Hereditas. 3. 1922
The Genotypical Response of the Plant Species to the Habitat
Institute of Genetics, Åkarp, Sweden
THE study of the variability of the plant species in relation to its environment, or its habitat, might be pursued along two lines of research, viz. the study of the effect of various environmental factors upon the individual plant, and the study of the effect of these environmental factors upon the genotypical composition of a species-population in a certain habitat. The one line of research is primarily a study of the modifications of the plant, the other is manifestly a study of the hereditary variations. The two groups of variation have long ago been dealt with from the point of view of the environment, but while the experimental study of the former problem has, received much attention the latter problem, the hereditary variation of wild plants in relation to habitat, has remained experimentally almost unattacked.
The classical transplantation experiments with Lowland plants in Alpine situations performed by KERNER (1891) and BONNIER (1895) belong to the important body of facts relating to, the modifications of plants. The modern study of the problem is particularly connected with the names of KLEBS (1903, 1906) and GOEBEL (1908, 1913). Our knowledge of the power of modification, or rather of the range of possible reactions, of the plant when exposed to different environmental factors has. been greatly increased by all this work. Its important bearing upon other lines of studies has also been felt, and, the data obtained through the experimental inquiries into this field have been extensively employed when attempts have been made to throw light Upon less known subjects, especially in the field of ecology. This is particularly true of the cases in which the supporters of the theory of the inheritance of acquired characters endeavour to find new support for the theory in the increasing mass of data obtained (WARMING, 1909, p. 373; WIESNER, 1913, p. 314). That the fact of the great plasticity of the plant has led to undue generalizations even in matters of a less speculative nature will be evident from the following instances, which also serve to bring out the necessity of a closer study of the hereditary variation in relation to habitat.
MASSART (1902), in his well known paper on the modifications of Polygonum amphibium, shows that the plant in question might be readily modified into a land form, a water form and a dune form by exposing cultures from one and the same individual to the proper environmental conditions. He then concludes that the three similar forms of this species found in nature are simply modifications of this kind brought about by direct hydromorphosis and aeromorphosis. It is clear, however, that the experiment does not prove anything of the sort. There may exist genotypical differences in the three forms, as found in nature, which would play an important part ecologically. Extensive work including cultivations under the same conditions of a great number of individuals of the plant, collected in different habitats in nature, and preferably supplemented by breeding experiments is necessary in order to settle the point. A similar generalization of facts, to give another example, has been made by SCHMIDT (1899). He finds that Lathyrus maritimus from the coasts of the Baltic differs in the anatomical structure of its leaves (these being dorsiventral) from the L. maritimus found along the North Sea coast of Denmark (which has isolateral leaves). Subsequent experiments with the former plant showed that watering with solutions of sodium chloride induced a leaf structure (viz. isolaterality) typical of the North Sea plant. He concludes that it is to be assumed that the direct effect of the sodium chloride, which is found in a higher percentage in the North Sea than in the Baltic, produces the North Sea type. It is at once seen that this is a generalization of facts similar to that criticised above, it is a generalization of about the same order as the statement that any white flowering Primula sinensis has been produced by the cultivation of normal, red flowering Primula sinensis at 30°C. Even if it is true that red flowering P. sinensis becomes white when cultivated at 30°C it is equally true that there exist hereditary, white P. sinensis forms (BAUR, 1914), e.g. white-flowering at a temperature at which the former plant has red flowers (viz. at 20°C).
Thus in the absence of critical acquaintance with the different forms of a plant species met with in nature, much speculation as to the origin of adaptive structures is to be found in writings on ecology. The following discussion includes a number of notorious adaptive forms, and an attempt is made to ascertain whether the existence in nature of such forms is the result of an advantageous response on the part of the individual, or whether these forms are brought into existence through a genotypical response of the species-population to definite habitat conditions.
Before going into details I wish to express my gratitude for the help received from so many sources during my work. The nature of the work necessitated considerable space. Professor H. NILSSON-EHLE, head of this Institute, has not only put the resources of the institute at my disposal; but to him I am also indebted for much inspiring advice given freely during the progress of this study. The necessary green-house space as well as other facilities have been kindly provided for me by Professor S. MURBECK, head of the Botanical Museum and Garden in Lund, who has taken much interest in my work. I also take this opportunity of acknowledging my indebtedness to Professor H. KYLIN, head of the Botanical Laboratory in Lund, for his kindness in promoting these studies in different ways.
