents. The some stunted tree

    form and low creeping branches that are found in extremely windy localities,

    and the some marked difference between the [ ?] branches which are covered with

    snow in winter and those lying above the snow (the height of [ ?] the snow cover is

    usually indicated by a branchless part of the stem) can be seen on almost all

    polar tree-line individuals.

            In arctic America the following species from the polar tree line: white

    spruce ( Plcea glauca ), black spruce ( P. mariana ), and tamarack or black larch

    002      |      Vol_V-0179                                                                                                                  

    Hustich: Tree line

    (Larix laricine) . It seems that in areas with sedimentary bedrock, or where

    the glacial till is of less acid origin, as also on alluvial soils, white spruce

    forms the polar tree line. Black spruce dominates in granite-gneise areas,

    and in moist places, the northernmost trees are tamaracks. In Alaska balsam

    poplar ( Populus balsamifera ) reaches the polar tree line, as also on the Labra–

    dor coast. White b ri ir ch and aspen reach almost as far.

            In Eurasia several species appear at the tree line. In Europe, going from

    west to east, Scotch pine ( Pinus silvestris ), Norway spruce ( Picea excelsa ),

    and the closely related P. obovate , are found along the line. Outside this

    coniferous tree line, low mountain birches ( Betula tortuosa agg. and related

    species) occur, particularly in northern Scandinavia and on the Kola Peninsula.

    In the Pechora region the tamarack-dominated Sibe e rian polar tree line begins; the

    tamarack species is Larix sukatschewii , a species that was some years ago sepa–

    rated from L. sibirica , which forms the tree line in the central part of north–

    ern Siberia. In eastern Siberia a [ ?] third tamarack species, L. dahurica ,

    appears at the tree line. A poplar, Populus suaveolens , goes as for north as the

    tamarack in subarctic eastern Siberia. The tree line on Figure 1 inclu ed de s in

    easternmost Siberia the bushy pine species, Pinus pumila . The fir species ( Abies ),

    such as balsam fir and Siberian fir, do not teach the tree line, and pine is also

    generally absent there, whereas birth, aspen, and balsam poplar go as far, or

    nearly as far, as the tree line.

            The forest region proper begins some distances from the tree line. The polar

    tree line in the north, and the northern limit of the continuous forest, the

    forest line, border a zone which in places is a hundred miles wide, being espe–

    cially broad on the northern plains of the Soviet Union. This zone, the forest

    tundra, is ecologically interesting; here arctic and forest plant and animal

    003      |      Vol_V-0180                                                                                                                  

    Hustich: Tree li ne

    species intermingle. Economically, however, this region is of little value;

    the trees grow slowly and are stunted and deformed. The treeless tundra

    easily invades a cleared area in this forest-tundra region where the forest goes [ ?]

    north only in the shape of narrow patches or scattered trees in sheltered valleys,

    these timbered tracts being bordered by large open tundra areas. This forest–

    tundra region was probably narrower in earlier centuries, but [ ?] the fuel and timber

    requirements of the nomadic northern tribes, such as the Samoyeds in the north–

    ern Soviet Union, may have formed large open tundra areas in the northernmost

    parts of what were once continuous forests.

            It seems appropriate to apply the common expression “timber line” only

    to the northern limits [ ?] of forests with good reproductive capacity, and this

    timber line , then, does not coincide with the forest line. The different ex–

    pressions used here - tree line, forest line, and timber line - are illustrated

    by Figure 2. Generally speaking, the northernmost trees flower richly, forming

    great quantities of cones, but the seeds do not germinate every year. An

    early investigation by Renvall of the Scotch pine forest in northernmost Finland

    showed, for instance, that there are 60 to 80 years between good seed years at the

    polar tree line. In this the author’s opinion the same [ ?] condition could, at least

    in some degree, be found on other polar tree lines.

            In the north, the summer is short and its temperature varies greatly from

    one year to another, a [ ?] good warm summer with rich flowering commonly alterna–

    ting with cold inhospitable ones - a fact well known to everyone who has spent

    more than one summer in the Arctic. In some places north of the present polar tree

    line one may find remnants of trees, indicating warmer periods in earlier centuries;

    one may also, in exposed forest areas that have been cut, observe a rate of re–

    growth too slow to enable the trees to hold their own. However, in the last

    004      |      Vol_V-0181                                                                                                                  

    Hustich: Tree line

    decades a slight climatic change, confirmed by several glaciologists and

    meteorologists, has [ ?] caused the forest line and tree line to move northward.

    This is the case particularly in areas near latitude 70° N., where continuous

    daylight may accentuate a rise in temperature. The seed years at the polar tree

    line have been more frequent in the latest decades than previously, and the growth

    in thickness of the northern forest trees has slightly increased.

            At the polar tree line the trees are on their climatic border and thus, ac–

    cording to a familiar e cological principle, react more freely to changes in cli–

    mate. The polar [ ?] tree line should , therefore, the considered not as something

    stable but as a dynamic phytogeographical border line that clearly oscillates in

    accordance with climate fluctuations.


    Ilmari Hustich


    Unpaginated      |      Vol_V-0182                                                                                                                  
    EA-Plant Sciences

    (R. Ross)




    Marine Phytoplankton 1
    Distribution of Marine Phytoplankton 9
    The Polar Basin 9
    Greenland and Barents Sea 10
    Davis Strait and Baffin Bay 11
    Hudson Bay and the Canadian Arctic Archipelago 12
    Freshwater Phytoplankton 13
    Bibliography 16

    001      |      Vol_V-0183                                                                                                                  
    EA-Plant Sciences

    (R. Ross)



            The plant life of the sea is divided into two main ecological groups:

    the benthos which lives on a solid substratum (whether this be the bottom

    of the seas, other plants or animals, or, in polar waters, ice), and the

    plankton which floats unattached. The term plankton is used to include

    both the plant and animal life which occurs free in the waters of the

    ocean and whose mobility is so limited that it drifts at the mercy of the

    currents. It is the plants of this community which constitute the phyto–

    plankton. Most bodies of fresh water larger than puddles also possess a

    phytoplankton. Both marine and freshwater phytoplankton consist entirely

    of members of the Algae; but there are great differences in their composi–

    tion and the factors affecting their distribution and abundance in the two

    types of water, which are accordingly treated separately.



            Almost all species of algae which to form the marine phytoplankton

    are unicellular, but a number of these unicellular types form chains or

    colonies. The groups of algae found in the marine phytoplankton of the

    Arctic, and the approximate numbers of species in each group, are as

    follows: Bacillariophyceae (diatoms), about 250 species; Dinophyceae,

    002      |      Vol_V-0184                                                                                                                  
    EA-PS. Ross: Phytoplankton

    about 200 species; Chrysophyceae, about 15 species; Chlorophyceae (Volvocales),

    2 species; Zanthophyceae (Heterokontae), 1 species.

            It is impossible in the present state of our knowledge to give more

    than approximate figures for the numbers of species in each group. There

    are certainly many more species to be found than have hitherto been de–

    tected. This is particularly true of the small unarmored flagellate forms

    usually spoken of as the “nannoplankton,” the investigation of which is

    still in its early stages and is beset with great difficulties. Most of

    the species are so small that they pass through the finest note, and so

    delicate as to be extremely diffi d c ult to preserve. A start has been made

    on their study by concentrating them, after killing, either by settling or

    by the use of the centrifuge. In coastal waters in the neighborhood of

    research stations, living material has also been examined and culture

    methods tried. From these studies it appears that these unarmored flagel–

    lates are an important element in the phytoplankton, particularly in the

    oceanic regions.

            The figures which have been given show that the diatoms and the

    Dinophyceae are by far the most important groups in numbers of species in

    the marine phytoplankton of the Arctic. They also form the bulk of the

    individuals, for Phaeocystis pouchetii and Dinobryon pellueidum of the

    Chrysophyceae, and Halosphaera viridic of the Xanthophyceae, are the only

    species belonging to other groups that have [ ?] so far been found in any

    great numbers.

            Most of the algae of the phytoplankton, in the manner of plants in

    general, manufacture their own food from carbon dioxide, water, and various

    inorganic salts, using the energy of sunlight for the purpose. Hence they

    003      |      Vol_V-0185                                                                                                                  
    EA-PS. Ross: Phytoplankton

    can only live in the upper layers of the sea where sufficient light for

    their needs penetrates. The L lowest level at which they can maintain

    themselves indefinitely is that at which the two processes of phytosynthesis,

    or the building up of food substances, and respiration, which is the break–

    ing down of those substances to provide the energy necessary for the

    various processes of life, just balance over the twenty-four hours. This

    depth, known as the compensation depth, varies with turbidity of the

    water and the strength and direction of the incident light. The turbidity

    of the water is itself greatly influenced by the amount of phytoplankton

    present. In the Arctic, because of the obliquity of the light, the frequent

    cloudiness, and the high turbidity due to an abundant phytoplankton, the

    compensation level is comparatively shallow, being often only about 20

    meters, while in low altitudes with almost vertical incidence of light and

    low plankton populations, it is often as deep as 200 meters. Another result

    of the need which plants have for light is that the abundance of phytoplankton

    in the Arctic shows great seasonal variation - in contrast to the tropics

    where the amount of the standing crop in any one place changes little except

    when hydrographic conditions alter.

            Storms and heavy seas have a detrimental effect on the growth of

    phytoplankton because of the turbulence in the upper layers of the sea

    which they cause. By means of this turbulence a proportion of the p phyto–

    plankton organisms are carried down below the illuminated layer. Some of

    them are able to regain the upper layers, but in s i o doing they have to

    expand energy which is accordingly no longer available for growth. Further–

    more, no growth is possible until the upper layers are once more reached.

    In addition, the total stock is depleted by those individuals which are unable

    to rise again above the compensation level.

    004      |      Vol_V-0186                                                                                                                  
    EA-PS. Ross: Phytoplankton

            Of the other requirements of phytoplankton organisms, water and carbon

    dioxide are always present in adequate, or more than adequate, quantities

    in both marine and freshwater habitats. On the other hand, the supply of

    mineral salts, of which nitrates and phosphates are the most important to

    the phytoplankton, often limits its growth. In the sea, nitrates and

    phosphates go through a complex cycle of assimilation by the phytoplankton,

    then consumption by animals, and later regeneration by bacterial action from

    the dead bodies of the plants or animals, or from animal feces. Much of

    this regeneration takes place below the illuminated zone, and consequently

    the maintenance of a supply of these salts in the upper layers of the sea,

    where the phytoplankton can use it, is dependent upon mixing of the upper

    and lower layers of the water or upon upwelling. During the winter in high

    latitudes, the cooling of the upper layers of the sea increases their

    density and thus reduces the stability of the water column. In those

    areas they are free from ice, mixing is further facilitated by the strong

    winds. At the same time, lack of light prohibits the growth of phytoplankton

    and considerable concentrations of nitrates and phosphates are built up in

    the surface waters. Even where the ice prevents the wind from having any

    effect, mixing does occur when the upper layer are cooled and their salt

    concentration is increased by ice formation, and there is a consequent

    increase, albeit a rather lesser one, in the nitrate and phosphate content

    of the upper layers.

            Throughout the Arctic a the development of the phytoplankton follows

    a similar cycle, but its timing varies considerably from place to place.

    The presence of ice, and especially of snow-covered ice, so reduces the

    light intensity in the water below it that the growth of phytoplankton

    005      |      Vol_V-0187                                                                                                                  
    EA-PS. Ross: Phytoplankton

    is almost entirely prevented. In the spring or early summer, when the

    ice begins to melt and open out, a rapid growth of phytoplankton occurs.

    This first outburst consists chiefly of diatoms, although these are

    preceded at times, and perhaps generally, by an abundance of minute

    flagellates belonging to the Dinophyceae, Chrysophyceae, and Chlorophyceae

    (2). These have been so little investigated, however, that the extent of

    their occurrence is virtually unknown. This rapid growth of diatoms

    depletes the water of nutrient salts and is followed by a decline in the

    number of diatoms, often equally rapid, to which grazing by the then

    abundant zooplankton contributes considerably. The diatom maximum is

    usually followed by a considerable growth of Dinophyceae which do not

    require such high concentrations of nutrient salts and which need a higher

    temperature for rapid growth. Ceratium arcticum is an especially important

    species among these. Certain less exacting diatoms may also be found

    in considerable numbers at this stage.

            This change from a predominantly diatom phytoplankton to one in which

    the Dinophyceae are the major element, normally occurs at about the time

    at which the surface layers become heated to such an extent that mixing

    with deeper waters by vertical circulation is stopped (6). The time at

    which the growth of phytoplankton begins in the spring appears to be

    controlled by temperature and light intensity, for it gets progressively

    later as one moves north. Thus in the Gulf of Maine and the English

    Channel, the diatom outb t u rst begins in late February or rarely March,

    while in Baffin Bay and the waters around Spitsbergen it does not start

    until late May or early June, and about the center of the polar basin

    it is as late as August.

    006      |      Vol_V-0188                                                                                                                  
    EA-PS. Ross: Phytoplankton

            High concentrations of diatoms are found chiefly along the coasts

    and close to the edges of the melting ice. In such places the numbers

    present are very great and only equallyed by those found in the Antarctic

    at the corresponding period of the year. Densities of the order of one

    million cells per liter have been recorded. Many factors combine to

    cause this high production of phytoplankton. The high concentration of

    nutrient salts built up by mixing during the long winter when lack of

    light prevents their utilization has been mentioned. The continuity of

    illumination when growth is taking place is another cause. Also it has

    been shown (9) that, other things being equal, the growth of diatoms is

    more rapid in waters that have recently been frozen. This is attributed

    to the stimulating effect of trihydrol, the trimolecular polymer state of

    water found in greater proportion in such circumstances. A further facto r d

    is the greater density of the cold polar waters, resulting in the expen–

    diture of less energy in maintaining the phytoplankton in the upper layers

    of the sea, so that a greater proportion of the food manufactured is

    available for growth.

            Protoplasm and the various constituents of the cell walls of the

    phytoplankton organisms are [ ?] denser than sea water and hence, although

    many plankton organisms have structures (giving “form resistance”) which

    retard their rate of sinking by increasing drag, they can only maintain

    themselves near the surface by active processes. Gross and Zeuthen (8)

    have recently shown that diatoms, which have no means of swimming, keep

    their density equal to that of the sea by reducing the density of the

    vacuolar sap - probably by eliminating bivalent ions such as calcium

    and sulphate - while maintaining it isotonic with sea water.

    007      |      Vol_V-0189                                                                                                                  
    EA-PS. Ross: Phytoplankton

            In addition to these favorable physical factors, two biological factors

    operate in the same direction. During the long arctic winter, when virtually

    no phytoplankton is present, the zooplankton population is also drastically

    reduced so that, when the phytoplankton begins to grow rapidly in the

    spring, there are few animals to graze on it. Some weeks are required

    befo d r e the zooplankton population has increased sufficiently for it to

    exercise a large effect on the numbers of phytoplankton organisms. This

    is in contrast to the conditions in warmer seas where the zooplankton is

    present throughout the year and is exercising a steady grazing pressure on

    a virtually constant phytoplankton population.

            Another necessity for the rapid increase of phytoplankton when condi–

    tions become favorable after having been unfavorable for a period, is the

    presence of sufficient individuals to initiate the increase. Many planktonic

    diatoms from resting spores, which sink to the bottom with the onset of

    unfavorable conditions. These species, which are normally found only in

    coastal and shelf waters shallow enough for the resting spores to be able,

    after germination, to reach the illuminated zone, are called “neritic”

    species, and plankton composed of them is termed neritic plankton. Others,

    which live in deep water and form no resting spores but maintain themselves

    in the upper layers of the sea throughout the year, are described as “oceanic.”

    In waters that can be colonized by neritic species, resting spores provide a

    considerable initial population, and the building up of large numbers with

    the onset of more favorable conditions is much more rapid than in oceanic

    waters. This is one reason why, in temperate waters, larger vernal growths

    of phytoplankton are found only in coastal waters.

            In the Arctic the resting spores of neritic species are able to overwinter

    008      |      Vol_V-0190                                                                                                                  
    EA-PS. Ross: Phytoplankton

    not only on the bottom in coastal waters but also frozen into the ice.

    When this melts the re [ ?] ting spores are set free and large population is

    able to accumulate rapidly. While many oceanic species are found within the

    Arctic, particularly in the Greenland and Barents Seas and in Davis Strait,

    into which they are carried by Atlantic water, the main bulk of the phyto–

    plankton, particularly during the spring maximum, consists of species

    which have overwintered in the ice. At the beginning of the spring out–

    burst, the most prominent diatoms in the phytoplankton are often species

    which can live equally well in the plankton or growing attached to ice

    floes. Melosira arctica , Fragilaria oceanica , and Achnanthes taeniata

    are the commonest of these.

            As a result of the operation of these factors, large populations of

    phytoplankton dominated by neritic species of diatoms (especially Melosira

    arctica and Fragilaria oceanica ) are found in the open waters of the Arctic

    during the spring and early summer. These high concentrations are usually

    localized, but where they occur they discolor the water noticeably and give

    it a characteristic smell. This type of water is the “black water” of the

    early whalers who used to seek it out because they knew that in and around

    areas of it they would find the largest concentrations of whales. These

    were attracted by the high concentration of planktonic animals feeding on

    the dense phytoplankton population (3).

