The New International Encyclopædia/Distribution of Plants
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Distribution of Plants
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DISTRIBUTION OF PLANTS, Phytogeography, Plant Geography, or Geographical Botany. That division of biological geography that deals with the distribution and with the causes of distribution of plants. The subject has been studied since the time of Linnæus, but its modern period dates from the works of Alphonse De Candolle and of Grisebach, about the middle of the nineteenth century. More recently, the works of Warming, Drude, Engler, and Schimper have added much to our knowledge of the subject. Phytogeography may be divided into two main heads: First, ecological phytogeography, which seeks to account for the distribution of plant-forms (e. g. the forms peculiar to water-plants, to desert-plants, etc.) and the causes, mainly climatic and meteorological, of such distribution; second, floristic phytogeography, which has for its aim the subdivision of the world into floral regions and districts, and the study of the distribution of plant-species in these regions, thus dealing with the species as units, and seeking to determine the geological and similar causes for plant distribution.
Schimper subdivides the world into tropical, temperate, and frigid regions, alpine districts, and the seas. Each of these divisions, especially the first two, is in turn redivided into formations. These formations, if determined by climatic factors, are termed climatic formations, if by local or soil factors, edaphic formations. For example, deserts and tropical rainy forests are climatic formations, their existence being determined mainly by the distribution of atmospheric moisture; on the other hand, the flora of a swamp or of a heath, being governed mainly by local soil factors, is spoken of as an edaphic formation.
Climatic Formations. In general, the climatic plant-formations of the world belong to one of three types; viz. forests, grass-lands, or deserts. This classification points to three great atmospheric factors — wind, moisture, temperature. Wind has acted as the great distributing agent, carrying seeds and spores from place to place. Moisture is not only necessary to the germination of seeds and the growth of plants, but its abundance or scarcity has a profound influence upon plant-growth; the moist belt bordering the equator is as remarkable for the luxuriance of its vegetation as the arid districts — the great deserts of the world — under the Tropic of Cancer, are noted for their barrenness; barrenness proved, by oases at springs and by the results of irrigation, to be due mainly, if not solely, to the absence of water. The range of temperature in which plants will grow and reproduce varies widely with different species; some are capable of withstanding very low temperature, others very high, and in many instances experiment has proved that members of either group will fail under conditions favorable to members of the other. A fourth factor, light, must be considered. Light is as essential to the growth of green plants as are moisture and heat, but the amount necessary for individual species varies greatly. Some are able to thrive in even the dense shade of tropical forests; others are unable to exist except in direct sunlight. Again, the effect of the amount of light is well illustrated by the shortening of the period of growth required by cereals cultivated in the Canadian Northwest, due to the extra hours of sunshine during the growing season. Although, as yet, but little knowledge has been gained by experiment to aid in determining the exact nature of the influence of these factors on the distribution of plants, there is no room for doubt that such influences do operate. Between the equator and the poles, between the base and the top of a mountain, between the moist coastal districts and the arid interiors of the continents, between the depths of a forest and the treeless plateau, there are gradual changes in the vegetation that mark the striking parallelism that exists between the distribution of wind, moisture, heat, and light, and the distribution of plants.
Edaphic Formations. As already noted, the edaphic formations are determined by local causes, mainly those which reside in the soil. Much dispute has arisen among authors as to whether chemical or physical causes are uppermost in determining distribution; dispute that obviously cannot be settled until knowledge based upon sufficient experimental data, at present sadly lacking in most instances, shall be gained upon this subject. It seems most in harmony with our present knowledge to hold with Warming that the water in the soil is the most potent factor in determining local distribution of forms. In passing from the centre of a pond to upland, plants may be encountered in more or less definite succession whose structures bear close relations to the water-supply. Forms, such as duckweed, which have no root-anchorage, but drift with the wind on the surface of the water, have large air-containing chambers, which greatly increase the transpiring surface. A similar open structure is observed in water plants whose roots are anchored on the bottom and in those marsh plants growing on the margins, but as the ground becomes more and more dry the open structure which favors transpiration gradually disappears, and devices for checking transpiration take their place. Careful study has shown clearly that there are other elements, however, which must be considered. For example, the vegetation of an undrained swamp is radically different from that of a similar moist situation on a river margin. Doubtless, certain conditions of drainage enter here to cause the difference in plant forms. Again, factors which are in a certain sense atmospheric often have a local influence. Plant forms found on the north side of a hill frequently differ from those on the south side because of possible differences in exposure to moisture and wind as well as to different exposure to the sun. All of these factors may, so to speak, be regarded as edaphic, since all are similar in being local rather than climatic. In general, the edaphic formations may be subdivided into coastal formations or societies, and inland formations or societies. The coastal societies may be destructive, as illustrated by the erosion of cliffs where they break down the rocks; or they may be constructive, as illustrated by swamps and beaches, where they act in building up the land. Inland societies may be divided into those associated with river activities and those that are not. In the life of a region all areas will ultimately be worked over by rivers, so that the stages which are not associated with rivers may be regarded as more or less temporal. The ultimate condition of a region is that of the base-level (the flat lowland resulting from completed erosion) toward which the other conditions are approaching, and which in a favorable climate will be covered in most instances by a mesophytic forest.