The collecting of the various plants in cultivation, discussed in the following, has been done chiefly by myself on journeys some of which have been rendered possible through economical support from the Physiographical Society and the Botanical Club of Lund. However, material of certain forms has also been supplied by friends interested in my work and to them I wish to extend my sincerest thanks for all their kindness. I am under obligation to Messrs. N. STENSSON, and K. B. KRISTOFFERSON, for the necessary photographic work.
I. THE MATERIAL AND THE CULTURES.
Transplantations on a small scale were begun in 1916. At that time isolations were also made of species of the genus Atriplex. The necessary ground for these cultures was obtained in my home garden in Malmö. With the increase in the number of cultivated species and in the number of individuals of each species the space became too small, and in 1,918 the cultures were moved to the Institute of Genetics at Åkarp. Since then the cultures have much increased; the area covered by individuals of perennials transplanted from various habitats is at present about 25 ares. The soil is a fine loam, and the conditions of the ground show satisfactory uniformity throughout. In order to maintain this uniformity no animal manure is used. The plants in the permanent cultures (that is the perennials) are planted in rows, usually with a distance of 40 cm. between the plants in the row and of 50 cm. between the rows. The more bulky ones are given distances of 30 x 60 cm. Each plant has then space enough to increase in size, and the cultivation with tools between the plants becomes easy. The cultures are kept free from weeds and are dug twice a year, in spring and in autumn. Among the species cultivated, only Leontodon autumnalis has been found to thrive less satisfactorily. The individuals of this species sometimes suffer badly from the attacks of cutworms (Agrotis spp.?) on the roots. A certain type of Hieracium umbellatum, viz. the broad-leaved sea-cliff type, is particularly damaged and greedily eaten by rabbits, which has necessitated the fencing of the field.
A few words should be said as to the kind of species used and as to the mode of collecting. It is clear from the nature of the study that only such species have been investigated as are very common and occur in different habitats. Such species are, for instance, Lysimachia vulgaris, Centaurea jacea, Solanum Dulcamara, Matricaria inodora, etc. It is further to be expected that if changes in the genotypical composition of a species result in response to climatic or edaphic factors these changes would be most clearly brought out and most easily demonstrated in species which have an extended and uninterrupted distribution running through areas of different climatic and edaphic character. The coast line of southern Sweden, to which region most of the species dealt with in the following are to be referred, is well suited for investigations of this kind. There are marked climatic differences between the east coast and vest coast, the latter being more maritime and exposed to the action of strong winds and atmospheric sodium chloride (cf. FRODIN, 1912), and the physiographical features of the coast lines vary much, cliffs, dunes and salt marshes alternating. There are plant species which occur throughout this varied stretch of land, and some of these, viz. species of the genus Atriplex, Armeria vulgaris, Hieracium umbellatum, have been found to furnish important data and have therefore been transplanted and cultivated on a large scale.
In the collecting, the individuals of a species from a certain habitat are carefully dug up, care being taken. that no selection is made; they are then numbered and packed in sacks and posted. They are immediately planted in the experimental field on arriving. In planting as much soil as possible is removed from the roots. When just planted they are watered once or twice and then left. When treated in this, way, only very few of the plants are spoiled and die.
In regard to the notes and measurements of the different cultures discussed in the following it should be said that the values given in the Hieracium tables (placed at the end of the chapter) for the length and width of the middle leaves 'are average values based upon the measurements of five leaves of each plant. The values given in the same tables of the magnitude of the angles formed by the stem with the perpendicular refer to the angles of the largest stem of the individual. These values have by tests been found to deviate ± 5 degrees from the correct value. Field numbers preceded by one or more 0 indicate that a corresponding number of individuals have died and left gaps in the row. It has been found practical to use this method in checking the individuals. A in the table columns indicates that measurements or notes have not been taken because of the wilted conditions of the leaves or of the whole plant (or because of damage by rabbits in some cases).
The cross-sections made of different leaves have for technical reasons been collected at the end of the Hieracium chapter.
The terminology followed in discussing the distribution of various shore species in the different zones of the shore is that worked out by SERNANDER (1917).
In arranging the material it has been thought best to group it under seven headings, the first four containing the results of the cultivations of shade forms, dwarfs, succulent shore forms of inland species, and halophytes, and the last three containing the data for inland and coast forms of Sedum maximum, Armeria vulgaris and Hieracium umbellatum respectively. The general discussion of the results and the bearing upon the problem which these results imply will then follow.