            The boundary regions between ocean currents flowing in different

    directions constitute another type of situation in which large concentra–

    tions of phytoplankton are often found. This is due to the turbulence

    which is found in these regions as a result of the mixing along the edges

    of the two water masses, and which brings water rich in nutrient salts to

    009      |      Vol_V-0191                                                                                                                  
    EA-PS. Ross: Phytoplankton

    the surface. Also it is often the case that one water mass, whose

    nutrient content has been greatly reduced by a “flowering” of phytoplankton,

    can contribute the initial population, while the other contributes the

    necessary salts. The nitrate and phosphate content of water masses can

    change considerably in the course of a season, and accordingly the role

    played by each mass in a boundary region may differ from month to month.


    Distribution of Marine Phytoplankton

            In reaind this account it must be remembers that, for most of the

    areas, the observations on which it is based refer only to one or two

    years and conditions may vary considerably from one year to another.

    Accordingly, although the available information is presented here in

    generalized form, the annual cycles described may well not be typical,

    and almost nothing is known of their variability from year to year. Also,

    this account does not refer to waters close inshore, nor to the fjords

    and narrow channels along the coasts. Observations on these show that the

    phytoplankton may very greatly over comparatively short distances, and it

    is clear that local conditions and topography are apt to be the determining

    factors in such localities. There are indications, however, that the

    diatom-dinophycean cycle is a normal feature here as in more open water.

            The Polar Basin . The results of Nansen’s expedition in the Fram

    (5; 11), and the observations made by Sverdrup during the drift of the

    Maud (15), suggested that there was very little development of phytoplankton

    in the permanently ice-covered regions about the center of the polar basin.

    During Papanin’s drift, however, a moderately large development of phyto–

    plankton was observed to begin during early August (14). This plankton

    is poor in species and consists principally of diatoms, Chaetoceros socialis

    010      |      Vol_V-0192                                                                                                                  
    EA-PS. Ross: Phytoplankton

    being the dominant form; Dinophyceae are unimportant in it. Active

    growth takes place between the depths of 3 and 30 meters. The start of

    this growth of phytoplankton coincides with the melting of the snow on

    the ice floes, which renders them much more transparent. This suggest

    that light is the determining factor.

            Around the edge of the polar basin, similar conditions are found,

    with the start of the outburst coming earlier in lower latitudes. Off

    Wrangel Island it is still around the beginning of August, but in the

    Chuk t o tsk Sea it occurs during July. In these areas, and apparently

    throughout the polar basin, there is a large phytoplankton population

    until the onset of winter. Thus Nansen (11) reported that north of the

    New Siberian Islands there was abundant Chaetoceros in October, with

    many individuals being frozen into the newly forming ice, and the Belgica

    found large phytoplankton populations to the N n orth of Novaya Zemlya in

    September . ( ).

            Greenland and Barents Seas. Owing to the influx of comparatively

    warm Atlantic water, the spring growth of diatoms occurs early in the

    eastern part of the Greenland Sea and in the Barents Sea. Thus dense

    populations of phytoplankton, consisting mainly of diatoms, with

    Chaetoceros spp. Forming the most prominent element, are found in late

    April and early May extending northward from the region of the Faeroes

    and Iceland to Jan Mayen and Spitsbergen and into the Barents Sea. In

    the latter sea, at least, the growth of diatoms occurs earlier and is

    denser near the coast and the ice edge than it is in the center of the

    area of Atlantic water which is ice-free throughout the winter (10).

    Throughout this area the diatoms are succeeded by a population dominated

    011      |      Vol_V-0193                                                                                                                  
    EA-PS. Ross: Phytoplankton

    Dinophyceae, of which Ceratium arcticum is the most prominent form, and

    in which species of Peridinium are often frequent. Halosphaera virdis

    is also a [ ?] frequent member of this community. It is a southern

    species which comes in with Atlantic water during the summer, and may also

    maintain itself throughout the year in the ice-free parts of the Barents Sea.

            In the western part of the Greenland Sea, in the waters of the south–

    ward-flowing East Greenland current, the seasonal cycle begins with the

    development of a phytoplankton consisting of minute naked flagellates,

    some being Dinophyceae and some Volvocales. This is found under close

    and thick pack ice near the Greenland coast of Denmark Strait at the end

    of June (2), and is succeeded, as the ice opens and the upper layers of

    the sea become better illuminated, by a rich growth of diatoms. Achnanthes

    taeniata and Fragilaria oceanic are the first dominants in this diatom

    growth, followed by Thalassiosira gravida accompanied by Chaetoceros species

    in large numbers. This growth of diatoms depletes the upper layers of

    nutrients and, owing to their dilution by melting ice, the water column in

    this area at this time is very stable and replenishment from deeper layers

    is probably negligible. This population accordingly lasts only a few weeks.

    In this East Greenland current it is followed by a phytplankton community

    dominated by Detonula confervacea , a much less exacting species of diatom

    than which precede it. The larger Dinophyceae are apparently less important

    in the later part of the phytoplankton cycle in these waters than elsewhere

    in the Arctic. This is possibly due to the very low surface temperature

    even in August when it scarcely reaches 3°C.

            Davis strait and Baffin Bay . As the ice breaks up along the west

    coast of Greenland in the spring, a rich plankton consisting principally

    012      |      Vol_V-0194                                                                                                                  
    EA-PS. Ross: Phytoplankton

    of neritic species of diatoms develops (7). It is probable that this is

    preceded by a development of flagellates as on the east coast, but this

    point has nto been investigated. In the southern part of Davis Straft

    the chrysophyceae Phaeocystis pouchetii is prominent along with the diatoms.

    After a period of between one and two months, the nutrients in the upper

    layers are much depleted and the plankton becomes much poorer in individuals,

    the Dinophyceae becoming the group principally represented in it.

            During the summer the West Greenland current penetrated further and

    further north, and eventually, in September, reaches Smith Sound. As it

    does so the same cycle of plankton development is followed. Meanwhile

    cold water is flowing through Smith Sound and the straits between the

    islands to the west. Being largely ice-covered, this is very poor in

    plankton but quite rich in mineral salts. Along the boundary between the

    two water [ ?] masses large phytoplankton populations develop, with

    Ceratium arcticum the most prominent species, and as the summer advances

    and the surface temperature of the water rises slightly, this community

    spreads throughout the whole area of the cold water mass. Just south of

    Cape York at the beginning of August a phytoplankton dominated by Detonula

    confervacea has been recorded. This may correspond in its place in the

    annual cycle to that observed in the East Greenland current by Braarud (2).

            Hudson Bay and the Canadian Arctic Archipelago . Almost nothing is

    known of the marine phytoplankton of this area. Davidson (4) investigated

    the diatoms of some plankton collections made in Hudson Bay in August and

    early September. Chaetoceros compressus , C. lacinosus , and C. convolutus

    were the most important species. The density fell off from the north and

    northeast of the bay toward the southwest, the distribution of diatoms

    013      |      Vol_V-0195                                                                                                                  
    EA-PS. Ross: Phytoplankton

    being very similar to that of salinity and temperature, with the densest

    populations in the colder and more saline waters. In addition Polunin (12)

    reports a phytoplankton consisting principally of Chaetoceros and

    Thalassiosira species as occurring at the end of August off Akpatok Island

    in Hudson Strait. This again was apparently not very rich.



            Our knowledge of the phytoplankton of arctic fresh wa r t ers is much less

    than of that from marine sources. It is limited to floristic lists from

    various localities and more or less subjective estimates of the abundance

    when the collections were made. For the most part only single collections

    have been made from the various lakes. The only set of observations made

    at intervals over a year are those of Vanhöffen (16), who made a series of

    collection from November to July in a small lake close to the Karajak Fjord

    just north of latitude 70° N. on the west coast of Greenland. In this lake

    Dinobryon bavaricum var. vanhoeffenii was the most important planktonic plant,

    being present in small numbers under the ice from November to March; in late

    May, when the lake was still ice-covered, and in early July, when the water

    was open, it was present in considerable quantity. Peridinium cinctum ,

    one of the Dinophyceae, was also present in considerable numbers in

    November in January, more sparsely in March, and again in considerable

    numbers in July. The diatoms, of which Eunotia and Tabellaria species

    were the most important, apparently take longer to develop after the melting

    of the ice, for none were found in July, although they had been frequent

    the previous November and January. The only other planktonic alga of any

    importance was the desmid Xenthidium fasciculatum , which was found not

    infrequently in July.

    014      |      Vol_V-0196                                                                                                                  
    EA-PS. Ross: Phytoplankton

            The floristic lists which are available for a number of lakes in

    Greenland (1), Novaya Zemlya, and Franz Josef Land (14) show that there

    is considerable variation in the phytoplankton of the arctic lakes. No

    work has, however, yet been published which enables a correlation to be

    made between plankton types and the hydrological features of the lakes,

    nor has the effect of the long periods of ice cover on the physical and

    chemical conditions yet been elucidated. It is possible, nevertheless,

    to make some suggestions based on analogies with conditions farther south

    in the Subarctic and North Temperate Zones. The majority of arctic lakes

    have a poor phytoplankton and desmids are frequently the most important

    group contributing to it. Lakes with a plankton dominated by desmids are

    also found quite commonly in western and northern Scotland and in the

    mountain regions of Norway. Such lakes are usually very poor in mineral

    salts and have rather said waters. Lakes in which the main component of

    the phytoplankton is one of the species of Dinobryon are also frequently

    met with. These are especially the shallower lakes with muddier substrata,

    and may be somewhat richer in nutrients than those characterized by desmids.

    Diatoms do not seem to [ ?] be a predominant element in the plankton of any of

    the lakes investigated. Almost always the phytoplankton has been poor

    in quantity; but a water bloom, due to the colonial chrysophycean

    uroglens volvox , was reported from a shallow lake in Greenland by Bachmann (1)

            The approximate numbers of species of the various groups of algae

    recorded up to the present from freshwater plankton in the Arctic is:

    Chlorophyceae, about 100 species; Bacillariophyceae (diatoms) about 50

    species; Cyanophyceae, about 20 species; Dinophyceae, about 10 species;

    Chrysophyceae, about 10 species; Xanthophyceae, about 5 species. These

    015      |      Vol_V-0197                                                                                                                  
    EA-PS. Ross: Phytoplankton

    figures are very tentative since it is not always possible to be certain

    which of the gatherings examined by Bachmann (1) and Shirshov (14) should

    be regarded as consisting of plankton. All further work will undoubtedly

    add many species.

    016      |      Vol_V-0198                                                                                                                  
    EA-PS. Ross: Phytoplankton


    1. Bachmann, H. “Beiträge zur Algenflora des Süsswassers von Westgrönland,”

    Mitt.Naturf.Ges.Luzern , vol.8, pp.1-181, 1921.

    2. Braarud, T. “The Øst expedition to the Denmark Strait, 1929.” Skr .

    vol.10, pp.1-173, 1935.

    3. Brown, R. “On the nature of the discoloration of the Arctic Seas,”

    Trans.Bot.Soc.Edinb . vol.9, pp.244-52, 1868.

    4. Davidson, V.M. “The planktonic diatoms in Hudson Bay (Biological and

    oceanographic conditions in Hudson Bay, 5)” Contrib.Can .

    Biol . and Fish ., n.s. vol.6, pp.497-509, 1931.

    5. Gran, H.H. Diatomacaae from the Ice-Floes and Plankton of the Arctic

    Ocean. London, N.Y., Longmans, Green, 1904. Norwegian North

    Polar Expedition, 1893-1896. Scientific Results, vol.4,

    no.11, pp.1-74, 1900.

    6. Gran, H.H. “Quantitative plankton investigations carried out during

    the expedition with Michael Sars, July-Sept.1924,”

    Conseil Perm.Internat.Explor.Mer. Rapport vol.56, no.5,

    pp.1-50, 1939.

    7. Grøntved, Jul. and Seidenfaden, Gunnar. The Phytoplankton of the Waters

    West of Greenland . København, Reitzel Medd.Grønland vol.82,

    no.5, pp.1-380, 1938.

    8. [ G ?] ross, F., and Zeuthen, E. “The buoyancy of plankton diatoms: A problem

    of cell physiology,” Proc.Roy.Soc . London, B, vol.135,

    pp.382-89, 1948.

    9. Harvey, H.W. “Substances controlling the growth of a diatom,”

    J.Mar.Biol.Ass.U.K ., n.s. vol.23, pp.499-520, 1939.

    10. Krebs, E., and Verbinskaya, N. “Seasonal changes in the phosphate

    Barents Sea,” J.Cons.Int.Explor.Mer . vol.5, pp.329-46, 1930.

    11. Nansen, F. The Oceanography of the North Polar Basin . Norwegian North

    Polar Expedition, 1893-1896. Scientific Results , vol.3,

    no.9, pp.1-429, 1902.

    12. Polunin, Nicholas. “The vegetation of Akpatok Island. Part II,” J.Ecol .

    vol.23, pp.161-209, 1935.

    13. Shirshov, P.P. “Ecologic geographical essay on the fres y h /-water algae

    Novaya Zemlya and Franz-Joseph Land,” Leningrad.

    Arkticheskii Nauchn.-Issled Inst. Trudy , no.14, pp.73-162,


    017      |      Vol_V-0199                                                                                                                  
    EA-PS. Ross: Phytoplankton

    14. ----. “Oceanological observations,” C.R.Acad.Sci. U.R.S.S ., vol.19,

    pp.569-80, 1938.

    15. Sverdrup, H.U. “The waters on the North-Siberian Shelf,” Norwegian

    North Polar Expedition “Maud” 1918-1925. Scientific Results

    vol.4, no.2, pp.1-131, 1929.

    16. Vanhöffen, E. “Die fauna und flora Gr ö nlands,” Grönland-Expedition

    der Gesellschaft für unde zu Berlin 1891-1893 . Berlin, 189 8 7 .

    vol.2, pp.1-383.


    Robert Ross

    Arctic Plant Physiology

    Unpaginated      |      Vol_V-0200                                                                                                                  
    EA-Plant Sciences

    (J. Warren Wilson)




    Growth 1
    Photosynthesis and Respiration 4
    Nitrogen Relations 13
    Experimental Culture of Plants 15
    Observation of the Effect of Fertilization 15
    Observations of Late-Snow Areas 16
    Plant Analysis 17
    Water Relations 19
    Summary and Conclusions 20
    Bibliography 22

    001      |      Vol_V-0201                                                                                                                  
    EA-Plant Sciences

    (J. Warren Wilson)



            The foremost feature of arctic vegetation is perhaps its poverty; the

    most obvious factor peculiar to the arctic environment is low temperature.

    One of the main objects of the physiologist must be to find the mechanism

    of the relationship (if any exists) between these two features. An exami–

    nation of growth rates appears likely to be a profitable approach.

            Practical difficulties have so discouraged work with plant physiology

    in the Arctic that only four real contributions have been made - those of

    Müller and Wager in Greenland, and of Russell and Warren Wilson on Jan Mayen

    Island. These four investigations form a rather inadequate basis for generali–

    zation about the physiology of plants in the Arctic as a whole. Low-temperature

    studies made in laboratories of temperate countries can be used to supplement

    field investigations in the Arctic; but as temperature is only one of a complex

    of factors affecting arctic plant growth, such studies have a limited value in

    this connection. Several aspects of arctic plant physiology remain completely




            Wager (40), by weighing pressed plants belonging to various age groups,

    found that in several species growing at Kangerdlugssuaq, East Greenland

    002      |      Vol_V-0202                                                                                                                  
    EA-PS Warren Wilson: Arctic Plant Physiology

    (lat. 68° N.P, the weight was approximately doubled each year, and in especially

    warm or moist habitats it was trebled. Warren Wilson (44) estimated annual

    growth to be of the same order at Jan Mayen (lat. 71° N.). Such increases

    are found normally in a single week’s growth of plants in temperate regions.

            These low values are related to the short growing season, which lasts

    only some two or three months for arctic plants. A slow rate of growth even

    at the height of the season is probably an additional factor. Few data are

    available on this point. Warren Wilson (44) determined the relative growth

    rates at midsummer of plants growing at Jan Mayen. Even in favorable habitats,

    values for Saxifraga tenuis were less than one-tenth of those typical of tem–

    perate plant growth, and those for Koenigia islandica about one-third. Plants

    growing in unfavorable habitats gave lower figures, falling in one especially

    barren area to a negative value.

            The relative growth rate is a measure dependent not only on the rate of

    assimilation; it is also a function of plant form — of the relationship of

    total weight to leaf area. The net assimilation rate (increase in dry weight

    in grams per square decimeter of leaf area per week) has the a s d vantage of being

    independent of form; it measures the efficiency of what might be termed the

    photosynthetic apparatus.

            Heath and Gregory (13) believe “that the net assimilation rate in the

    most diverse types of plant under very various conditions approximates to a

    constant mean value” — about 0.55. Watson (46), on the other hand, found

    seasonal trends and differences between species. It is of some general interest,

    therefore, to find out whether values under the extreme conditions of the Arctic

    approximate to the “usual” value.