In a general presentation of the distribution of plants, the algæ, fungi, liverworts, and mosses may be dismissed with a few statements. These groups are found everywhere. It should be noted, however, that the brown algæ (kelps) reach their greatest development in the colder waters; and that the lichens and mosses are more abundantly displayed in temperate and arctic than in tropical regions.
Pteridophytes. In number the ferns so far exceed all other pteridophytes that they may be taken as representing the group. Their greatest display is in the tropics, where they often assume the tree form and develop extraordinarily large fronds. In temperate regions they are neither abundant nor conspicuous; while in arctic and alpine conditions they are hardly at all represented. It will be noted that the distribution of ferns is almost in direct contrast to that of mosses.
Gymnosperms. In presenting the distribution of gymnosperms, the three principal living groups must be considered separately. The cycads are strictly tropical forms, being distributed about equally between the Oriental and Occidental tropics, the genera for the most part being definitely restricted to certain regions. For example, while the genus Cycas ranges throughout the Oriental tropics, and Zamia throughout the Occidental tropics, one genus is strictly Australasian, two are African, two Mexican, and one is Cuban. The conifers form the largest group of gymnosperms, and their distribution contrasts sharply with that of the cycads, being entirely absent from the tropics and massed in the temperate regions, especially of the Northern Hemisphere. The broad tropical belt separating the conifers of the north and the south temperate regions is traversed in only two places, namely by a southern genus, Podocarpus, that reaches China and Japan through the East Indies, and by a northern genus. Libocedrus, that reaches into temperate South America by way of the Andes. By far the greatest conifer display, with respect to number of genera and of species, is found in the districts that border the Pacific Ocean, the chief areas being the China-Japan region, the Australasian region, and western North America. The most remarkable displays of endemic genera are in the China-Japan and the Australasian regions, the former containing eight such genera, and the latter five. The other regions of endemic genera are North America, with its redwood (Sequoia), and bald cypress (Taxodium), and South America, with a peculiar genus in the mountains of Patagonia. Throughout the north temperate regions the dominant and widely distributed genera are the pine (Pinus), juniper (Juniperus), fir (Abies), spruce (Picea), cypress (Cupressus), and larch (Larix); the order of citation indicating their relative abundance. There is also a remarkable pairing of western North America and eastern Asia in the display of certain genera, no less than six genera being common to these two regions and occurring nowhere else. The distribution of the conifers of the Southern Hemisphere is modified by the temperate conditions that occur in three great isolated areas. The dominant genus, Podocarpus, the ‘pine’ of the Southern Hemisphere, is the only one represented in all of these regions; but in the display of certain other genera there is a pairing of the continents, the Australasian region always being one member of the pair, and, with one exception, South America the other member. In conifers, therefore, there is much more in common between Australia and South America than between either of them and Africa. The Gnetales, constituting the third prominent group of gymnosperms, embrace three genera of very distinct characters and distribution. Ephedra occurs under both tropical and temperate conditions in the arid regions of Mediterranean Europe and adjacent Asia, and in arid parts of America; Gnetum ranges through the moist tropics of both hemispheres; while the monotypic Tumboa (Welwitschia) is narrowly restricted to certain extremely arid regions of Western Africa.