1. MATERIAL AND CULTURES OF SHADE FORMS OF MESOPHYTIC SPECIES.
A great number of mesophytes are known to develop shade forms in response to subdued light, and the morphological and anatomical changes brought about in such forms, especially in the leaves, are well known. The further question, whether or not all shade forms occurring in nature are shade-modified open air plants, or whether hereditary shade varieties also exist within 'certain species, has received but little attention. The cultures made in order to settle this point cannot yet be said to be conclusive, since no crossings have been made, but they are calculated to throw some light on the problem. The plants employed are Lysimachia vulgaris L., L. nummularia L. and Dactylis glomerata L.
A. LYSIMACHIA VULGARIS.
The extreme shade form of this plant has been found most typically in the moist Alnus swamps of Hallands Väderö. The habitus of the shade form differs markedly from the ordinary form, as has already been pointed out, although rather incompletely, by GLÜCK (1911). The stems of the shade form are thin and slack, and, in their upper half, horizontally expanded. The leaves stand horizontally and almost in one plane; they are considerably larger than in the ordinary form but very much thinner. Flowers are rare.
A dozen of these plants, collected in different spots within the swamp area, were brought home in 1920. By the following year (1921) the habitus of these plants was already changed and corresponded now in all essentials with the ordinary form. The changes brought about in the culture are best followed in regard to the leaf structure. Fig. 75 a (p. 329) represents a cross section of a leaf of the shade form. They were found to be between 123-157μ thick and did not have any typical palisade layer. The figure to the left is a cross section of a leaf of the same plant from the culture in 1921. The leaf thickness was now found to vary between 358-368μ, and powerful palisades in two layers are found throughout. The anatomical leaf structure as well as the habitus of the separate plants belonging to this series do not at present exhibit any observable differences from sets of the ordinary form brought home and transplanted from the beach at Ringsjön (middle part of Scania) in 1920.
It should not be thought that Lysimachia vulgaris might not be found to show hereditary shade forms, but the evidence adduced above points to the fact that the Hallands Vãderö shade form is merely a shade modification of the ordinary form.
B. LYSIMACHIA NUMMULARIA.
Series of this plant were collected and brought home from various points in Scania in 1920. The cultivated material thus includes series from moist pastures at Åkarp, from similar habitats in the neighbourhood of Malmö, and from the woods south of Kivik (on the east coast of the province of Scania). The latter locality is heavily wooded with oak, elm, Norway maple, ash and linden, while the former localities consist of open, grassy, and somewhat moist pastures. Careful examination reveals differences between the pasture nummularia and the plant from the woods. The latter has somewhat larger leaves than the former; they are further convexly bent and of a deep green colour in the woods, while the leaves of the pasture plant are somewhat concave and light green in colour.
Fig. 1. Lysimachia nummularia. Forest type (the upper) and pasture type.
The separate plants of the pasture series cannot be seen to differ from each other in culture. The series from the Kivik woods, in all 16 individuals, cultivated at the side of the former does not show any observable variation within the series either. It is a remarkable fact that the Kivik nummularia retains its distinctive marks in culture. The small pieces of turf originally brought home have increased in size ten-fold without showing any tendency to lose their characteristics. Clones raised in 1922 from cuttings from both the pasture and the forest nummularia likewise retain the characteristics of the respective mother plants. Fig. 1 represents pieces of the two plants taken from the clone cultures in 1922. The differences between the pasture plant and the plant of the woods as to the leaves are at once seen. Cross sections of leaves from these cultivated plants show much the same structure (both forms having one layer of palisades), but the cuticle of the upper epidermis is found to be thicker throughout in the cultures of the pasture plant than in those of the plant from the woods. Repeated measurements have given a mean value of 7.8μ. for the former and 5.6μ. for the latter. The outer wall of the lower epidermis is also thicker in the former than in the latter, especially in places below the vascular bundles, where it attains a considerable thickness and becomes much folded. Thus the results of the cultivations point .strongly to the assumption that there are in Sweden at least two hereditary types of this species, the one growing in meadows and pastures, the other in the woods. The differences in the leaves show that the latter is manifestly a shade plant as compared with the former.
DAHLGREN (1922 a) has recently traced the distribution of L. nummularia in Sweden and pointed out that the plant has to a large extent been spread by human agency. It remains to be settled whether the above described shade type from the woods is native in our country, while the type from the meadows and pastures is introduced and spread through man, a view, which might be an approximation to the truth. A further point of interest will be afforded by the crossing-experiments between the two types to be started a following year. The experiments made by DAHLGREN (1922 b) have shown that plants from widely different parts in Sweden are sterile when crossed with each other, while fruit develops when Swedish plants are crossed with German and Austrian specimens. It seems reasonable to assume that the failure in the case of the crossings where Swedish material was exclusively used was due to the fact that the plants were members of the same clone, in this case- probably the pasture nummularia, as this type has no doubt the largest distribution in the country.