            Russell (29) obtained a value of 0.30 for the net assimilation rate of

    003      |      Vol_V-0203                                                                                                                  
    EA-PS. Warren Wilson: Arctic Plant Physiology

    Oxyria digyna in mid-August at Jan Mayen. Because of various deficiencies in

    his experimental technique (a detached leaf method was used), he regarded this

    value as minimal, and concluded that “the net assimilation rates of arctic

    plants are at least comparable with those of plants in temperate regions.”

            Similar studies, in greater detail, gave values near 0.55 for four species

    and indicated, for Oxyria digyna (45); ( 1 ) a downward trend from 0.9 at the

    end of June to 0.4 at the end of August; ( 2 ) low net assimilation rate, 0.2 to 0.3,

    in midseason of leaves from ce d r tain especially poor habitates; and ( 3 ) a 25% lower

    net assimilation rate in an exposed as compared with a sheltered locality doing

    leaves from one source.

            Net assimilation rates are usually determined on whole plants, and the use of

    detached leaves gives spuriously high results, as it neglects losses due to respi–

    ration in stem and root. Thus net assimilation rates of detached leaves under

    temperate conditions are of the order of 1 to 2 (7; 32). The approximation to

    0.55 of net assimilation rates as determined at Jan Mayen on detached leaves

    suggests therefore that estimates on whole plants would give low values. Deter–

    minations for whole plants of three species growing in favorable habitats gave

    net assimilation rates between 0.18 and 0.23; even lower values were found for

    plants in poor habitats (44).

            At Jan Mayen, therefore, and probably in the Arctic generally, plants appear

    to have abnormally low net assimilation rates which are subject to further

    depression by poor conditions of soil or climate.

            The figures quoted suggest that although Koenigia islandica and Saxifraga

    tenuis have roughly similar net assimilation rates, the former has a much higher

    relative growth rate than the latter. This discrepancy is attributable to

    differences in form. In the minute Koenigia (at Jan Mayen, plants are about

    004      |      Vol_V-0204                                                                                                                  
    EA-PS. Warren Wilson: Arctic Plant Physiology

    5 mm. tall), a higher proportion of material can be devoted to photosynthetic

    structures than in the larger Saxifraga , which necessarily has a greater propor–

    tion of structural tissue. The ratio of leaf area to total dry weight is some

    four times higher in Koenigia islandica than in Saxifraga tenuis , and the

    Saxifraga in turn gives a ratio rather higher than those typical of crop plants

    in temperate regions. The relative growth rates, for a certain net assimilation

    rate, vary correspondingly. This association of high relative growth rate with

    small size may determine in part the smallness of arctic plants generally; it

    may also explain the capacity of Koenigia to grow farther north a than any other


            Wager (41) has pointed out that the ability to become active rapidly in

    spring is a characteristic feature of arctic plants which enables them, by an

    early expansion of leaves, to exploit the short arctic growing season to the

    full. Sørensen (34) has reached the same conclusion; he mentions two ancillary

    features — the production of leaves simultaneously on several shoots (as opposed

    to successive expansion on one shot), and the aperiodical development of organs,

    which allows even short favorable periods to be utilized.

            Such considerations are clearly related to the dominance of perennial plants

    in arctic regions; their rapid spring growth is made possible by food stored in

    perennating organs. Annual species with no such store are at a disadvantage

    unless, as Russell (28) points out, the seed contains a large proportion of

    stored food relative to the total growth of the plant.


    Photosynthesis and Respiration

            Net assimilation and growth depend on the balance of photosynthesis and respira–

    tion. Arctic conditions may be expected to affect these processes both directly

    and also through genetic changes in the physiology of arctic species. Müller (23)

    005      |      Vol_V-0205                                                                                                                  
    EA-PS. Warren Wilson: Arctic Plant Physiology

    and Wager (41) have studied gas exchange at controlled temperatures in East

    and West Greenland, respectively latitude 68° N., and their results appear

    to lead to an appreciation of the more important effects of arctic conditions

    on phytosynthesis and respiration. Müller and Wager used temperatures of

    10° C. and 20°C., and less frequently of 0°C. Observations on the balance

    of phytosynthesis and respiration under field conditions of temperature and

    illumination are lacking for arctic regions, apart from a single observation

    on detached leaves of Oxyria digyna at Jan Mayen (45). The ratio of photosyn–

    thesis to respiration obtained at midsummer was roughly 6: 1. Anderson (2)

    usefully summarizes the literature on gas exchange at low temperatures, and

    the results of his experiments with winter wheat confirm those of Müller and

    Wager with arctic plants.

            Kostychev, Chesnokov, and Bazyrina (17) at Portshnicha (lat. 69° N.) on

    the Murmansk coast estimated rates of [ ?] photosynthesis by a method of their

    own, which has been criticized by Müller and Wager, and which gave unusually

    high results with temperate material. Their observation of lower rates of

    photosynthesis in four arctic species than in temperate plants does not agree

    with results of other workers and must be treated with reserve.

            Using gas analysis methods, Müller and Wager found that the maximum rates

    of photosynthesis (under high light intensity and at 20°C.) of arctic and

    temperate plants were similar. Lower temperatures, however, reduced the rate

    of photosynthesis less for arctic than for temperate species; thus Wager found

    the Q 10-20 of arctic and temperate plants averaging about 1.2 and 2.0,

    respectively. These figures are from several sources.

            Respiration behaves quite differently. Stocker (35), using Muller’s figures,

    and Wager (41) have shown that, at a particular temperature, respiration is

    006      |      Vol_V-0206                                                                                                                  
    EA-PS. Warren Wilson: Arctic Plant Physiology

    more rapid for arctic than for temperate and tropical species. Wager, using

    figures from several sources, obtained the following averages for respiration

    at 20°C: Arctic, 2.3 milligrams per square decimeter per hour; Temperate,

    1.25 mg. / sq.dm. / hr., and Tropical, 0.5 mg. / sq. dm. / hr. The same

    trend is apparent when a fresh-weight basis ms used. Especially striking is

    the fact pointed out by Anderson that the respiration of arctic plants in

    Greenland at 10°C. may have the same intensity as that of tropical plants in

    Java at 30°C.

            The Q 10 of respiration, on the other hand, appears to be similar for

    arctic and temperate species, at least when active summer material is used (41).

    Winter material has given slightly higher values, probably owing to saturation

    of enzyme systems by the high carbohydrate concentrations known to occur in

    arctic plants in winter; analysis of Wager’s figures in fact shows a correla–

    tion of high respiration rate with high Q 10 .

            Thus temperature has a greater effect on respiration than on photosynthesis.

    This is confirmed many times in literature. Zakharova (50) considers assimila–

    tion in Pinus sylvestris and Picea excelsa to be independent of temperature over

    a wide range around zero centigrade; Müller (23) found for Epilobium (Chamaenerion )

    latifolium and Salix glauca that the Q 10-20 for photosynthesis at high light

    intensities was 1.25 and 1.48, respectively; but for respiration, 2.5 and 1.8

    in the same plants, Stålfeld (36) found for certain lichens a Q 2-10 of photo–

    synthesis of 1.23 and 1.20, but of respiration, 2.29 and 2.80; Anderson (2),

    investigating the hardening process in winter wheat, stated that “the respira–

    tion is far more influenced by the temperature than the assimilation.”

            The observations mentioned above permit the following general conclusions:

    arctic plants are capable of photosynthesizing rapidly — as “sun” plants --

    007      |      Vol_V-0207                                                                                                                  
    EA-PS Warren Wilson: Arctic Plant Physiology

    given relatively favorable conditions of temperature and light intensity;

    under such conditions, loss of energy in rather rapid respiration is not

    dangerous to the plant; it may indeed be related to a capacity for rapid

    growth in the arctic spring. With lower temperatures, the rate of photosyn–

    thesis falls, but, as the respiration rate falls more markedly, the efficiency

    of the over-all exchange is decreased less than proportionately. Synthesized

    materials appear as accumulating sugars rather than as new growth.

            Müller (23) has shown that the compensation point falls with temperature.

    Thus at low temperatures, relatively little illumination may “saturate” the

    photosynthetic apparatus. Müller demonstrated, in various arctic species under

    low light intensities, an apparent assimilation more rapid at 10°C. than at

    20°C. Stålfelt (36; 37) has found the same effect in a number of mosses and

    lichens. It is clearly of importance in relation to assimilation under arctic

    conditions, where temperatures are low, and where light, though of low inten–

    sity, is more or less continuous.

            Anderson (2) found in winter varieties of wheat and rye many of the

    physiological features of arctic plants: a maximum assimilation rate at

    15°C. of the same order as in sun plants, and the rate at 2°C. only 25%

    less rapid; a very high respiration / maximum assimilation quotient at low temperatures;

    a normal compensation point at 15°C., and a low or very low compensation point

    (as for a shade plant) at 2°C. He traced the physiological changes accompanying

    lowered temperatures more thoroughly for his winter varieties than has been

    done for arctic plants; as the same main features are likely to occur in both

    groups, his observations are summarized here.

            He found that at 7° to 8°C. growth practically ceased, and carbohydrate

    formation and utilization balanced. A decrease in temperature disturbed

    th d e balance, resulting in a surplus of assimilates and an increase in the

    008      |      Vol_V-0208                                                                                                                  
    EA-PS Warren Wilson: Arctic Plant Physiology

    sugar content of the plant. After some time the rate of photosynthesis

    began to fall off gradually and at the same time the respiration rate began

    steadily to increase. The daily surplus consequently diminished, finally

    reaching zero, so that a balance between production and consumption was

    again reached. The level at which this new balance was reached depended

    upon light and temperature and also upon inherent characters of the plant,

    both genetic and historical.

            Anderson showed that the depression in photosynthesis and increase in

    respiration which he found at low temperatures were caused, directly or

    indirectly, by accumulation of sugar. This is interesting in view of a

    comparison by Russell (28) of plants of Oxyria digyna growing at Jan Mayen

    under continuous daylight, with some occurring at 3,800 meters in the

    Karakoram-Himalaya where there was an eleven-hour night. Temperatures in

    both localities were low. A surprisingly close similarity was found in con–

    tents of starch and of reducing and nonreducing sugars, suggesting “that when

    the level of carbohydrate is high it may tend to reduce the assimilation rate.”

            The accumulation of sugar in plants at low temperatures is well estab–

    lished as a result of investigations on frost hardening (2). The sugar

    is derived from hydrolysis of starch and also directly from photosynthesis;

    the former source is probably the more important. The only observation on

    carbohydrate levels of plants under arctic conditions are those of Russell (29)

    at Jan Mayen.

            Russell analyzed Oxyria digyna roots at four stages of development,

    from the start of activity in spring to the fruiting condition of mid-August.

    His values are expressed as percentages of alcohol-insoluble material (see

    Table I). The earliest estimate suggested that during the winter, with

    010      |      Vol_V-0209                                                                                                                  
    EA-PS Warren Wilson: Arctic Plant Physiology

    Table I. Analysis of Oxyria Digyna Roots.
    Development stage Total

    sugar, %
    Starch, % Total

    carbohydrate, %
    Break of dormancy 36.6 1.9 38.5
    Flowering 22.8 3.2 26.0
    Fruiting 12.4 6.0 18.4
    Late fruiting 13.8 13.0 26.8

    temperatures averaging −5°C., starch was largely converted to sugar. During

    rapid growth, in spring and early summer, (the third sample was taken on

    July 21), sugar was used. Subsequently carbohydrate was built up again in

    shoot as well as root. Throughout the season, startch content gradually

    increased, even when the total carbohydrate level was falling; summer tempera–

    tures were apparently sufficiently high to promote the conversion of sugar to


            Indirect confirmation of these results is provided by the work of Wager

    and Wager (42) which showed that, in seven species growing at latitude 68° N.

    in East Greenland, osmotic pressures were high during the winter, fell rapidly

    in spring to a low summer value, and built up again d r u ring [ ?]

    autumn and winter (owing, it was suggested, to physiological drought as well

    as to temperature effects). Arrhenius and Sőderberg (3) found high osmotic

    pressures in mountain plants growing at Abisko in Swedish Lapland (lat. 68° N.)

    at temperatures around zero centigrade.

            Russell’s analyses of Polygonum viviparum showed carbohydrate levels

    rather lower than in Oxyria digyna , but yet quite high. Leaves of Ranunculus

    glacialis contained about 50% total carbohydrate (alcohol-insoluble material

    011      |      Vol_V-0210                                                                                                                  
    EA-PS Warren Wilson: Arctic Plant Physiology

    basis), mostly as reducing sugar. He concludes from these high carboh d rate

    levels that arctic plants do not suffer from carbohydrate deficiency; and

    that the hypothesis of Wager (40), that the poor vegetation of fjeldmark

    is related to carbohydrate starvation, is false.

            Russell (29) found a diurnal rhythm in carbohydrate content during the

    arctic summer day, when light, though of low intensity at midnight, is con–

    [ ?] tinuous. In mid-August at Jan Mayen, sugar and starch levels in Oxyria

    digyna leaves rose during morning and afternoon, and fell again during evening

    and night. In the absence of data on translocation, interpretation in terms

    of assimilation is difficult. It seems likely that at midsumer, when light

    intensity varies less, the diurnal rhythm would be less marked.

            Curtel (9), using rye an [ ?] Hieracium pilosella , investigated assimilation

    during the “night” (August 1) at 900 meters in mountains of Norway (lat. 62° N.);

    but his method did not give reliable quantitative results, and his estimates do

    not cover the darkest period of the night. Kostychev (16), working at

    St. Petersburg (lat. 60° N.), found that assimilation, as indicated by gas

    analysis, had ceased half an hour after sunset in four herbaceous species, though

    not in Pinus strobus and Abies sibirica . Kostychev, Chesnokov, and Bazyrina (17),

    at Portshnicha (lat. 69° N.), found assimilation generally continuing throughout

    the 24-hour day in July.

            Kjellman (15) provided 24-hour daylight artificially and found that arctic

    species could use the additional light, whereas other plants grew only slightly

    better in continuous than in interrupted light. Fries (12) at Abisko in

    Swedish Lapland (lat. 68° N.) agreed with Kjellman that plants could assimi–

    late during a light “night”, but found that temperate as well as arctic species

    were capable of doing so. His results were confirmed by Polunin (25a) working

    in Norwegian Lapland.

    012      |      Vol_V-0211                                                                                                                  
    EA-PS Warren Wilson: Arctic Plant Physiology

            It seems likely that arctic plants have low optimum and minimum tempera–

    tures for assimilation as compared with temperate plants. Direct evidence is,

    however, lacking. Lundegårdh (20) states that many plants can assimilate

    at temperatures below zero; he mentions the low temperature-assimilation

    optima (around 10°C.) found by Henrioi for alpine plants. Beliakov (4)

    claims that his figures for two varieties of barley show a difference of 10°C.

    in assimilation optima, corresponding to differences in geographical distribution.

    Kreusler (19) found assimilation at temperatures just below zero in several

    species native to warm regions. Jumelle’s observations (14) of assimilation

    in certain conifers and lichens at or near −40°C. are probably false, owing

    to certain features of his method. Matthaei (21) demonstrated photosynthesis

    in Prunus laurocerasus at −6°C., and Zakharova (50) showed that there was

    photosynthesis at temperatures several degrees below zero in Pinus sylvestris

    and Picea excelza . Walter (43), using winter wheat and varley, detected assimi–

    lation at −2° to −3°C., and respiration at −6° to −7°C. It seems likely that

    the cold-enduring plants of arctic regions will, as Matthaei suggests, photo–

    synthesis at even lower temperatures. Indeed, Wager’s figures (41) suggest

    that Ranunculus glacialis , Oxyria digyna , and Sexifraga cernua in Greenland

    assimilate almost as fast between 0° and 1°C. as at 10° or 20°C. In this

    connection also it is known that leaf temperatures under field conditions are

    generally higher than air temperatures, because of radient energy absorbed

    from insolation. Thus Ehlers (11) has shown that at temperatures between

    −20° and +5°C. pine leaves may be 2° to 10°C. warmer than the surrounding air.

            Müller (23) at Disko, East Greenland, and Kostychev, Chesnokov, and

    Bazyrina (17) at Portsnicha, Murmansk, both at latitude 69°N., have found

    stomata wide open in all plants examined throughout the 24-hour daylight

    013      |      Vol_V-0212                                                                                                                  
    EA-PS Warren Wilson: Arctic Plant Physiology

    of arctic summer. Stålfelt (38) at Abisko (lat. 68° N.) found some species

    with open stomata and some with stomata partly closed at midnight in July.

    Russell (29), using a porometer, found that at Jan Mayen (lat. 71° N.) during

    July and August the stomata of Oxyria digyna and Taraxacum [ ?] lapponicum

    ( T. croceum ) were always open, but less widely so at night. Polunin (25a),

    on the other hand, found in plants of Norwegian Lapland, stomata open at night

    but closing during the day in relation to dryness of the woil and atmosphere.

    Stomata of snow-covered plants remained inactive and for the most part closed.