It should perhaps be said that the shade variety of L. nummularia, discussed above, has not been found described in the literature. DOMIN (1904), who lists a number of forms of the species, does not mention any such form.
C. DACTYLIS GLOMERATA.
A shade form of this grass is known to systematists under the name D. glomerata var. lobata Drej. It occurs in the beech woods of southernmost Sweden, as well as in Denmark, and differs from the ordinary type in having culms taller and slacker and loosely tufted, long, drooping and dark or light green leaves, slack, long and somewhat drooping, never violet-coloured, panicles, smaller spikelets, and smooth flowering scales. Sets of this form have been collected and transplanted from Denmark (near Copenhagen) in 1919 (17 individuals), and from Dalby in Scania in 1921 (16 individuals). The changes brought about in the cultivated shade form as compared with the plants in their natural habitat are the following. The culms become more tufted, although the tuftiness of .the main type is not attained. Both leaves and the panicles become brightly violet-coloured, and more so than is generally seen in the main type. The thickness of the leaves increases. The mean thickness of the basal leaves of two individuals from the Dalby series was found to be 135-155μ. and 140-150μ. in their natural habitat in 1921, while the mean leaf thickness of the same plants the following year (1922). in culture was found to be 150-175μ. in the one plant and 165-185μ. in the other. Whether the leaf thickness of the main type, the leaves of which are considerably thicker, has been attained in any of the plants of the Copenhagen series, which has been in culture since 1919, has not been ascertained with exactness, but to judge from superficial examination this is not yet the case.
The length of the culms as well as of the leaves has been found to increase in the cultures. The mean leaf length, for instance, in the Dalby series (measuring 10 of the longest basal leaves of each plant) was found to be 425 mm. in 1921, while it was 530 mm. in 1922. An increase in the length of the panicles has also been found to take place in culture.
The above results favour the assumption that D. glomerata var. lobata is a hereditary shade variety and not a modification due to the direct effect of the environment. Although undoubtedly certain characteristics ascribed to the form are the results of the extreme environment, as for instance the great looseness of the tuft and the pure green colour of the leaves and panicles, which characteristics disappear in culture, other characteristics, as the length of the leaves, culms and panicles, increase in magnitude upon cultivation. This would not be likely to take place had the plant in question developed its characteristics in direct response to subdued light.
The variation within the series is otherwise rather large. Fig. 2 represents the shape of the panicle in fruit from 4 individuals of the Copenhagen series. The same relation as to the degree of spreading of the panicle is seen every year in these individuals, so there is little doubt as to the hereditary nature of the variations seen between the different plants. A few of the plants in the series have somewhat hairy flowering scales and should not be referred to var. lobata according to the diagnosis given of the form in the floristic handbooks. They correspond in all other respects, however, with the shade variety, and thus illustrate the difficulties and perhaps the impossibility with which the systematists are confronted in their efforts to draw hard and fast lines between a certain variety and its supposed main type.
Fig. 2. Dactylis glomerata. Illustrating different shapes of panicle found within the shade type.
It should not be thought that modificatory shade forms of D. glomerata do not exist. Such forms are, on the contrary, recorded several times. They are often much like the var. lobata, although they are usually furnished with some hairs on the flowering scales. From what has been said above with regard to the occurrence of hairy scaled individuals within the hereditary shade variety it will at once be seen that the difficulty with regard to the separating of the modificatory forms from the hereditary ones without cultivating the plants must be very great, or rather insurmountable.
2. MATERIAL AND CULTURES OF LOWLAND DWARF FORMS.
In our country dwarf forms are especially met with in Alpine habitats, in the so called Alvar vegetation and in the salt meadows along the coast. Representatives of the two former groups have been cultivated for too short a time to be considered here, and therefore only salt meadow dwarfs, already in culture for some years, will be discussed. The plants to be discussed at any, length are Aster tripolium L., Succisa pratensis Moench. and Centaurea jacea L.
A. ASTER TRIPOLIUM.
A dwarf form of this plant is known to systematists under the name A. tripolium var. diffusus DC. It is a much branched plant, not more than 5 cm. high, and occurs in somewhat drier spots of the salt meadow than the ordinary, tall growing tripolium. A series of the form (in flower) was collected at Vellinge, south of Malmö, and transplanted in the autumn of 1019 together with a few rosettes of the ordinary form. The latter attained a height of between 45-70 cm. in 1920, then flowered, and died in the autumn. The dwarf, although flowering in 1919, lasted through the autumn and winter and flowered again 1920. The height of the 1919 plants varied between 3-5 cm. The plants grew taller in 1920, and the height now varied between 10-16 cm. They were found to fruit freely. Fruits were collected from one of the individuals; they were sown in the autumn, and the seedlings were forced in the green-house and transplanted into the open in May the following year. They all flowered in the autumn. Fig. 3 illustrates some of the resulting types in the series raised. The height of the plants in the series varies between 3-18 cm. There is great variation between the different individuals, especially in branching. The plant to the left in the fig. has a wholly prostrate main axis, the plant in the middle has a number of ascending, equivalent branches, while the plant to the right has an erect main axis and smaller, ascending side branches. The same characteristics of the different plants both as to height and branching are seen this year (1922). The same is true of the original dwarfs transplanted in 1919, which do not show any tendency to. change the habitus attained in the culture 1920.
Fig. 3. Aster tripolium. Hereditary dwarfs.
It seems therefore safe to conclude that the series in question is made up of hereditary dwarfs, which, at least in the experimental fields at Åkarp, are perennials, as distinct from the ordinary tall growing A. tripolium. Within the dwarf series, moreover, all kinds of branching types might be found just as in the ordinary form showing that certain characteristics vary as much in the dwarf variety as in the ordinary tripolium. Modificatory dwarfs of A. tripolium, called forth by insufficient water and food supply, might also probably be found in nature. As to modificatory perennials of A. tripolium the statement is made by BUCHENAU (1896) that the plant becomes perennial when eaten by animals or cut down. Both modificatory and hereditary dwarfs might be contained in the var. diffusus of the floristic handbooks. As to the latter group it should be noted that it is not the dwarf as seen in the culture which conforms to the var. diffusus; it is only the plant that results from a modificatory dwarfing of this dwarf in the natural habitat which fulfils the prescription given in the diagnosis.
B. SUCCISA PRATENSIS.
This species is rather common in the upper part of the salt meadows along the coast of Scania. It is usually dwarfed in these localities, and the most extreme form, which only attains a height of at most 85 mm., is known under the name f. nana Bolle. Series of this extreme dwarf were collected and transplanted from Torekov and Hallands Väderö (N. W. Scania) in 1919, and a set of the ordinary swamp plant, as found in the inland, was transplanted from Stehag (middle part of the province) in 1920. Table 1 lists some of these series (series 34 from Torekov, 35 from Hallands Väderö, 118 from Stehag) and gives the length (in mm.) of the longest flowering stem of each plant when transplanted and as found in 1921 and 1922.
Fig. 4 illustrates the habitus of the dwarfs used in the series 34 and 35. They have all increased in length under culture, as may be seen from the table. The increase in the case of series 34 and 35 was greatest the year after the transplanting. The dwarf habit was at that time thought to be an unquestionable case of modificatory dwarfing, which would disappear in time. For this reason no measurements were made in 1920. In 1921 the height of the plants was surprisingly like that already attained the preceding year, and the suspicion arose that the variation seen in the series as to the height of the different individuals was of a hereditary nature. The different heights observed in 1921 do not fall much lower than those observed in 1922. As the summer of 1922 was considerably wetter than that.-of the preceding year the slight increase in the height of most of the individuals in 1922 is most probably due to differences in weather conditions.
It is further seen from the table that series no. 35 includes a number of individuals much taller than any of those of series no. 34. It would be difficult to understand why this should be the case were it not that the genotypical constitution of the plants differed in the two series. When it is remembered that the series, as well as the different plants of each series, grow under almost identical conditions of culture, little doubt remains as to the hereditary nature of the differences seen between the plants in these two series, as well as of the differences between these salt meadow series and the inland series. (no. 118).
Fig. 4. Succisa pratensis f. nana. Plant to the left 10 cm. high. Torekov 1919.
Fig. 5. Succisa pratensis. Behaviour of a dwarf series (no. 35) when cultivated.
The differences in height between the plants in series nos. 34 and 35 may probable be accounted for when the nature of the two original habitats is considered. No. 34 comes from a very sterile salt meadow, while no. 35 comes from a much less extreme salt meadow where species of the inland swamp (Ranunculus flammula, Lycopus, Scutellaria, Caltha, Comarum etc.) occur mixed with typical salt meadow plants (Plantago maritimum, Glaux, Armeria). The point in question will be discussed more fully in a following chapter.