    Nitrogen Relations

            Available information suggests that the growth rates of plants in the

    Arctic are low or very low; also, that carbohydrates are in excess rather than

    in short supply. Some factor other than reduced assimilation rate must there–

    fore be looked to for the limitation of growth.

            Summerhayes and Elton (39) describe how, in bird-manured areas of Svalbard,

    the general dward-shrub heath is replaced by rich herbaceous (often grassy)

    communities. Such enriched vegetation around localities frequented by birds,

    foxes, or men is a general feature of arctic regions (27; 31). Summerhayes

    and Elton ascribed the change to increased available nitrogen, stating that the

    supply of nitrogen in arctic soil d s is very small They had no direct analytical

    evidence and mentioned that other factors (e.g., phosphate) might be involved.

            Russell et al . (30) also believed that nitrogen deficiency was widespread

    and of foremost importance in controlling the density and floristic composition

    of arctic vegetation. They attributed the lack of available nitrogen mainly to

    slow rates of bacterial activity, due in turn to low temperatures and small

    nutrient supply.

    014      |      Vol_V-0213                                                                                                                  
    EA-PS Warren Wilson: Arctic Plant Physiology

            The only available estimates — those of Acock (1) for Svalbard, and

    those of Russell et al . (30) and Warren Wilson (44) for Jan Mayen — suggest

    that total nitrogen contents of arctic soils lie between 0.05 and 0.1% dry

    weight, though higher and lower values are occasionally found. These figures

    are somewhat lower than those characteristic of temperate soils, which usually

    fall between 0.1 and 0.4%. Soluble nitrogen contents of Jan Mayen soils

    (which are exceptionally young) are of the order of 10 parts per million of

    which only about [ ?] one-tenth is inorganic.

            These low values of soil nitrogen correspond to the small quantities of

    nitrogen present (per unit area of ground) in the generally sparse and low

    vegetation. Russell et al. (30) have shown a correlation of soil nitrogen

    content with density of vegetation; and these two features are in turn core–

    lated with numbers of microorganisms and rates of biological processes —

    respiration, nitrification, etc. — in the soil (44).

            Such a correlation of vegetation with soil [ ?] nitrogen would be expected,

    if nitrogen supply were inadequate; but a deduction in the reverse direction

    (that if a correlation is found, nitrogen supply must therefore be deficient)

    cannot be made. The presence of vegetation, controlled by whatever factor it

    may be, will promote the accumulation of soil nitrogen by stabilizing soil

    and reducing leaching effect, and by improving conditions for microorganism

    activity through the presence of moisture and humic substrates, and by

    rhizosphere effects.

            Luxuriance of arctic vegetation seems to be generally related not only

    to soil nitrogen but also to local microclimatic and topographic effects - –

    such as shelter (reinforced by snow cover), aspect, and moisture (5; 26; 45).

            It seems that where climate, vegetation, and nitrogen supply are correlated,

    015      |      Vol_V-0214                                                                                                                  
    EA-PS Warren Wilson: Arctic Plant Physiology

    the primary differentiating factor is most likely to be climate, which will

    affect vegetation and hence soil nitrogen; interaction between these three

    features will of course occur. However, it is not yet shown at all clearly

    that such a relationship is existent. Work concerned with evaluating the

    relative importance of climatic severity and nitrogen deficiency in limiting

    arctic vegetation has so far been confined to Jan Mayen, and has not been

    carried much beyond a preliminary stage. Further work, along lines which are

    indicated in this article, and in the various parts of the Arctic, is much needed.

    In general, the following methods have been used.

            Experimental Culture of Plants . Winter rye, grown simultaneously at Jan

    Mayen and in England, under identical condit i ons of sand culture, showed

    development some six times as fast in England as at Jan Mayen. The arctic climate

    slowed down hydration of the seed; translocation of materials from endosperm to

    embryo; and, when leaves had expanded, assimilation. These differences were

    even more marked when spring rye was used instead of the winter variety.

            Turnip seedlings, culture on an exposed hilltop and in a sheltered

    lowland habitat at Jan Mayen, grew some 50% faster in the more favorable

    lowland climate (44; 45).

            Though the results of such experiments would be more significant if species

    native to the Arctic were used, it is clear that climatic effects along (even

    at midsummer) may seriously hinder arctic plant growth.

            Observation of the Effect of Fertilization . Manuring by bird, fox or man

    results in increase in total and soluble nitrogen content of the soil, and in

    numbers and rates of biological activity of soil microorganisms. Accompanying

    these effects are characteristic changes in floristic composition of the

    vegetation. Plant growth also becomes more dense — as it does with manuring

    016      |      Vol_V-0215                                                                                                                  
    EA-PS Warren Wilson: Arctic Plant Physiology

    in temperate countries — but appears not to attain as rich a condition

    (measured as dry weight of plant material per unit area) as is found in more

    favorable, unmanured habitats. This is especially brought out by the fact

    that luxuriant vegetation examined by Russell et al . (30) in bird-manured areas

    owes its richness in fact to favorable aspect coupled with outcropping of a

    rock type producing particularly suitable soil. Moreover, while the [ ?]

    rich vegetation developed in naturally favorable (but unmanured) localities

    persists, that developed under manuring gradually shows less and less differen–

    tiation from the general vegetation, if the addition of manure ceases.

            Seedlings of several species, cultivated on various types of soil at

    Jan Mayen, grew much faster on bird cliff soils than on others. Growth was

    also increased, on all soils, by the addition of phosphate, potassium, and

    especially nitrogen fertilizers. In the field, however, these fertilizers

    rarely had any effect. The appearance of differences in culture was probably

    related to the absence of competition (44; 45).

            Observations of Late-Snow Areas. R ussell et al. pointed out that the

    level of available nitrogen in the soil as measured at any one time is dependent

    on the balance between formation and utilization, and forms a poor basis for

    assessing deficiency. The measurement of rates of turnover at different

    seasons being impossible, Russell ingeniously compared the level of inorganic

    nitrogen at the edge of a late-snow area with that 20 meters away, where the

    plants had been free of snow for some three weeks and had developed rapidly.

    Values of inorganic nitrogen of 0.39 and 0.02 parts per million were found,

    and the difference was attrib y u ted to nitrogen uptake by the growing plants

    proceeding to a point at which concentrations were too low for further absorption.

    Warren Wilson (44) failed to find this effect.

    017      |      Vol_V-0216                                                                                                                  
    EA-PS Warren Wilson: Arctic Plant Physiology

            Late-snow areas are also of interest at Jan Mayen in that, where they

    face south, they support the richest vegetation found on the island. This

    is correlated with high soil nitrogen contents, and with air and soil tempera–

    tures as much as 2° and 6°C., respectively, above those of surrounding un–

    sheltered areas (45). While such microclimatic differences will come into

    force as soon as a late-snow depression is formed, and will favor plant growth,

    it is difficult to understand why a high soil nitrogen content should be

    differentiated in such places except as a result of successful plant growth.

            Plant Analysis . Analyses of Oxyria digyna leaves (44) showed progressively

    better growth and higher total nitrogen content correlated with successively

    more favorable habitat conditions as regards shelter and soil nitrogen content;

    soluble nitrogen was, however, lowest in leaves of the best-developed plants,

    and progressively higher (a three-fold difference) in the poorer plants. This

    suggested that poor growth was not a product of nitrogen deficiency.

            Polunin (personal communication), using the diphenylamine test for nitrates

    plus nitrites on Cornwallis Island, north of Baffin Island, found in 1949 “no

    marked deficiency, at least in the plants of some habitats.”

            Russell et al. (30) believe that the activity of soil microorganisms in

    the Arctic must be limited for a considerable part of the year by cold;

    observations on temperate microorganisms are quoted, which suggest that

    activities are slight below 5° to 7°C. However, Warren Wilson (44) has shown,

    in microorganisms isolated from Jan Mayen, soils, that there is activity at

    rather lower temperatures than in their temperate counterparts. Fungi had

    cardinal points about 5°C. lower, and ceased activity around zero centigrade

    (mean for 20 isolates); for bacteria the optimum (mean of 10) was 18°C.,

    certain species having an exceptionally low optimum of 11°C. All bacteria

    examined appeared to remain active at temperatures several degrees below zero


    018      |      Vol_V-0217                                                                                                                  
    EA-PS Warren Wilson: Arctic Plant Physiology

            Observations on microorganisms in late-snow areas give additional infor–

    mation; for while snow persists, conditions in the soil beneath must be

    similar to those generally prevalent during winter. Warren Wilson (44)

    found that numbers of fungi in soil beneath the melting snow (at 0°C.) were

    very low’ numbers continued low a few meters beyond the edge of the snow, and

    then increased suddenly (approximately sevenfold) to a steady high value. This

    increase presumably corresponded to germination of fungal spores, which is

    depressed by cold even more than is vegetative growth. Bacteria, on the other

    hand, were present in considerable numbers under the snow, and increased

    approximately twofold immediately on melt, to a steady figure. These results

    suggest that, during winter, soil fungi may be largely inactive, but that soil

    bacteria (which are physiologically more important than fungi in the soil) may

    remain to a considerable extent active.

            As, moreover, arctic soil temperatures are usually several degrees higher

    than air temperatures, owing to insolation (1; 34; 45), it seems by no means

    certain that limitation of the activity of microorganisms occurs to such an

    extent as to cause nitrogen deficiency.

            Although numbers of microorganisms are relatively low in many soils at

    Jan Mayen, rates of biological activities appear to be reduced less than

    proportionately (47).

            It is clear that both climatic and edaphic factors may limit the growth

    of arctic vegetation; but the evidence is too scanty to permit of a decision

    as to which factor is generally the more important. An interesting possibility

    is revealed by physiological studies on frost hardening. In plants at low

    temperatures, soluble nitrogen content increases at the expense of protein

    nitrogen. This change has been demonstrated for winter wheat by Mudra (22)

    019      |      Vol_V-0218                                                                                                                  
    EA-PS Warren Wilson: Arctic Plant Physiology

    and by Dexter (10); for the bark of certain trees by Siminovich (33); for

    va d r ious crop plants by Wilhelm (49); and in a number of other instances (24).

    Accumulation of soluble nitrogen to the abnormal level of 50% of total nitro–

    gen in plants of Oxyria digyna and Koenigia islandica under arctic conditions

    is indicated by the results of Warren Wilson (44). This soluble nitrogen may

    be accumulated as nitrate (24) or as amino acids (49). Accumulation of sugars,

    previously, occurs simultaneously with that of soluble nitrogen, and is in fact

    attributed by Nightingale to decreased utilization in organic nitrogen synthesis.

            Put in oversimplified terms, protein synthesis involves the combination

    of simple carbohydrates with soluble nitrogenous compounds. The availability

    of these substances determines the rate of synthesis, which is reduced when

    one or other group of materials becomes limiting. The situation discovered

    in plants at low temperatures and in the Arctic, of accumulation both of simple

    carbohydrates and of soluble nitrogen, is therefore surprising. It seems

    possible that protein synthesis or some related process may be limited by low

    temperatures. Protein synthesis is an immediate requisite for protoplasm forma–

    tion and growth, and its depression would explain the reduction of growth rates

    observed by Tumanov (48) and Andersson (2) to occur at about 6° to 7°C. and to

    be associated with carbohydrate accumulation.

            This working hypothesis, that low growth rates in arctic plants are caused

    by slowing down of protein synthesis (in the widest sense) by low temperatures,

    seems worth examination; the progress of one such attempt was reported by the

    author in 1950 (45).


    Water Relations

            Low temperatures impede water uptake, owing mainly to decreased permeability

    of root membrances and increased viscosity of water (18). Water contents of

    020      |      Vol_V-0219                                                                                                                  
    EA-PS Warren Wilson: Arctic Plant Physiology

    plants may thereby be reduced (2; 22) and transpiration rates lowered; wilting

    may be caused. Whitfield (47) and Clements and Martin (8) believe that low

    soil temperatures may limit growth of plants at high altitudes in this way.

    Such effects appear to commence at temperatures below about 10°C; however there

    is adaptation to low temperatures in species typical of cold habitats, and in

    the absence of direct evidence it is impossible to assess the importance of

    this effect in arctic regions.

            Polunin (25) has shown the conduction of water in roots of Betula odorate

    through 50 centimeters of frozen soil at 0°C.; absorption was occurring in soil

    at a temperature just above zero. Conduction through roots at 4° to 5°C. was

    little if at all more rapid than that through roots at 0°C.

            Bonnier (6) compared the anatomy, in certain species, of individuals from

    arctic and temperate localities. He concluded that the differences in structure

    sometimes to be found under arctic conditions (thinner cuticle, larger inter–

    cellular spaces, etc.) were symptomatic of a more humid environment.


    Summary and Conclusions

            The limited evidence at present available suggests that arctic conditions

    affect plant physiology as follows. Growth rates are generally reduced much

    below those typical of temperate plant growth, and are subject to greater

    depression by locally unfavorable conditions of soil or climate. These low

    growth rates result in accumulation of sugars and probably of soluble nitrogenous

    compounds, and may be caused, in part at least, by retardation of protein

    synthesis or some related process by low temperatures. High sugar levels are

    probably responsible for the low rates of assimilation which have been

    observed. There is no evidence of carbohydrate shortage in arctic plants, in

    spite of the fact that respiration rates are higher, at a particular temperature,

    021      |      Vol_V-0220                                                                                                                  
    EA-PS Warren Wilson: Arctic Plant Physiology

    in arctic species than in temperate or tropical species. Physiological adap–

    tation is shown by low Q 10 s of assimilation, by low compensation points at

    low temperatures, and probably by low optimum and minimum temperatures for

    photosynthesis. Photosynthesis has been shown in some cases to continue

    throughout the 24-hour arctic day. Morphological adaptation also is present

    in the form of leaf area / high total dry weight rations, in the rapid expansion of leaves

    at the start of the short growing season, and in the dominance of the perennials

    of small growth habit. There is some evidence of lack of available nitrogen,

    owing perhaps to low activity of soil microorganisms, but it is not clear how

    important this effect is.

    022      |      Vol_V-0221                                                                                                                  
    EA-PS. Warren Wilson: Arctic Plant Physiology


    1. Acock, A.M. “Vegetation of a calcareous inner fjord region in Spitsbergen,”

    J.Ecol . vol.28, pp.81-106, 1940.

    2. Andersson, Gősta. Gas change and frost hardening studies in winter cereals .

    Lund, 1944.

    3. Arrhenius, O., and Sőderberg, E. “Der osmotische Druck der Hochgebirgspflanzen,

    Svensk Bot. Tidsk . Vol.11,

    4. Beliakof, E. “Uber den Einfluss der Temperatur auf die Kohlensäureassim–

    ilation bei zwei klimatischen Pflanzenrassen,” Planta , vol.11,

    p.727, 1930.

    5. Bőcher, T.W. “Studies on the vegetation of the east coast of Greenland between

    Scoresby Sound and Angmagssalik,” Medd. Grønland , vol.104, no.4,


    6. Bonnier, G. “Les plantes arctiques comparees aux memes especes des Alpes et

    des Pyrenees,” Rev. egen. Bot . vol.6, p.505, 1894.

    7. Brown, H.T. and Escombe, F. “Researches on some of the physiological processes

    of green leaves, with special reference to the interchange of

    energy between the leaf and its surroundings,” Roy.Soc. Lond.

    Proc . B, vol.76, p.29, 1905.

    8. Clements, F.E. and Martin, E.V. “Effect of soil temperature on transpiration

    in Helianthus annuus ,” Plant Physiol . vol.9, p.619, 1934.

    9. Curtel, M.G. “Researches physiologiques sur la transpiration et l’assimilation

    pendant les nuits Norwegiennes,” Rev. egen. Bot . vol.2, p.7, 1890.

    10. Dexter, S.T. “Grwoth, organic nitrogen fractions and buffer capacity in

    relation to hardiness of plants,” Plant Physiol . vol.10, p.149, 1935.

    11. Ehlers, John H. “The temperature of leaves of Pinus in winter,” Amer. J. Bot .

    vol.2, p.32, 1915.

    12. Fries, T. Experimentella undesokningar őver dit arktiska Gusklimaets infly

    tarde paa vaxterna, Flora och Fauna . Stockholm, 1918.

    13. Health, O.V.S. and Gregory, F.G. The constancy of the mean net assimilation

    rate and its ecological importance. Ann. Bot . 2, p.811, 1938.

    14. Jumelle, Henri. “Recherches physiologiques sur les lichens,” Rev. egen. Bot .

    vol.4, p.305, 1892.

    023      |      Vol_V-0222                                                                                                                  
    EA-PS. Warren Wilson: Arctic Plant Physiology

    15. Kjellman, F. Ur polarväxternas liv. Stockholm, 1884. (Quoted by Müller,


    16. Kostychev, S. “Studien űber Photosynthese. III. Findet eine Kohlensäureas–

    similation während der Sommernächte in der subarktischen Region

    statt?” Deutsch. Bot. Ges. Ber . Vol.39, p.334, 1921.

    17. Kostychev, S., Chesnokov, W., and Bazyrina, K. “Untersuchungen űber den

    Tagesverlauf der Photosynthese an der Kűste des Eismeeres,”

    Planta , vol.11, p.160, 1930.

    18. Kramer, Paul, J. Plant and soil water relationships . New York, 1949.

    19. Kreusler, U. “Beobachtungen űber die Kolhlensäureaufnahme und -Au s gabe

    (Assimilation und Atmung) der Pflanzen,” Landwirtschaftl. Jb .

    vol.16, p.711, 1887.

    20. Lundegardh, H. Enivronment and plant development . London, 1931.

    21. Matthaei, L.C. “Experimental researches on vegetable assimilation and respire–

    tion. III. On the effect of temperature on carbon-dioxide assim–

    ilation,” Roy. Soc. Lond. Philos. Trans . B, vol.197, p.47, 1905.

    22. Mundra, Alivs. “Zur Physiologie der Kälteresistenz des Winterweizens,” Planta ,

    vol.18, p.435, 1932.

    23. Müller, D. “Die Kohlensäureassimilation bei arktischen Pflanzen und die Ab–

    hangigkeit der Assimilation von der Temperatur,” Ibid . vol.6,

    p.22, 1928.

    24. Nightingale, G.T. “The nitrogen nutrition of green plants,” Bot.Rev . vol.3,

    p.85, 1937.

    25. Polunin, Nicholas. “Conduction through roots in frozen soil,” Nature vol.132,

    1933, p. 313.

    25a. ----. Contributions to arctic botany . D. Phil. thesis, Oxford Univer–

    sity, 1935.

    26. ----. “The vegetation of Akpatok Island. Part I,” J.Ecol . vol.22, p.337, 1934.

    27. ----. “The vegetation of Akpatok Island. Part II,” Ibid . vol.23, p.161, 1935.

    28. Russell, R.S. “The effect of arctic and high mountain climates on the carbohy–

    drate content of Oxyria digyna ,” Ibid . vol.36, p.91, 1948.

    29. ----. “Physiological and ecological studies on an arctic vegetation.

    III. Observations on carbon assimilation, carbohydrate storage

    and stomatal movement in relation to the growth of plants on

    Jan Mayen Island,” Ibid . vol.28, p.289, 1940.

    30. ----, etx al . “Physiological and ecological studies on an arctic vegetation.

    024      |      Vol_V-0223                                                                                                                  
    EA-PS. [ ?] Warren Wilson: Arctic Plant Physiology

    II. The development of vegetation in relation to nitrogen

    supply and soil micro-organisms on Jan Mayen Island,” Ibid .

    vol. 28, p.269, 1940.

    31. ----, and Wellington, P.S. “Physiological and ecological studies on an

    arctic vegetation. I. The vegetation of Jan Mayen Island,”

    Ibid . vol.28, p.153, 1940.

    32. Sachs, J. Ein Beitrag zur Kenntniss der Ernahrungshatigkeit der Blätter.

    ärb. Bot. Inst. Wurzburg, 1884.

    33. Scarth, G.W. “Cell physiological studies of frost resistance: a review,”

    New Phytol . Vol.43, p.1, 1944.

    34. Sørensen, Thorvald. “Temperature relations and phenology of the northeast

    Greenland flowering plants,” Medd.Grønland , vol.125, no.9, 1941.

    35. Stocker, O. “Assimilation und Atmung Westjavanischer Tropenbäume,” Planta ,

    vol.24, p.402, 1935.

    36. Stalfelt, M.G. “Der Gasaustausch der Flechten,” Ibid . vol.29, p.11, 1938.

    37. ----. “Der Gasaustausch der Moose,” Ibid . vol.27, p.30, 1937.

    38. ----. “Die Lichtokonomie der Arktischen Pflanzen,” Svensk. Bot. Tidskr .

    vol.19, p.192, 1925.

    39. Summerhayes, V.S., and Elton, C.S. “Further contributions to the ecology of

    Spitzbergen,” J.Ecol . vol.16, p.193, 1928.

    40. Wager, H.G. “Growth and survival of plants in the arctic,” J.Ecol . vol.26,

    p.390, 1938.

    41. ----. “On the respiration and carbon assimilation of some arctic plants

    as related to temperature,” New Phytol . vol.40, p.1, 1941.

    42. Wager, H.G., and Wager, E.M. “Annual changes in the osmotic value of some

    arctic and temperate plants,” Roy.Soc.Dublin Sci.Proc . vol.21,

    p.641, 1938.

    43. Walter, H. “Über die Assimilation und Atmung der Pflanzen im Winter bei

    tiefen Temperaturen,” Deutsch. Bot. Ges. Ber . vol.62, p.47, 1949.

    44. Warren Wilson, J. Botanical Results, Oxford University Expedition to

    Jan Mayen , 1947.

    45. ----. Botanical Results, British Expedition to Jan Mayen , 1950.

    46. Watson, D.J. “Comparative physiological studies on the growth of field

    crops. I. Variation in net assimilation rate and leaf area

    between species and varieties, and within and between years,”

    Ann. Bot . vol.11, p.41, 1947.

    025      |      Vol_V-0224                                                                                                                  
    EA-PS. Warren Wilson: Arctic Plant Physiology

    47. Whitfield, C.J. “Ecological aspects of transpiration. II. Pikes Peak and

    Santa Barbara regions: edaphic and climatic aspects,” Bot. Gaz .

    vol.94, p.183, 1932.

    48. Whyte, R.O. Crop production and environment . London, 1946.

    49. Wilhelm, A.F. “Untersuchungen űber die Kaltresistenz winterfester Kultur–

    pflanzen unter besonderer Berucksichtigung des Einflusses

    verschiedener Mineralsalzernährung und des N-Stoffwechsels,”

    Phytopath. Z . vol.8, p.111, 1935.

    50. Zakharova, T.M. “Über den Gastoffwechsel der Nadelholzpflanzen im Winter,”

    Planta , vol.8, p.68, 1929.


    J. Warren Wilson

    Frost Resistance

    001      |      Vol_V-0225                                                                                                                  

    (J acob Levitt)


            Even in temperate and semitropical climates, plants are injured by the in–

    frequent, mild frosts. The occasional loss of a major part of the citrus fruit

    crop is a case in point. Arctic plants must therefore possess pronounced adaptations

    to survive the extremely severe and long-lasting frosts to which they are exposed.

    In other words, they must be frost-resistant to the highest degree.

            One of the earliest attempts to explain such frost resistance was the caloric

    theory, which maintained that resistant plants produced enough heat to keep them

    from freezing. But it was soon discovered that plants are not like warm-blooded

    animals. They are unable to keep their temperatures appreciably above that of

    their environment, except insofar as their temperature lags behind during a change.

    Thus bulky plant parts such as tree trunks may be temporarily several degrees above

    or below that of the air, depending on whether the air temperature is falling or

    rising (22; 26). But it is the cambium layer of tree trunks that is most in

    need of frost resistance, as future growth of the tree is dependent upon (and to

    some extent takes place in) this layer. The readiness with which cambium tempera–

    ture changes is shown by the fact that on the sunny side of a tree on a cold winter

    day it may be 60° to 80°F. higher than on the shady side, when the latter is about

    0°F. (6).

            Frost resistance therefore cannot be due to any ability of a plant to keep

    its temperature above that of its environment. In fact, even the early observers

    noticed that resistant plants do freeze solid during winter without being injured.

    The term resistance is therefore used in the sense of tolerance. Furthermore, a

    002      |      Vol_V-0226                                                                                                                  
    EA-PS Levitt: Frost resistance

    resistan ce t plant must be able to tolerate very rapid temperature fluctuations,

    such as occurred in the case of the cambium layer described above.

            That the speed of temperature change is an important factor has been re–

    peatedly shown. Even plants with a high degree of frost resistance may be killed

    if frozen very suddenly. Apple buds that normally withstand −20° to −30°F. with

    little injury, may be killed if frozen suddenly at 0°F. (3). Similarly, sudden

    thawing may frequently cause more injury than gradual thawing. Alfalfa plants

    frozen at −0.4°F (−18°C.) nearly all survived when thawed gradually, but most

    were killed when thawed rapidly . (19).

            To add to the complexity of the problem, it has been found that plants may

    be fully frost-resistant in one climate yet may be killed by frost in a more

    moderate climate. Thus, some clover varieties that are quite hard i y in Canada

    were found to be winterkilled in England. It is even conceivable that some arctic

    plants may not be able to survive the milder winters of other climates. This is

    to be expected from the fact that even the most frost-resistant plants are quite

    sensitive to frost at some stage in their development. Thus the Siberian pea

    tree ( Caragana arborescens ) is capable of surviving temperatures as low as −80°

    F. in its native habitat, yet when new growth has just been produced in spring it

    is readily killed by freezing at + 24°F. Similarly, it has [ ?] been found that

    perfectly hardy shrubs may suffer severe injury if growing near street lamps (11).

            Neither of these cases of injury could occur under natural conditions, yet

    they point to a fact that is of both practical and theoretical importance, namely,

    that plants must undergo a hardening-off process before they can withstand [ ?]

    freezing. Newly formed tissues have not developed this hardiness and are, as a

    rule, sensitive. Similarly, many woody plants cannot develop hardiness unless

    subjected to a “short day” (i.e., a daylight period of less than 12 to 14 hours)

    for a period of some weeks. This is the reason for the injurious effect of street

    003      |      Vol_V-0227                                                                                                                  
    EA-PS Levitt: Frost resistance

    lamps mentioned above. These daylight requirements vary with the plant. Vari–

    eties from southern climates may need the short day earlier in the season than

    varieties from northern climates (17).

            Not only must plants develop frost hardiness, they must also retain it until

    danger of frost is past. This retention depends partly on internal plant charact–

    eristics that are not understood. Thus some trees (e.g., the blue or Colorado spruce,

    Picea pungens ) fail to grow at any time during winter even if transferred to

    spring or summer temperatures. Others (e.g., Indian bean, Catalpa hybrida ) will

    sprout as early as January if given the opportunity. The former have a longer

    “rest period” and therefore stay hardy for the length of the winter. The latter

    will begin to grow in an unseasonably warm period during winter and will then

    lose much of their frost resistance. A subsequent cold snap will destroy a

    tree like the Indian bean but will fail to injure the blue spruce.

            This may explain why certain varieties of plants hardy in Canada fail to sur–

    vive the more moderate English climate. If the winter remains severe, they re–

    tain their hardiness, but if moderate temperatures occur during winter, they

    lose their hardiness. This is apparently true even of different parts of the

    same plant. Thus injury to the south side of trees may well be due to the fact

    that the tissues on this side lose their hardiness on sunny days when their tem–

    perature rises far above that of the north side. However, this kind of injury

    may also be caused by rapid freezing about sunset and thawing after sunrise.

            It might be questioned why the term “frost resistance” is used rather than

    “cold resistance.” There is good reason for the choice. At the beginning of

    the twentieth century, it was clearly shown that the cold itself may be completely

    innocuous, for if a frost-sensitive plant is kept absolutely quite during the

    temperature fall it supercools and on rewarming shows no sign of injury (16;27).

    004      |      Vol_V-0228                                                                                                                  
    EA-PS Levitt: Frost resistance

    If, however, owing to shaking or some other factor, f ice forms in the tissues,

    the same plant is killed almost instantly. This kind of plant is therefore cold–

    resistant but frost-sensitive. An extreme example is afforded by the seeds of even

    the most frost-sensitive plants, which can be cooled to as near absolute zero as has

    been attained without suffering injury (14). Such seeds do not contain any free

    water and cannot freeze. If, however, they are allowed to imbibe water beforehand,

    they will readily freeze at temperatures below the freezing point of water and in

    many cases will be killed. It is true that some plants are cold-sensitive and may

    be injured or even killed at temperatures above the freezing point, but these are

    all native to tropical or subtropical regions.

            It is possible that supercooling may be of some importance in the case of

    certain arctic plants. In the less severe climate of the state of Washington,

    pine and f u i r needles do not appear to freeze, judging by their pliability at low

    temperatures (4). Further o more, even very frost-sensitive plants can be plunged

    into liquid air without injury if certain precautions are taken that prevent ice

    formation in their tissues (15). One of these precautions is to use a layer of

    tissue only one cell thick in order to get the extremely rapid temperature fall

    necessary to avoid ice formation. Another requires plunging the liquid-air-cooled

    tissue into a salt solution to prevent ice formation during the rise in temperature.

            It is, perhaps, possible that some arctic plants may be capable of supercooling,

    but they would have to remain supercooled all winter. Otherwise, this adaptation

    would do more harm than good, for if a supercooled plant f r eezes, the ice formation

    in its tissues occurs rapidly and is more likely to do damage than if the plant

    froze gradually without supercooling. On the basis of observations that have been made,

    we must conclude that all or nearly all plants do freeze when exposed to temperatures

    a few degrees below the freezing point of their sap. Thus all or near l y all the native

    005      |      Vol_V-0229                                                                                                                  
    EA-PS Levitt: Frost resistance

    trees have ice in their tissues on a normally cold winter day in the state of New

    York (28). It seems certain, therefore, that all or nearly all overwintering

    arctic plants must possess frost resistance in order to survive.

            An understanding of frost resistance is possible only if we know why frost

    causes injury. One of the earliest ideas, still current among some practical men

    (as well as some scientists), is that ice formation in plants ruptures the cells.

    This explanation was long ago given for frost splits of trees (5). It was also sug–

    gested that frost-resistant plants survive owing to the ability of their cells to

    expand without rupture. Like many other theories based largely on speculation,

    this one was exploded by the very first careful observations. Instead of expanding

    on freezing, plant tissues usually contract (2;8). It is this contraction of the

    outer part of a tree trunk without much contraction in the center that seems to

    cause the frost splitting (2).

            The most conclusive evidence against the cell [ ?] -rupture theory was produced

    by the painstaking observations of Göppert in 1830 (7), which were later confirmed

    by other workers. He examined thousands of cells of frost-injured plants and failed

    to find any cell [ ?] rupture. The reason for this and for the contraction that

    actually does occur is now clear. Ice does not form inside the cell, but arises

    in the intercellular spaces (18). As the temperature drops, these intercellular ice

    crystals grow at the expense of water drawn from the cells. Consequently, in frozen

    plants the cells are actually severely contracted instead of being expanded. The

    outer cutinized or suberized layer surrounding a leaf or twig contains no water

    and therefore contracts as all normal solids do when their temperature drops. It

    seems that this outer layer acts like a tightening belt around the plant organ, and

    squeezes air out of the intercellular spaces, which gradually become filled with ice

    formed from the cell’s water.

    006      |      Vol_V-0230                                                                                                                  
    EA-PS Levitt: Frost resistance

            Under artificial conditions it is possible to induce ice formation inside

    the cells, for instance by supercooling the tissues well below their freezing point

    and then inoculating with an ice crystal (25). The subsequent ice formation is so rapid

    t j h at t j h ere o i s [ ?] insufficient time for the water to come out of the cells and solidify

    on the [ ?] intercellular ice crystals. Consequently, instead of relatively few,

    large intercellular crystals, a vast number of minute ice cr u y stals form inside the

    [ ?] cells. In almost all cases, such ice formation inside the cells is fatal, even

    if the tissues are frost-hardy. This may well be the reason for the fact that

    rapid freezing is more injurious than slow freezing.

            Plants that have just thawed frequently have water-soaked, flaccid leaves.

    This was originally thought to be due to the supposed cell [ ?] rupture, but the true

    explanation is now known to be the melting of the intercellular ice crystals. If

    the cells are dead, they cannot absorb this intercellular water and the flaccid

    leaves soon dry up. If the cells are alive, they gradually reabsorb it, regain–

    ing their turgor and permitting air to flow back into the intercellular spaces.

    It is an interesting fact that very frost-resistant tissues, such as the buds of

    native trees and shrubs, do not show any such water-soaked appearance on thawing.

    Instead, they reabsorb the intercellular water just as soon as the ice melts(28).

    It is quite possible that the injury occurring during rapid thawing may be re–

    lated to this formation of intercellular water.

            The fact that ice formation normall y occurs outside the cells leads to the

    conclusion that frozen tissues are just as truly dehydrated as are tissues of

    plants wilted during a drought. In both cases water is removed from the cells and

    as a result the cells collapse. It is therefore not surpris e ing that frost and

    drought resistance frequently go hand in hand. The normal way to harden off a

    plant against frost injury is to expose it to low temperatures (above freezing)

    007      |      Vol_V-0231                                                                                                                  
    EA-PS Levitt: Frost resistance

    for several days. But it is also possible to increase frost resistance by allowing

    the plant to dry out for the same period at normal growing temperatures. Furthermore

    when cells are frost-resistant, they are also resistant to other forms of dehydra–

    tion injury (20;21). In fact, one of the best indirect methods of measuring frost

    resistance is to determine how severely the cells can be dehydrated by “plasmoly–

    sis” without in f j ury. This is done by placing sections of the tissue in a series of

    sugar solutions of different strengths, and finding the weakest solution that de–

    hydrates the cells to the point from which they are unable to regain their turgor

    on transfer to pure water (24).