Fig. 5 illustrates some of the different types now seen in the cultivated series no. 35. It should not be thought, however, that variation does not occur with regard to other characteristics. Differences between the plants are also seen in the matter of the hairiness of the leaves, in the shape of the leaf margin, in the shading of the blue flower colour, etc. These characteristics have been found to repeat themselves, in those plants observed, every year, and thus strengthen the belief in the hereditary nature of the characteristics in question.
In summarizing the results of the cultivations the following conclusions seem most reasonable. The hereditary variation in height becomes covered by the modificatory dwarfing of the plants when exposed to the more or less extreme conditions in the natural salt meadow habitat, resulting in a seemingly homogeneous population of dwarfs known under the name f. nana. When brought into culture, the population breaks up into its component parts, and the great hereditary variation as to height becomes visible. The fact that individuals, found upon culturing to be constitutionally as tall as those of the inland swamp population, are found to be contained in certain dwarf populations, while they are excluded from others, points to the controlling effect of the habitat factors upon the hereditary composition of the population.
C. CENTAUREA JACEA.
This species is also found growing in the salt meadows along the coast in a much dwarfed form, f. humilis Schrank., about 10 cm. high, or even less. Fig. 6 illustrates the general appearance of this dwarf as it is found growing at Vellinge. One of these Vellinge dwarfs was brought into culture as long ago as 1916. It increased in height the following year, and then measured about 35 cm. In 1918 it attained a height of about 50 cm., and has since kept this height, with slight fluctuations in response to the yearly fluctuations of weather conditions. The plant was divided in 1920, and fig. 7 represents an individual of the resulting clone. A dwarf transplanted in 1917 from Torekov behaved in a different way. It should be said that f. humilis grows abundantly at this place together with the dwarf Succisa dealt with above. It is apparently the locality already mentioned by NEUMAN (1884) as densely populated by Centaurea dwarfs. The dwarf could not be seen to differ in any respect from the original Vellinge dwarf, but on cultivation a prostrate habit of growth was taken up. This plant was also divided in 1920, and fig. 8 represents one of the clonal individuals. This clone does not attain half the height attained in the Vellinge clone, and the tall, nearly erect form of the latter individuals contrasts sharply with the prostrate, spreading growth of the Torekov clone.
Fig. 6. Cenlaurea jacea f. humilis. Plant in the upper left corner 12 cm. high. Vellinge 1916.
In the summer of 1919 the Torekov locality was visited once more and a larger col lection of the dwarf was made, in order that the nature of the dwarf population might be studied more closely. The collection included 40 individuals, the majority of which, or 35, are thriving well. The behaviour of this population in culture has been found to be much the same as the Suc cisa population, series no. 35. Fig. 9 represents four of the types making up the population. The extremes, the dwarf in the upper left corner with stems only 15 cm. long, and the ascending-erect plant in the lower right corner with stems 60 cm. long, are rare. The former type, shown in the photograph, is the only one present in the population, while there are four individuals attaining the height of the latter type. The intermediates, two of which are shown in the photograph (pag. 228), predominate. As to the position of the branches, individuals wholly prostrate are in the minority, while plants with procumbent and ascending branches predominate. None has been found to be quite erect. There are additional differences between the plants in this series, involving the hairiness of the leaves, the position of the leaves (prostrate or ascending), the shape of the bracts, etc.
Fig. 7. Centaurea jacea. Behaviour of Vellinge dwarf in culture.
In order to make clear the genotypical constitution of the plant as to the form of growth, a cross was made in 1019 between the nearly erect Vellinge plant (fig. 7) and the spreading Torekov plant (fig. 8). The resulting F1 was intermediate, none of the bastards obtained (12 in number) being as erect as the one parent, and none as prostrate as the other. Several of the F1-plants were crossed with each other (C. jacea is wholly self-sterile) but the fruits were found to be eaten by a larva and only one of the plants was found to have some intact fruits left. The 30 fruits sown gave 26 individuals, which were already brought to flower in 1921. They show segregation into 2 nearly erect, 20 intermediates and 4 spreading. Although this F2-generation is too small to allow of any factorial scheme, the segregation evidently involves several factors; it is mentioned here to show the hereditary nature of the characteristics in question. The cross has been repeated this year.