            Such evidence leads to the conclusion that frost injury is really a dehydration

    injury. Unfor u tunately, the reason why dehydration in general causes injury is

    not any better understood than the reason why frost itself causes injury; but

    recent experimental evidence has at least led to the proposal of a working hypoth–

    esis (9;10). According to this concept, it is the physical effects of the cell

    collapse and the subsequent cell expansion that [ ?] injures the protoplasm.

            The properties of frost-resistant cells explain how they avoid such injuries

    injury. Frost-resistant cells are more permeable to water than frost-sensitive cells

    (13). This may protect them from injury in two ways: ( 1 ) it may permit the water

    to leave the cells rapidly enough during freezing to prevent the fatal ice forma–

    tion inside the cells; and ( 2 ) it may permit sufficiently rapid reabsorption of

    intercellular water during thawing to prevent thawing injury.

            It has also been shown that other physical properties of the protoplasm are

    markedly altered during frost hardening. If the protoplasm of frost-sensitive cells

    is dehydrated by placing them in a strong sugar solution, it becomes brittle and

    snaps readily. In the same sugar solution, the protoplasm of hardy cells can still

    be stretched and appears to retain its normal fluidity (24).

    008      |      Vol_V-0232                                                                                                                  
    EA-PS Levitt: Frost resistance

            These and many other tests indicate that the protoplasm of frost-resistant

    cells can retain considerable amounts of water in the presence of dehydrating for–

    ces that remove nearly all the water from the protoplasm of frost-sensitive cells.

            Recently it has been possible to show that such frost-resistant cells have a

    much larger fraction of water-soluble protein than do the frost-sensitive cells (23).

    This leads to the conclusion that the ability of frost-resist na an t protoplasm to hold

    onto more water is due to the hydrophilic (water-loving) nature of its proteins.

    This may also explain the higher permeability of frost-resistant cells to water.

            Plant breeders have succeeded in proving that more than one factor is involved

    in frost resistance; so in spite of the importance of protein hydration there must

    be some other factor as well. Physiologists long ago succeeded in recognizing another

    factor — the sugar content of the plant. As a general rule, the more frost-resistant

    a plant is, the greater will be the concentration of sugars in its cells. It has,

    in fact, been possible to arrange several varieties of wheat in decreasing order

    of frost resistance simply by arranging them in order of decreasing sugar content

    (1). Similarly, the sugar content of the cortical cells of trees rises from a

    minimum at the time of greatest sensitivity to frost in spring to a maximum (which

    may be five times the minimum) during midwinter when frost resistance is highest

    (13). But this relationship does not always hold. Some plants with high sugar

    content are completely lacking in frost resistance (e.g., sugar cane); and there

    are cells that survive severe freezing with little or no sugar (e.g., xylem paren–

    chyma of many trees). Moreover, some varieties fail to show the relationship

    mentioned above.

            From all these results we can safely conclude that sugar content is only a

    secondary factor in frost resistance. It enhances the frost resistance of cells

    having moderately hydrophilic protoplasm, but it fails to affect the resistance

    009      |      Vol_V-0233                                                                                                                  
    EA-PS Levitt: Frost resistance

    either of protoplasm that is very readily dehydrated or of protoplasm that is ex–

    tremely hydrophilic.

            In order to understand the interaction of these two factors, we have to consider

    quantititat a ively what happens during freezing. The formation of ice in the intercel–

    lular spaces dehydrates both the protoplasm of a cell and the vacuole enclosed by it.

    As a result, the cell collapses and is exposed to mechanical stresses (9;10). At a

    certain degree of dehydration (and therefore at the subzero temperature just suffi–

    cient to produce it), the protoplasm becomes sufficiently brittle to be injured by

    these stresses. The injury therefore depends on two factors: ( 1 ) the degree of brittle–

    ness of the dehydrated protoplasm; and ( 2 ) the degree of contraction of the cell,

    which sets up the stresses.

            Increasing the hydrophily of the protoplasm will result in a lack of brittleness

    even in severe frosts which set up strong dehydrating forces; on the other hand,

    increasing the sugar content of the vacuole increases the osmotic pressure of the

    cell sap. The degree of contraction of a cell on freezing is inversely proportion [ ?] al

    to this osmotic pressure (3). Theoretically, one might expect this to be true only

    down to the eutectic point of the sugars — i.e., the temperature at which the sugars

    crystallize out. But sugars do not possess a definite eutectic point, and, on concen–

    tration, they form a thick sirup from which they fail to crystallize out, even at

    −7.6°F. (−22°C.) (3). Thus, if one cell has twice the sugar [ ?] content of a sec–

    ond cell, it will have nearly twice the osmotic pressure, and at any one freezing

    temperature the second cell will contract approximately twice as severely as the

    first. Consequently, protein hydration and sugar content may act as two independent

    factors that have an additive effect on the frost resistance of the cell.

            It may be not only the sugars that have this secondary effect on frost resistance.

    Cortical cells of frost-resistant trees also have an accumulation of colloidal mate–

    rial in their vacuoles. As a result, it is impossible to contract them to less than

    010      |      Vol_V-0234                                                                                                                  
    EA-PS Levitt: Frost resistance

    40% of their normal volume, even if all the water be removed (13).

            The problem of frost resistance is thus a comple s x one; but much is now

    known, and the way appears to be open to a fuller understanding of the mechanism.


    Jacob Levitt

    011      |      Vol_V-0235                                                                                                                  
    EA-PS Levitt: Frost Resistance


    1. A Å kerman, A. Studien uber den Kältetod und die K a ä lteresistenz der

    Pflanzen . Lund, 1927.

    2. Caspary, Robert. “Bewirkt die Sonne Risse in Rinde und Holz der Bäume?”

    Botanische Zeit . vol.15, pp.153-56, 329-35, 345-50, 361-71, 1857.

    3. Chandler, W.H. “The killing of plant tissue by low temperature,” Missouri

    Agric. Exp. Sta. Res. Bull . no.8, 1913.

    4. Clements, Harry F. “Mechanisms of freezing resistance in the needles of

    Pinus ponderosa and Pseudotsuga mucronata ,” Washington. State

    College. Res. Stud . vol.6, no. 1, pp.3-46, 1938.

    5. Du Hamel et de Buffon. “Observations des diff e é rents effects que produisent

    sur les [ ?] V e é g e é taux les grandes gel e é es d’Hiver et les petites

    gel e é es du Printemps.” Histoire de l’Acad e é mie Royale des Sciences

    avec les M e é moires de Mathematique et de Physique, pp.273-298, 1740.

    6. Eggert, R. “Cambium temperatures of peach and apple trees in winter,”

    Amer. Soc. Hort. Sci. Proc. vol. 45, pp.33-36, 1944.

    7. Göppert, H.R. Über die Wärme-Entwickelung in den Pflanzen deren Gefrieren

    und die Schutzmittel gegen Dasselbe . Breslau, 1830.

    8. Hoffmann, Hermann. Witterung [ ?] u nd Wachsthum, oder Grundzüge der Pflanzenkli

    matologie . Leipzig, 1857, pp.312-34.

    9. Iljin, W.S. Über den Kältetod der Pflanzen und seine Ur as sa chen,” Protoplasma ,

    vol.20, pp.105-24, 1933.

    10. ----. “Die Ursache der Resistenz von Pflanzenzellen gegen Austrocknung,”

    Ibid . vol.10, pp.379-414, 1930.

    11. Kramer, P.J. “Photoperiodic stimulation of growth by artificial light as

    a cause of winter killing,” Plant Physiol . vol.12, pp.881-83, 1937.

    12. Levitt, J. Frost Killing and Hardiness of Plants . Minneapolis, Burgess, 1941.

    13. ----, and Scarth, G.W. “Frost-hardening studies with living cells,”

    Canad. J. Res . Ser.C, vol.14, pp.267-305, 1936.

    14. Lipman, C.B. “Normal viability of seeds and bacterial spores after exposure

    to temperatures near the absolute zero,” Plant Physiol . vol.11,

    pp.201-5, 1936.

    15. Luyet, B.J. “The vitrification of organic colloids and of protoplasm,”

    Biodynamica , vol.29, pp.1-14, 1937.

    012      |      Vol_V-0236                                                                                                                  
    EA-PS Levitt: Frost Resistance

    16. Molisch, Hans. Untersuchungen über das Erfrieren der Pflanzen . Jena, 1897.

    17. Moschkov, B.S. “Photoperiodismus and Frosthärte ausdauernder Gewächse,”

    Planta , vol.23, pp.774-803, 1935.

    18. Müller-Thurgau, H. “Uber das Gefrieren und Erfrieren der Pflanzen,”

    Landwirtschaftl . Jahrb., vol.9, pp.133-89, 1880.

    19. Peltier, G.L., and Tysdal, H.M. “Hardiness studies with 2-year-old alfalfa

    plants,” J. Agric. Res . vol.43, pp.931-55, 1931.

    20. Scarth, G.W. “Cell physiological studies of frost resistance,” New Phytol .

    vol.43, pp.1-12, 1944.

    21. Scarth, G.W. “Dehydration injury and resistance,” Plant Physiol . vol.16,

    pp.171- [ ?] 79, 1941.

    22. Schübler, G. “Beobachtungen über die Temperatur der Vegetabilien und

    einige damit verwandte Gegenstände,” Annalen Phys ., Leipzing, vol.

    10, pp.581-92, 1827.

    23. Siminovitch, D., and Briggs, D.R. Arch. Biochem . Vol. 23,pp. [ ?] 8-17, 1949.

    24. Siminovitch, D., and Levitt, J. “The relation between frost resistance

    and the physical state of protoplasm. II,” Canad. J. Res .

    Ser. C, vol.19, pp.9-20, 1941.

    25. Siminovitch, D., and Scarth, G.W. “A study of the mechanism of frost

    injury to plants,” Canad. J. Res. Ser .C, vol.16, pp.467-81, 1938.

    26. Solom e é . “Observations sur la temp e é rature interne des v e é g e é taux, compar e é e

    a á celle de l’atmosph e è re,” Annales Chim . (Phys.) vol.40, pp.113-22,


    27. Voigtlander, H. “Unterkühlung und Kältetod der Pflanzen,” Beitrage z. Biol.

    Pfg . vol.9, pp.359-414, 1909.

    28. Wiegand, K.M. “The occurrence of ice in plant tissue,” Plant World , vol.9,

    pp.25-39, 107, 1906.


    J. Levitt

    Thermoperiodism, Vernalization, and Photoperiodism in the Arctic

    Unpaginated      |      Vol_V-0237                                                                                                                  
    EA-Plant Sciences

    (A. Lang)




    Introduction 1
    Thermoperiodicity 3
    Vernalization 4
    Photoperiodism 9
    Ecological Aspects 18
    Adaptive Significance 20
    Significance for Arctic Agriculture and Horticulture 24
    Bibliography 26

    Unpaginated      |      Vol_V-0238                                                                                                                  
    EA-Plant Sciences

    (A. Lang)



            With the manuscript of this article, the author submitted 5 figures

    (in duplicate). Because of the high cost of reproducing them, one set

    will accompany the first copy of the manuscript and the other held at

    The Stefansson Library.

    001      |      Vol_V-0239                                                                                                                  
    EA-Plant Sciences

    (A. Lang)





            It is generally agreed that plants for their growth and development

    require a definite set or constellation of environmental conditions, among

    which the most important are the conditions of temperature and light.

    [By growth, we mean the quantitative increase of a plant in size, leaf

    number, etc., under development qualitative changes, as the transition

    of the dormant seed to germination or of a vegetative plant to flower

    formation.] In some cases, this growth and development requirement can

    be expressed in a very simple manner. For example, some plants (pears and

    other fruit trees, etc.) will reach a definite stage in their annual cycle

    of development, when the sum of the average daily temperatures, which

    surpass a certain level, reaches a definite value, the “temperature sum”

    characteristic of that stage. Let us assume, for example, that, for some

    plant, temperatures of more than 5°C. are effective and that the tempera–

    ture sum required for flowering is 1000°. Then this plant will flower after

    50 days at 25° (1000/(25 – 5)) = 50 or after 100 days at 15° (1000/(15- 5)) = 100

    A similar situation is found in certain cases in relation to light duration.

            Both light and temperature conditions in such cases have to be adequate

    throughout the entire life of the plant. If a green plant, at any point in

    002      |      Vol_V-0240                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    its development, is exposed to an inadequate temperature or is deprived

    of sufficient light, it sooner or later will cease growing and may eventually

    die. This however represents only the simplest instance of relationship

    between plant and environment. In the course of the last three decades

    it has become increasingly apparent that the relationships of plant life

    to environment are in numerous cases much less simple and straightforward.

    Many plants exhibit certain specific environmental requirements, and unless

    these are met they may stop developing even though the general temperature

    and light conditions, etc., seem optimal. A particularly important feature

    of these requirements is that they frequently can be satisfied in a rather

    short time. To pass through its entire life cycle a plant always will

    require favorable temperature and light, but for a certain time it may need

    low temperature or a certain number of dark hours per day. During the action

    of these specific conditions visible changes may not take place at all, but

    the subsequent development may be profoundly different depending on whether

    or not those conditions were met. Such type of action may be called inductive .

            We know at present of three instances of specific environmental require–

    ments in flowering plants, which are of general occurrence and which therefore

    play an important part in determining plant life under natural conditions:

            (1) Very many [ ?] plants require different day and night temperatures

    and will develop optimally only if the temperature is subject to diurnal

    fluctuation: This is thermoperiodicity.

            (2) Many plants, for passing their life cycle, particularly for forma–

    tion of flowers, require a certain period of low temperatures: This is


            (3) Many plants for flower formation require a definite day-length

    003      |      Vol_V-0241                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    range: This is photoperiodism.

            It is obvious that in the Arctic, where temperature changes enormously

    both in the course of the day and of the year and where day-length also

    undergoes tremendous seasonal variations, all three of these phenomena will

    be of the utmost significance for plant life. Investigations devoted to

    this question are, however, remarkable by their fewness, due no doubt

    largely to the lack of appropriate experimental facilities. This situation

    seems to be improving; nevertheless, all that is possible at present is a

    brief outline of our knowledge of the phenomena enumerated and an indication

    of some general ideas about their bearing for Arctic plant life.



            It had been known for some time, particularly to practical horticulture,

    that many plants thrive best if night temperatures are comparatively low.

    Thus, the following plants show optimal development at the temperatures indicated

    (in degrees Centigrade)*:

    Day Night
    Violets 8.5-14 4.5-1.0
    Snapdragon 14-16 7-9
    Sweet peas 13-15.5 7-10
    Roses 21-23 14.5-16.5

    * Data after Went (1948) from A. laurie and D. C. Kiplinger: Commercial

    Flower Forcing , Philadelphia, Blakiston, Fourth Edition, 1944.

            F. W. Went showed, from 1944 on, that this is a widespread, perhaps general,

    phenomenon and explained at least some of its physiological nature. In

    tomatoes actual stem growth takes place exclusively during night, and the

    optimal temperature is comparatively low. The value changes during development;

    in seedlings it may be as high as 30°, in older plants it may drop to as low

    as 8°. The levels seem to be different in different varieties; this indicates

    004      |      Vol_V-0242                                                                                                                  
    EA-PS. Labg: Thermoperiodism, Vernalization, and Photoperiodism

    genetic control. On the other hand, growth also depends on the con [ ?] itions

    to which the plant is exposed during the daytime, and here the optimal

    temperature is considerably higher (26°). Therefore, if day and night

    temperatures are equal, growth is inferior; certain other combinations

    (low day and high night temperatures) may even cause pronounced damage.

    Apart from stem growth, flowering and in particular fruit-set are also

    affected. In all species from temperate zones and higher latitudes which

    have so far been investigated basically f t he same situation was found. The

    physiological cause of thermoperiodicity is probably that those processes

    on which growth depends during day and during night, have somewhat different

    temperature optime. Also, in some cases excessive night temperature may

    induce excessive respiration which will cause a general depletion of

    material required for growth.



            Low temperature requirement has become best known in cereals. All

    small-grain cereals (wheat, barley, rye, oats) include both summer (spring)

    and winter (fall) varieties. The former are sown in spring and harvested

    in summer and fall, while the latter have to be planted in the fall of the

    preceding y ear; summer varieties thus pass their development within one

    growing season, winter varieties require (parts of) two. If a pronounced

    winter variety is planted in spring, it will give, that summer, excellent

    vegetative growth, producing abundant leaves and side shoots (tillers), but

    will neither head nor flower and fruit. If plants of the same variety are

    artificially exposed to low temperature for a certain time, they will

    behave as plants of a summer variety. This change, of course, is individual and

    not hereditary; seeds harvested from [ ?] plants of a winter variety cold treated

    and grown as a summer crop will give rise again to typical winter plants.

    005      |      Vol_V-0243                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

            That a specific “cold requirement” is the physiological basis of the

    winter habit of cereals and other plants was first conclusively demonstrated

    in 1918 by the German plant scientist G. Gassner. Later, Russian authors

    introduced the term yarovizaciya (from yar = summer). Which was translated

    into English as “vernalization.” This term is used to cover both the

    phenomena of cold requirement as such and the actual cold treatment, either

    experimental or by the low temperatures of the season from fall through

    early spring. For the cold treatment, the term “thermal induction” also

    is used.

            We can distinguish two groups of cold-requiring plants. In the one,

    the winter annuals (e.g., winter cereals, white and black mustard, Vicia

    villosa ), cold requirement has a quantitative or facultative character,

    i.e., cold treatment hastens flowering, but if the growing season is arti–

    ficially extended, by greenhouse culture with supplementary light, the plants

    will ultimately proceed to flowering even if continuously grown at tempera–

    tures of 20°C. or above. In the second group, the true biennials (e.g.,

    beets, turnips, many cabbage varieties, carrots, and many other Umbelliferae ,

    the biennial variety of the henbane Hyoscyamus niger ), cold requirement is

    qualitative or obligatory; without thermal induction the plants will remain

    vegetative indefinitely. Most biennial plants have a tuber and a pronounced

    leaf rosette.

            Plants not exhibiting any cold requirement are called summer annuals

    (summer cereals, cotton, tobacco, annual henbane, etc.). One and the same

    species may comprise both summer and winter annuals, or summer annuals and

    biennials; in several cases (certain cereal varieties, beets, henbane) the

    genetic basis of the difference is simple (1 or 2 genes). Moreover,

    006      |      Vol_V-0244                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    particularly in cereals, there are all intermediates between summer and

    pronounced winter annuals, between varieties that usually behave as

    summer crops but may to some extent be accelerated in flowering by low

    temperatures, and such that without thermal induction would never conclude

    development if planted under natural conditions in spring. Usually, the

    less pronounced winter types require shorter induction times and are

    favored by comparatively higher induction temperatures (sometimes up to

    + 15°), whereas typical ones as well as biennials require longer periods

    (in general, around 6 to 8 weeks, but sometimes several months) and lower

    temperatures ( + 1° to + 10°, mostly closer to the lower limit).

            Temperatures below 0° have at most only a slight vernalizing action,

    but if vernalization has been started at a temperature above freezing point,

    it may be concluded at temperatures below. The low temperatures act on the

    growing region (growing points) themselves, that is on the regions which

    later produce the flowers; whether other organs or tissues of the plant

    are also exposed to cold or not is of no importance. On the other hand,

    low temperature need not act continuously, but may be interrupted daily,

    by some (although not too many) warm hours; therefore, vernalization is

    completed under natural conditions even if temperatures rise during the

    day periods to levels which, in themselves, would be too high.

            The purpose of this review as well as space preclude a detailed

    discussion of our understanding of the physiological basis of vernalization,

    which, incidentally, is still far from complete. Quite [ ?] generally, the

    situation may be as follows: For flowering to set in, a certain condition

    (III) must be reached in the plant; it is attained starting from an initial

    condition (I) and passes through an intermediary one (II). However,

    007      |      Vol_V-0245                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    can also be converted along another pathway, leading to IV, which is

    inactive as to flower formation:


    Each one of the changes involved would, as usual for physiological processes,

    be accelerated with increasing temperature; but the degree of this increase

    is different, so that at high temperatures only IV is reached, at lower ones

    (also) III. Temperature, thus appears to act by adjusting the balance of

    these changes. The nature of the changes is yet unknown. Presumably they

    are chemical reactions which lead to the formation of definite substances

    (II IV may also be the destruction or inactivation of a compound). In

    further consequence of III, a substance seems to be produced which causes flower

    formation and which can move within the plant (a “flowering hormone”). As

    was first shown (in 1936/37) by G. Melchers in Germany, by grafting a cold–

    treated biennial henbane plant, or an annual one, on a non-cold-treated

    biennial this latter is caused to flower itself. This means that some sub–

    stance (or substances) move from the plant which is ready to flower (the

    “donor”) to the plant which, by itself, would not be able to do so (the

    “receptor”). Similar results have been obtained in beets, cabbage, mustard,

    and carrots. In grasses (cereals) grafts are not possible, but it is rather

    certain that formation of substances inducing flowering is a general phenomenon

    in plants, biennial and winter-annual plants being capable of forming these

    substances only after exposure to low temperature, summer annuals without.

    Donor and receptor in the graft experiments may belong to different species

    and even genera; the flower-forming substance therefore must be identical

    in different plants.

    008      |      Vol_V-0246                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

            One physiological trait of vernalization remains to be stressed. This

    is the fact that in vernalization the inductive type of action is particu–

    larly strongly pronounced. The cold treatment results in an acceleration

    of flowering particularly if the plant afterwards is exposed to higher

    temperatures, namely, those temperatures which constitute the temperature

    range ‘generally’ adequate for plant growth and development in the climate

    given. If, for example, a winter annual plant with an optimal vernalization

    temperature of 5° were kept at this temperature permanently, it would grow

    very slowly and flower - if at all - probably much later than an individual

    which was kept permanently at 20 to 25°, a temperature which is ineffective

    in vernalization. But if such a plant is kept at the low temperature for the

    period necessary to complete thermal induction, and then transferred to the

    higher one, it will proceed to flowering much faster than the nonvernalized

    individual. Under natural conditions, too, the actual flowering of cold–

    requiring plants usually takes place when the temperature is considerably

    elevated as compared with the vernalization temperature. On the other hand,

    the low temperatures are already effective at amazingly early stages of the

    development of the plant, at least in winter-annuals. Usually, winter cereals

    and other winter annuals undergo the cold treatment in the seedling stage,

    but it can applied also to seed which barely has started germination and

    even to seed which is developing on the mother plant (though not to the dry

    ripe seed). Flowering, on the other hand, never occurs before the plant has

    made a certain amount of vegetative growth. Throughout this period of

    development the effect of the cold treatment, then, remains “latent”, i.e.,

    is carried in the plant without becoming visible; as long as actual flower

    formation has not set in, a winter-annual plant grown from vernalized seed

    009      |      Vol_V-0247                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    cannot be distinguished from another grown from nonvernalized seed. Thus,

    the effect of the low temperature of flowering in winter-annual and biennial

    plants becomes manifest only under elevated temperatures and in winter-annuals

    frequently only after a pronounced latency period. Biennials, as a rule,

    have to make some vegetative growth before they become responsive to ver–


            Of the different stages within the flowering process, it is particularly

    the onset of flowering which is hastened by vernalization. In the process

    of flower formation, several steps can be distinguished. The first is the

    formation of microscopic flower initials of flower primordia . In numerous

    plants, including cereals, beets, etc., it is usually accompanied by stem

    elongation (“heading” or “bolting”). Subsequent steps are the growth of

    the primordia to visible flower buds, the opening of the buds to flowers

    proper ready for pollination (anthesis), and, after fertilization had been

    accomplished, fruit development and seed ripening. In a not cold-treated

    winter-annual plant, flower initiation is delayed as compared with a vernal–

    ized individual. But if this nonvernalized plant is allowed, by greenhouse

    culture, to attain this stage at all, the later stages of flowering will be

    passed at much the same rates as in vernalized individuals, or at any rate

    the differences will be less pronounced than in flower initiation.



            The discovery that day length , i.e., the relative duration of day and

    night, is a factor controlling development in a great many plants, was made

    in 1920 by the American plant physiologists W. W. Garner and H. A. Allard.

    Garner and Allard established the two types of day-length-dependent plants

    010      |      Vol_V-0248                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    which are recognized today; long-day plants and short-day plants . Both

    long- and short-day plants possess a critical day length . In long-day

    plants, in day lengths below the critical one, flowering is either delayed

    or does not occur at all; in day lengths above it is promoted. For example,

    the long-day plant henbane will not flower if given 10 hours of light per

    day or less. However, if plants grown on 10-hour days are transferred

    to longer days, microscopic flower buds will be formed in the following

    periods, while visible buds will appear some 2 or 3 weeks and open flowers

    some 6 weeks later (all this at a temperature of approximately 22°):

    11-hour days about 40-45 days
    12-hour days about 15 days
    13-hour days about 12 days
    14-hour days about 10 days
    15-hour days about 9 days
    18-hour days about 8-9 days
    20-hour days about 8-9 days
    24-hour days (continuous

    about 8-9 days

            Short-day plants, on the contrary, will flower in day lengths below their

    critical day l -length value and will fail to flower above; or will be more

    or less delayed in flowering. For example, a fall chrysanthemum variety

    for the formation of visible flower buds

    in 14-hour days about 65 days
    in 13-hour days about 35 days
    in 12-hour days about 25 days
    in 10-hour days about 25 days
    in 8-hour days about 25 days

            It will not flower in 15-hour days. As can be seen from these figures,

    differences in day length have a particularly great influence close to the

    critical day-length value. If day length is increased or decreased beyond

    a certain limit, flowering no longer is much affected. If henbane, this

    limit is reached at approximately 15 or 16 hours of light, in chrysanthemum

    at 12 hours.

    011      |      Vol_V-0249                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

            It is not possible — although it is often done, even in textbooks —

    to give a generally valid demarcation line of the long- and the short-day

    range. The critical day length may be different in different representatives

    of both photoperiodic types. It is a specific or vertical character, and

    the critical day-length values of long-and short-day plants overlap.

    Therefore, many short-day plants will still flower more or less readily in

    day lengths in which many long-day plants, in their turn, are already capable

    of flowering, too. Thus, henbane and chrysanthemum will both flower in day

    lengths of 11 to 14 hours. The short-day [ ?] plant cocklebur ( Xanthium spp.)

    flowers even in a 15 1/2-hour day, whereas some stonecrop ( Sedum ) species —

    all long-day plants — flower only if the day length exceeds 14 hours.

    Therefore, if the plants are grown on a 13-hour day, one might easily be led

    to consider the cocklebur as the long-day and Sedum as the short-day plant.

    The essential thing is that in henbane and stonecrop flowering will be

    promoted if the plants are transferred to comparatively longer days, in

    chrysanthemum and cocklebur it will be delayed or suppressed.

            In an over-all fashion, however, one may say that for most day-length

    dependent plants, both of the long- and the short-day type, the limit between

    the flower promoting and flower inhibiting day lengths lies in the range

    from 11 to 14 hours of light per day. Examples of long-day plants, besides

    those already quoted are: most small-grain cereals, lettuce, radish, spinach,

    China aster, Rudbeckia , Stenophragma thalianum, etc. Short-day plants are:

    rice, sorghum, millet, teosinte, poinsettias, Maryland-Mammoth tobacco, many

    Chenopodiaceae and Amerantaceae , etc. Plants in which flowering is independent

    of day lengths are called indeterminate or day-neutral plants; some examples

    are corn, tomato, chili pepper, cucumber, snapdragon. One and the same species

    012      |      Vol_V-0250                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    may include day-neutral and long- or short-day strains. In side-oats

    grama ( Bouteloua curtipendula ) both long- and short-day strains have

    been described; but this seems to be rather an exceptional situation.

    The differences long-day vs. day-neutral habit and short-day vs. day-neutral

    habit may be caused by a single gene.

            Only a brief outline can be given of the physiological basis of

    photoperio d ism. Those features which have ecological significance will

    be particularly stressed. The foremost feature of this sort is the induc–

    tive nature of day-lengths action. As with low temperature in vernalization,

    the appropriate day length need not act continuously, until visible response

    occurs, but only for a comparatively short time, after which the plant will

    respond even if it is then transferred to a day length in which flowering

    would never have occurred. This is known as photoperiodic aftereffect or

    photoperiodic induction. Day lengths favorable of flower formation, therefore,

    are frequently referred to as “inductive,” those unfavorable as “noninductive.”

    Day length s itself is often called “photoperiod,” a photoperiod of, e.g.,

    15 hours meaning 15 hours of light + 9 hours of darkness. The time necessary

    for photoperiodic induction is generally much shorter than that for thermal

    induction in vernalization. In particularly sensitive plants, as cocklebur,

    one single inductive photoperiod may be sufficient (although it will not give

    maximal response). In contrast to vernalization photoperiodic induction is

    not permanent in most cases. With but a few exceptions, most notably again

    with the cocklebur, g the plant returns to vegetative growth after having been

    a longer or shorter time under noninductive conditions.

            It must be emphasized that photoperiodic responses actually involve a

    periodic element. One might assume — and this has indeed been done — that,

    013      |      Vol_V-0251                                                                                                                  
    EA-PS. Lang: Thermoperio i d ism, Vernalization, and Photoperiodism

    for flowering, long-day plants require a definite amount of light and

    short-dayplants similarly a certain amount of darkness. Thus the dark

    periods in the former and the light periods in the latter would cause

    only an interruption of the favorable condition and thus delay the response.

    This however, is not true, as can be shown by experiments in which the

    cycle length is changed. For instance, henbane flowers in cycles of 16

    hours light + 32 hours darkness, but not in 8 + 16 hours. Soybean behaves

    in the opposite way. In all cases the total amounts of light and darkness

    are equal. Thus, both light and dark conditions are somehow involved in

    the outcome, and the response depends not only on the total duration, but

    also on the timing of light and darkness.

            Wherever light exerts as effect on an organism, it must be absorbed by

    some pigment. The pigment responsible for the adsorption of the photoperiodi–

    cally effective light has recently been shown to be not identical with any

    of the pigments already known in flowering plants. Its chemical nature has

    not yet been identified, but it seems to be identical in long- and short-day

    plants. In the light of this discovery another interesting and significant

    feature of photoperiodism can probably be explained. If a long-day plant or

    a short-day plant is grown in short-day conditions, but illuminated with

    supplementary light, very low intensities are sufficient to promote flowering

    in the former and to inhibit or to delay it in the latter. Even the light

    of the full moon may cause some photoperiodic response. Increasing the

    intensity of the supplementary light beyond a certain limit has no further

    photoperiodic effect. This limit is still very low, far below the light

    intensities which are required to maintain plant life. Thus the photoperiodic

    responses are caused by comparatively small quantities of light, and the

    014      |      Vol_V-0252                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    reason for this is probably that the pigment responsible is present only

    in small quantities and therefore able to utilize, at a time, only small

    amounts of light.

            The interaction of light and darkness in long-day plants may be

    visualized perhaps as follows: In the light, some substance or substances

    are produced which serve as precursor for a specific substance required

    for flower formation. In darkness, this substance is destroyed or inactivated;

    this latter process seems to be localized in the leaves:


    If the light periods are short and dark periods long, i.e., in short

    days, all of the “precursor” is subject to destruction or is inactivated;

    if the relation is reverse, i.e., in long days, the flower-forming substance

    can be made.

            This explanation is based, mainly, on two observations: (a) If certain

    long-day plants (henbane), which are able to stand such treatment, are

    deprived of all leaves, they produce flowers independently of day lengths,

    that is, in long as well as short days. (b) As the temperature is decreased,

    long-day plants become capable of flowering in progressively shorter photo–

    periods. For example, henbane, as mentioned, has a critical day length

    of about 10 hours. This is true, however, only at a temperature of about

    22°; at 15.5° the plants will form flowers even in 9-hour photoperiods,

    whereas at 28.5° they require at least 12 hours of light per day. The

    effect of temperature is particularly exerted during the dark period. This

    means that the processes which are going on in the dark phases and which

    are inhibitive to flowering, are accelerated with higher temperature and thus

    015      |      Vol_V-0253                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    become effective in shorter times. The explanation also accounts for the

    fact that long-day plants flower fastest in continuous light, i.e., need

    no dark periods whatever for flower formation, whereas they fail to flower

    if the dark periods are too long. One may say that the photoperiodic

    response of long-day plants is not flower formation is long days, but its

    inhibition in long nights. In the action of the light most probably two

    separate effects have to be assumed. One action of the light consists in

    supplying, assimilates by photosynthesis. The assimilates are the material

    on which all processes which are going on in the plant are dependent. This

    light action, therefore, is not specific for photoperiodism. The second

    light action is involved either in the production of the special flowering

    substance precursor or it consists in the prevention of its [ ?] destruction

    or inactivation. It is thus the specific photoperiodic light effect; it is

    probably carried out by the light absorbed by the “new” pigment. As either

    light effect, however, is ultimately directed toward promotion of flowering,

    the twofold character of light action in long-day plants is difficult to

    see and to separate.

            In short-day plants the situation is different. These plants require

    an alternation of light and dark periods of appropriate length. Soybeans,

    for example, will flower under regimes in which light periods of 3 or 4

    hours to 12 hours are combined with 10 1/2 to 14 hours of darkness; if

    either of the periods is longer or shorter, flowering will be greatly delayed

    or suppressed. In short-day plants it is therefore evident that light has

    two different actions in regard to flower formation. These two effects are,

    respectively, (a) a promoting one, which accounts for the need of light

    periods, and (b) an inhibitive one, which accounts for the need of dark periods.

    016      |      Vol_V-0254                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    As in long-day plants, the promoting actions seems to consist in the supply

    of assimilates by photosynthesis, whereas the inhibitive effect again seems

    to be tied in with the specific photoperiodic light action perceived by the

    special pigment. Long- and short-day plants, then, differ primarily in

    that the specific light effect in the former is promoting, in the latter

    inhibitive with regard to flower formation. Nevertheless it seems in either

    case to be mediated by one and the same pigment.

            As the final outcome of photoperiodic induction, both in long- and

    short-day plants, a special flower-forming substance appears to be formed.