Fig. 8. Centaurea jacea. Behaviour of Torekov dwarf in culture.
In summarizing the results of the cultivations it should be said that it might be safely assumed that the dwarf C. jacea f. humilis of the salt meadows is made up of a heterogeneous assemblage of most diverse, genetically different types. They all react upon the extreme habitat conditions with dwarf growth, thus giving the impression of a homogeneous population just as in the case of the dwarf Succisa. The question whether or not the erect Centaurea jacea of the inland might in nature become modified to the same extent has not yet been investigated.
Fig. 9. Centaurea jacea. Behaviour of a dwarf series (no. 37) when cultivated.
Additional dwarf forms, including those of Veronica spicata L. from sand dunes, Achillea millefolium L. from the same habitat, and Prunella vulgaris L. from salt meadows, have all upon culturing been found to be modifications presumably called forth by insufficient water and food supply in the respective habitats (JOST, 1913; WITTE, 1906).
3. MATERIAL AND CULTURES OF SUCCULENT COAST FORMS OF MESOPHYTIC INLAND SPECIES.
It is well known from works on ecology that certain species are described as dimorphous (WARMING, 1909), showing a halophytic form with succulent, thick leaves, and an inland form with thin leaves. It is also known (BATALIN, 1876; LESAGE, 1890; BOODLE, 1904 etc.) that fleshiness, increased development of palisades, reduction of intercellular spaces, etc. might be readily induced in many species by watering with solutions of sodium chloride. From these facts it might seem plausible that the halophytic, succulent forms of ordinarily mesophytic, thin-leaved inland species have been called forth by the modificatory action of the salt on the plant when growing in saline soil.
In order to test to what extent such forms are merely modifications or hereditary, some species showing "dimorphism" in this characteristic were brought under culture. The species to be dealt with at some length in the following are Solanum Dulcamara L., Matricaria inodora L., Leontodon autumnalis L. and Melandrium rubrum (Weig.). Garcke.
A. SOLANUM DULCAMARA.
Sets of this widely distributed plant have been collected in the inland and at different points on the east and west coasts. The inland series (coll. at Krageholm, Scania 1919 and at Stehag, Scania 1920) include plants (20 in all) with smooth and rather thin leaves. Fig. 75 h illustrates the anatomical structure of the inland leaf type as seen in the cultures in 1922. The thickness of the middle leaves has been found to vary between 193μ. and 245μ. The sets from the east coast of Sweden (from Getå, 1920 and Västervik, 1920) include plants with both hairy and smooth leaves. The leaves of this type are usually somewhat thicker than the inland type both in the natural habitat and in the cultures. The most remarkable type, however, is the one that inhabits the west coast of Sweden. The leaves of this type are more than twice as thick as those of the inland type and always more or less hairy. It is found on the exposed, rocky shore (in the upper supralittoral belt) from north-western Scania and north-wards. A series of 16 plants of this type from Hallands Väderö was brought under culture in 1919. These 16 plants have all retained their hairy and fleshy leaves, although the fleshiness is somewhat less in the cultures than in the natural habitat. One of the plants with very thick leaves, field no. 11, has been specially followed as to this point. The thickness of the middle leaves of this plant was found at the time of collecting to vary between 560μ. and 613μ. The value of the leaf thickness of the same plant in the culture in 1921 was found to vary between 438μ. and 507μ. In 1922 the value was found to be about 500μ.
Another proof of the hereditary nature of this succulent type is given by the cultural experiments with shade forms of the same type. Such shade forms are found growing in the Alnus swamps of Hallands Väderö a few hundred metres distant from the shore. The leaves of these plants (the physiology of which has been discussed by LUNDEGÅRDH, 1919) are thin and smooth. Fig. 75 e represents a cross section of a typical leaf of the shade form, showing that it is considerably thinner than the leaf of the cultivated inland type. A series of these thin-leaved shade forms has been cultivated since 1920. Fig. 75 f shows the appearance of the leaves the following year (1921). The increase in thickness is considerable, and it is now found to be much thicker than the leaf of the inland type. It has also become hairy. In 1922 the thickness shown in fig. 75 g, which is a cross section of a leaf of the same plant, is attained. The original thin-leaved shade plant has now attained the same fleshiness of leaves as has been found typical of the fleshy coast (and sun) plant when cultivated, thus showing that the shade form in this case is a shade-modified plant of the coast type. It is seen from the figure that the thickness of the leaf is brought about by an elongation of the palisades and an enlargement of the sponge cells.