    This material moves from the leaves in which it is generated to the growing

    points and causes these to change into flowers. The possible existence of

    such a “flowering hormone” was first suggested to early workers by the fact

    that the photoperiod effect is perceived by a part of the plant which is

    separated from the part where the response occurs. In vernalization, both

    perception of the cold stimulus and the response to this stimulus are

    carried out by the growing points themselves. In photoperiodism, the

    effect of the light is perceived by the leaves . It is of no consequence

    whether the growing points of the plant are also exposed to the inductive

    photoperiod or not. The flowers, however, are naturally formed by the

    growing points. A mediator of the flowering response must therefore

    pass through the plant.

            This was borne out by grafting experiments, similar to those done with

    cold-requiring plants (first by J. Kuijper and L. K. Wiersum in Holland,

    and by M. H. Chilahian and B. S. Moshkov in Russia, in 1936 and 1937).

    If a short-day plant is induced to flower by short-day treatment and is

    then grafted to an individual which has been kept on long day, the latter

    017      |      Vol_V-0255                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    individual, too, will produce flowers. The same can be done using a

    day-neutral or long-day plant as conor. Vice versa, a long-day plant

    maintained in short-day conditions can be caused to flower by graft union

    with a short-day plant. As in the grafts with cold-requiring plants, donor

    and receptor may belong to different species or genera (provided they can

    be grafted together satisfactorily).

            Since short- and long-day plants are thus capable of mutual flower

    induction and since the taxonomic relationship of the grafted plants is of

    no importance for the effect on flowering, the flowering substances of all

    photoperiodically sensitive plants must be identical or at least very closely

    related. The flowering substance demonstrated in grafts with cold-requiring

    plants may be different. Even names for the two “flowering hormones” have

    been introduced: “florigen” for the one suggested by the experiments with

    photoperiodically sensitive plants, “vernalin” for the one suggested by

    those with cold-requiring plants. However, so far neither of these substances

    has been isolated and identified, and therefore this question, and the whole

    question, whether specific flowering hormones indeed exist, is not yet

    finally settled.

            Flowering although the most spectacular, is not the sole photoperiodic

    response in nature. Among the animals, aphids, snails, birds, mammals show

    photoperiodic responses. In the plant world photoperiodic phenomena other

    than flower induction are also known. Further steps in the process of

    flower formation itself have a different day-length dependence. For example,

    the formation ( flower primordia may be promoted by one day-length condition,

    their further development until anthesis by another day-length condition

    Furthermore, the fertility and, in monoecious plants (plants having separate

    018      |      Vol_V-0256                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Phtoperiodism

    male and female flowers on the same individual), the sex of the flowers

    may depend on day length. In grasses with a tendency to vivipary, as

    Deschampsia caespitosa, this tendency becomes much more pronounced in

    short days. The production of cleistogamous and chasmogenmous flowers

    in violets is also regulated by day-length, although frequently in close

    intersection with temperature effects.

            In cleistogemous flowers pollination takes place in the bud, and the

    corolla does not open at all; chasmogamous flowers are the ones developing

    normally. Thus, Viola hirta and V. silvestria , if grown in photoperiods of

    12 to 14 hours at elevated temperatures, from exclusively cleistogamous

    flowers; in 8-hour photoperiods at such temperatures they do not flower

    at all; but by a low-temperature treatment are caused to form chasmogamous

    flowers. V. odorata forms chasmogemous flowers occasionally also without

    cold treatment.

            In addition, various vegetative morphological responses to photoperiod

    have been observed. Among such cases are: 1 . Tuber formation is a

    short-day response in many species as in certain wild potatoes, in yams,

    arrowroot, Jerusalem artichoke, etc. The flowering response of the plant

    may be the opposite. 2 . Bulb formation in onion is a long-day response.

    3 . In numerous strawberry species and varieties, in hawksbeard ( Hieracium

    pilosella ), and in other plants runners are formed in long days. 4 . In

    various succulent plants (species of Bryophyllum , Sedum , etc.) the habit

    may be deeply modified by day length, short-day causing more pronounced

    succulence. 5 . In numerous other plants the general habit, too, is modified

    by day length, although the differences are not very pronounced and therefore

    are frequently overlooked.

    018      |      Vol_V-0257                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

            As a rule, long-day grown specimens exhibit one or more of the follow–

    ing features: greater stem-elongation, longer petioles, leaves narrow and

    more erect; short-day specimens are less tall and have shorter petioles

    and broader and more horizontally arranged leaves. Quite generally, plants

    grown in long-day conditions are more vigorous than individuals grown in

    short-day conditions, although here the total amount of light and not only

    the day length may be of significance. Finally, in some cases day length

    has been found to effect physiological characters of a plant without changing

    its morphology. Thus, in locust trees (Robinia pseudoacacia) of southern

    origin, but grown in higher latitudes, short days increase frost and drought



    Ecological Aspects

            The information summarized in the preceding sections has been obtained

    nearly exclusively under experimental conditions. What implications do the

    results have for plant life in the natural environment, and most particularly

    in the Arctic? These implications can be divided into two groups. The first

    comprises the consequences of changes of the temperature and day length regimes

    for the development of a given plant growing in higher latitudes as compared

    with lower ones. Evidence on this point, as mentioned above, isstill very


            There are, however, a few observations which indicate two things:

    (a) Winter-annual plants, when moved farther to the North, frequently behave

    as summer annuals. (b) Plants which do not possess a pronounced cold re–

    quirement, frequently shorten their development time at higher latitudes.

    An example of the first phenomenon is constituted by certain Russian wheats

    which in temperate or southern regions of the country are i u sed as winter crops,

    019      |      Vol_V-0258                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    but which in the Russian Subarctic can be sown in the spring. Apparently,

    the cold requirement of these varieties can be satisfied by the low tempera–

    tures both during the early and the late parts of the growing season, the

    former acting on the seedlings, the latter on the developing seed. It may be

    that, at least in some cases, the second effect is the essential one; if seeds

    harvested in the North are planted in the South the plants still be have as

    summer annuals, and only their offspring again exhibits the winter habit.

            Similar observations have been made in mountain regions which, as far as

    the temperature factor is concerned, can be compared with the North. Wheats

    which in Switzerland are used as summer varieties behave in southern Germany

    as winter crops. In the opposite way, some Italian wheat and oats as well

    as turnips, which grow as winter annuals or as biennials at an elevation of

    160 m. can be treated as summer crops at 1,600 m. It is very significant in

    this connection that vernalization can be achieved with intermittently low

    temperatures. Likewise, it becomes significant that vernalization, as explained,

    primarily accelerates the onset of flower in t i tiation and has no comparable

    effect on the further stages of flowering, which, in addition, ordinarily

    takes place at already rather elevated temperatures. It is, therefore, not

    surprising that it is the heading and earing which are particularly promoted

    in cereals in the North; the subsequent stages from anthesis to seed ripen–

    ing may even be somewhat delayed.

            In the acceleration of the development of summer-annual plants in the

    North two factors may be operative: (1) longer days, and (2) higher daytime

    temperatures, possibly in combination with lower night temperatures. Many

    plants which grow both in moderate and in subarctic to arctic regions are of

    long-day character, and since the day length in summer increases with latitude,

    020      |      Vol_V-0259                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    they may find conditions more favorable for flowering in the higher latitudes.

    It is of importance in this connection that photoperiodic responses are

    caused by very small amounts of light. It has been shown, by the Swedish

    scientists Kjellman and Thore Fries, that arctic plants are capable of carry–

    ing out photosynthesis and growth in the light of arctic nights. In photo–

    periodic responses considerably smaller light quantities are already effective

    than those required for phytosynthesis. As far as photoperiodism is concerned,

    the “White Nights” of the North are continuous light, which is the optimal

    light regime for flowering in long-day plants.

            The temperature conditions of the Arctic may favor development in two

    ways, thermoperiodically and photoperiodically. Summer day temperatures in

    the North may reach values considerably above the average temperatures of

    lower latitudes, and such a temperature periodicity will be favorable to

    plants requiring comparatively high day and low night temperatures. Low night

    temperatures, on the other hand, may act by improving the photoperiodic

    conditions, since the photoperiodic response of long-day plants is favored

    by low dark-phase temperatures.


    Adaptive Significance

            The possibility of transferring plants from lower to higher latitudes

    (and altitudes) and vice versa, discussed in the preceding section, still

    is a limited one. A plant which requires elevated temperatures in all its

    stages of development will not grow in the Arctic, even though the day

    temperatures in summer may be more favorable than at a lower latitude. This

    failure is due to the fact that the temperature average through the entire

    growing season (the “temperature sum”) will be too low. An Arctic or

    subarctic plant may fine, farther south, a better average temperature, but

    021      |      Vol_V-0260                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    its low temperature and day-length requirements may remain unsatisfied.

    Plants can grow and reproduce only under those environmental conditions to

    which they are adapted.

            Adaptation is a genetical phenomenon. Natural selection, acting on a

    variety of hereditarytypes, chooses that one best suited for the particular

    habitat. Frequently, if not generally, adaptation is adjusted to rather

    specific conditions, and even within a comparatively limited area there

    may be several types of a plant species, each adapted to “its own” environment.

    Whether a plant is capable of inhabiting a smaller or greater area, or

    areas with less or more diversified conditions, depends on how wide its

    potential range of hereditary variation is. One may say, if a given plant

    is to be able to succeed in a given area, it must be able to produce

    hereditary types suited for the conditions of this area. This is evident

    from the situation as encountered in nature. Species with wide or environ–

    mentally diversified distribution areas usually comprise numerous different

    strains, varieties, etc., which may differ only physiologically or both

    morphologically and physiologically. Sometimes, a plant which appears

    farily uniform in its natural habitat, if transferred to new conditions,

    “breaks up” into numerous genetically different stains. This is easy to

    visualize. Let us assume we are dealing with a long-day plant, different

    strains of which have different critical day lengths, ranging between 14 and

    16 hours. If this plant is grown at Montreal, where in summer day length

    exceeds 16 hours, it will develop uniformly; if it is grown in Texas, where

    the summer days never are longer than 15 hours, some individuals will flower,

    others not. On the other hand, species with uniform and rigidly fixed

    environmental requirements inhabit either limited areas, or [ ?] at least

    022      |      Vol_V-0261                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    areas with uniform environmental conditions.

            Vernalization, thermoperiodism, and photoperiodism have a strictly

    adaptive character. For example, in northern region a plant without

    specific cold and day-length requirements (provided its seeds have no

    dormancy period) might germinate shortly after fruit bearing, some time

    in summer, and its development might fall into the increasingly unfavorable

    conditions of autumn. Both cold and long-day requirement will result in

    delaying the development until the favorable season sets in. This, then,

    is the second aspect of the significance of the phenomena discussed for

    plant life. Vernalization, photoperiodism, and thermoperiodism, all three,

    have an outstanding importance in the adaptation of plants to environment,

    and thus are operative in determining and limiting the distribution of

    plants. For such a role, these “specific” environmental requirements are,

    actually, far better suited than the “general” ones of rather elevated

    temperature and adequate light. These are operative permanently and by

    the virtue s of this will have no specific regulative value.

            Specific requirements, particularly those with the inductive type

    of action, constitute, on the contrary, blocks in the course of development

    which have to be released if the development is to proceed. This significance

    of the specific environmental requirements is again evident from the geo–

    graphical situation. Plants from higher latitudes have low night temperature

    optima, and are frequently winter-annual or biennial and long-day [ ?] plants.

    Plants form low latitudes prefer high night temperatures, never possess low

    temperature requirements, and are either short-day or day-neutral plants.

    Within one single species, there may be a fine gradation. For example,

    strains of Bouteloua curtipendula from southern Texas and Arizona proved

    023      |      Vol_V-0262                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    to be short-day types flowering on photoperiods of not over 13 hours,

    whereas a strain from North Dakota was a long-day plant flowering only

    on photoperiods of 14 hours or more.

            This role of our phenomena in the adaptation of plants to environment

    is, in the long run, much more deep seated that the effects on the life

    of the individual plant. It may even appear that for most arctic plants

    vernalization, thermo- and photoperiodism are no longer of direct importance,

    because these plants are so well adapted to the conditions of their habitat

    that their temperature and day-length requirements are always satisfied.

    This, however, is only the consequence of the fact, that all plants which

    were not capable of such adaptation have long been eliminated by selection.

            While the general significance of vernalization, photo- and thermo–

    periodism in adaptation is thus unquestionable, their individual shares are

    sometimes hard to evaluate in a particular case. The three factors are

    related by close interactions. Thus all, or nearly all, winter-annual and

    biennial plants also have the long-day habit. They require, for normal

    development, first, exposure to low temperature, second exposure to long days.

    Long-day treatment without preceding cold treatment is without effect on

    flowering as is cold treatment without subsequent long-day treatment

    (both requirements may be qualitative or quantitative). It is, however,

    impossible to tell by inspection which inductive factor or factors are

    lacking in a nonflowering specimen of such a plant. Between photo- and

    thermoperiodism there may be likewise an interrelation. The adverse effect

    of excessively high night temperatures in tomatoes may be compensated for

    to a certain extent by reducing the photoperiod. In many individual instances,

    an experimental investigation is the sole way to establish the condition or

    conditions operative.

    024      |      Vol_V-0263                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism


    Significance for Arctic Agriculture and Horticulture

            The question of application of our phenomena to practical problems can

    be discussed in the same manner as, and with close reference to, that of

    their bearing on wild plant life. There are certain direct applications,

    i.e., the possibility of regulating the individual development of some

    crops. In the early thirties of this century, great hopes were held for

    vernalization in this respect. In the Soviet Union, in 1937, 10,000,000

    acres were planted with winter cereals vernalized as soaked seed. The

    reason underlying these attempts is that vernalized winter varieties in

    yield and sometimes also in quality are superior to summer varieties. At

    present, however, these attempts appear to have been entirely abandoned.

    The reason for this probably is that large-scale cold treatment of seed is

    rather difficult. Direct application of vernalization will be limited to

    special cases, as in breeding work with winter varieties or seed production

    of biennials. Photoperiodic treatment is used in floriculture and to some

    extent may prove useful in horticulture, with crops as lettuce, which can

    be prevented from premature flowering (“bolting”) by short-day treatment,

    or strawberries in which short-day treatment extends flowering time.

    Thermoperiodic measures are also used in floriculture, and as a matter of

    fact were so used long before the significance of the phenomenon was

    discovered. They are also useful for some horticultural crops,

            By far the greatest significance is, however, as with wild plants,

    or a genetical nature. As in wild plants, genetically fixed thermo- and

    photoperiodic and low temperature requirements or operative in determining

    adaptation to a given region. Before the significance of these requirements

    was realized, it was sometimes unaccountable why certain crops, introduced

    025      |      Vol_V-0264                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism

    to certain areas, yielded only complete failure. After this realization

    has been made, breeding can be attempted along adequate lines. One also

    will have some indications where to look for suitable material. For

    instance, for introduction to subarctic or arctic regions one will preferably

    try varieties grown in mountain regions rather than varieties adapted,

    through long periods of time, to the higher and more uniform temperature

    conditions of plants. As cold and day-length requirements were discovered

    but approximately 30 years ago, and thermoperiodicity even more recently,

    it is understandable that at present the application of our knowledge to

    plant breeding still has a chance and casual character. Nevertheless, in

    several crops (cereals, sugar beet, onion, soybeans, forage plants) it has

    already been marked with considerable success. Of the northern countries,

    Russia and more recently also Finland have been doing pioneer work in this

    field, particularly in their extreme northern parts, up to the 70 th parallel.

    026      |      Vol_V-0265                                                                                                                  
    EA-PS. Lang: Thermoperiodism, Vernalization, and Photoperiodism


    1. Fries, Th. C.E. Abisko Naturwetensk. Stat., Meddel.1, Stockholm,

    pp.20, 1918.

    2. Kivinen, E. Medd. Norske Myrselsk. Vol.46, p.1, 1948.

    3. Kjellmann, F.R. Ur polarvexternas liv. Stockholm, 1884.

    4. Lang, A. Chapter on “Developmental Physiology” in “Fortschritte der

    Botanik,” vol.11, p.268, 1944, and vol.12, p.340, 1949.

    Berlin-Heidelberg-Göttingen: Springer.

    5. Lona, F. Nuovo Giorn. bot ital. vol.56, p.516, 1949.

    6. Malyshev, A.A. Acad.Sci.USSR, Doklady, vol.59, p.771-791, 1948;

    vol.60, p.153, 1948.

    7. Melchers, G., and Lang, A. Die Physiologie der Blutenbildung.

    Biolog.Zentralbl, vol.67, p.105, 1948.

    8. Olmsted, Ch.E. Bot. Gaz. 106 , p.46 and 382, 1944/45.

    9. Smirnov, L.A. Bot. Zhurn. (Russia) vol.33, p.559, 1948.

    10. Went, F.W. Thermoperiodicity. In “Vernalization and Photoperiodism”

    (A.E. Murneek and R.O. Whyte, edit.), Lotsya vol.1, p.145,

    Waltham, Mass.: Chronica Botonica.

    11. Whyte, R.O. Crop production and environment. London: Faber, 1948.


    A. Lang

    Back to top