The great differences in the anatomical structure of Dulcamara leaves from inland and coast habitats have been particularly discussed by WARMING (1906), although the question as to the hereditary nature of these differences has had to be left unanswered by him. The results of the above mentioned cultivations, however, favour the following conclusions as to the presence of different, hereditary types within S. Dulcamara:
B. MATRICARIA INODORA.
A succulent, halophytic variety of this species is known to systematists under the name var. maritima (L.). It differs also from the inland form growing as a weed throughout Sweden in being perennial and bushy in growth, in having bracts with broad, dark-coloured margins and, usually, in having broader and shorter rays and less erect growth. It is for the rest rather variable as to the length and shape of the leaf segments and rays, etc. (see NEUMAN, 1882). The different forms of this variety are common on the west and east coasts of Sweden, and series of these forms have been transplanted from Hallands Väderö, Kullen and Kristineberg in Bohuslän (on the west coast) and from the islands off Stockholm, Karlskrona and Västervik (on the east coast). Some of the original plants have been divided and clones have been raised. Table 2 gives the characteristics of five of these clones (with 10-20 individuals in each) as seen in the cultures in 1922, together with the place and year of collection. Field no. 1 represents the ordinary inland weed type growing on the same bed, and raised from seed collected in 1921 at Lund.
|Thickness of leaf
segments in μ
There are additional differences between the clones, but those cited are the most characteristic as to the vegetative parts. The differences between the inland type and the succulent type are striking enough and need not be further discussed. The point of interest is the difference between the var. maritima from the east coast and from the west coast. The latter have thicker and shorter leaf segments and are more depressed. These differences are seen not only in the above clones but also in most of the cultivated single plants transplanted from both coasts. The west coast forms have been found throughout to represent more extreme forms than those of the east coast, as compared with the inland type. Less prostrate and less fleshy-leaved forms may also be found on the west coast, and prostrate and very fleshy-leaved forms may conversely be found on the east coast, but west coast sets have always been found to contain more numerous individuals of the latter type, and vice versa with regard to east coast sets.
The anatomical structure of the leaf of the inland type differs much from that of the maritima type. Fig. 74 a represents a cross-section of the marginal part of a leaf segment belonging to a plant raised from seed collected from the cultivated Hallands Väderö series in 1919, from which the clones represented in field nos. 2 and 3 have been raised. As regards shape and leaf thickness it much resembles the clone represented in field no. 2. Fig. 74 b is a cross-section of a leaf segment from the inland type raised from seed collected at Lund in 1910 and grown on the same bed as the former. Besides the very great difference in thickness the maritima leaf is found to be isolateral, while the inland leaf is found to have only one layer of palisades on the lower side.
Fig. 10. Malricaria inodora. Prostrate form of the coast type from the west coast.
Fig. 11. Matricaria inodora. Procumbent form of the coast type from the west coast.
Fig. 10 represents an individual from one of the clones cultivated in 1922 (viz. field no. 3), and fig. 11 shows an individual from another clone (field no. 2). This latter was crossed in 1920 with the inland type. The F1-generation was found to be uniform and intermediate as to shape and thickness of leaf segments. The plants were as erect as the inland type, and, like the latter, had a single main axis, but the crown of the plants was very bushy. Fig. 12 illustrates one of these F1-plants. They all died in the autumn, thus being annual like the inland type.
Fig. 12. Matricaria inodora. A F1-plant from a cross between the inland type and the shore type represented in fig. 11.
The F2-plants obtained in 1922 from the crosses between F1-individuals (the plants were found to be wholly self-sterile) is at this moment still in the rosette stage. However, segregation is very evident. As to the shape of the leaf segment there are (out of a total of 163 plants) 28 short and blunt (like the maritima parent), 56 short and pointed, 58 elongated, 17 long segmented (like the inland parent), and 4 very long and narrow. The segregation is evidently polyhybrid showing transgression in one direction. The grouping of the individuals as to leaf thickness is rather peculiar; there are 9 very thick and fleshy (transgressions), 62 thick-leaved (like the maritima parent), 59 intermediately thick and 33 thin leaved (like the inland parent). The segregation, although probably polyhybrid, is presumably disturbed by modificatory influences. It will be followed up closely when flowering has commenced, but it is mentioned here as a proof of the hereditary nature of the characteristics involved.
The different hereditary forms of Matricaria inodora transplanted from various places are thus found to group themselves into the following types according to the habitat:
It should be added that in places where cultivated fields run down to the shore bastards representing different combinations of the inland and the halophytic types might be found.