Angiosperms. The immense number of angiosperms, or true flowering plants, makes their distribution a difficult subject to present in a few statements. It must be understood, therefore, that the following presentation is very general, considering large masses of plants rather than species or even genera. It must be remembered, also, that in general the conditions of plant life are most favorable in the tropics, where there is apt to be massing; and that under Arctic conditions, which are very unfavorable, the vegetation is apt to be scant. The three prominent groups of angiosperms, viz. the monocotyledons and the two divisions of dicotyledons (Archichlamydeæ and Sympetalæ) must be considered separately.
Monocotyledons. The most conspicuous general facts in connection with the distribution of monocotyledons are as follows: Cosmopolitan groups. — Four great families (grass, sedge, lily, and iris), including almost 10,000 species or about one-half of the monocotyledons, are world-wide in their distribution. This means that they have succeeded in adapting themselves to every condition of soil and climate possible to flowering plants; and in this feature the grasses easily lead not only monocotyledons, but all other seed-bearing plants. Beyond the natural massing toward the tropics, the distribution of these four families is fairly uniform. Aquatic plants. — The monocotyledons include an unusual number of purely aquatic families, although among the other families the number of water-species is remarkable. A distinct water habit is associated with world-wide distribution, not merely of families, but often of species. For example, the common cat-tail rush (Typha) of the United States is also found in Europe, Asia, and Africa. But it must be noted that although these aquatic families of monocotyledons are world-wide in their distribution, their number of species is remarkably small, being about 180. In contrasting this with the 10,000 species of the four terrestrial families above referred to, it becomes evident that the relatively uniform conditions of aquatic life do not result in the evolution of species as do the extremely varied conditions of terrestrial life. Tropical Plants. — The remarkable massing of monocotyledonous families in the tropics, far in excess of any normal ratio of tropical increase, makes it evident that the monocotyledons as a whole must be considered the leading tropical group of angiosperms. This tropical massing is indicated by the fact that, excluding the ten families of world-wide distribution, twenty-three out of the thirty-three remaining families are tropical. It is even more strongly indicated by the fact that, excluding the 10,000 species that were noted above as belonging to the four worldwide families, all but a few hundred of the remaining 10,000 are tropical. The approximately equal distribution of tropical forms between the two hemispheres is noteworthy. This is true not merely in the number of families, but in genera and species as well. There are four or five families peculiar to the Orient, seven or eight peculiar to the Occident, and perhaps ten or eleven common to both. The palms may be taken as an illustration of the distribution of tropical monocotyledons. There are approximately a thousand species of palms, almost exactly divided between the two hemispheres, but not a single native species is common to the two. The genera are about 130, approximately, 75 of them being Oriental and 55 Occidental, the apparent inequality being accounted for by the more numerous monotypic genera in the Orient, due to the larger and more broken tropical area, but the two hemispheres have not a genus in common. The family is divided into great tribes, but not one of these tribes is common to the two hemispheres. These tribes are groups of genera on the way to becoming families, and by some are regarded as such already. The palms thus notably illustrate the effect of long separation in plant adaptation to continental diversities, temperate and arctic conditions long having cut off the connecting land between the tropical regions of the two hemispheres, which are believed to have been united in prehistoric times and between which plant and animal forms could migrate. The preponderance of epiphytic forms in the Occidental tropics is probably associated with the great development there of the rainy tropical forest. The two great epiphytic groups of flowering plants are bromelias and orchids, both monocotyledons, the former family being restricted to the Occidental tropics, and the latter much more abundant there than in the Orient. The general adaptation of the monocotyledons to tropical conditions is emphasized by the extreme dearth of monocotyledonous species in arctic and alpine regions, only the few species belonging to the four families of universal distribution being present. The feeble representation of monocotyledons in the Southern Hemisphere outside of the tropics is noteworthy. There are only four small families belonging exclusively to this region; and in Australia, a continent prolific of endemic forms among gymnosperms and dicotyledons, there is but a single endemic family of monocotyledons, containing only four species. A family of most peculiar distribution is the Stemonaceæ, comprising only seven or eight species, but serving to illustrate how a family once widely distributed may later occur only in widely separated areas. One genus, Stemona, occurs from the Himalayas to southern Australia; another, Croomia, in Florida, Georgia, and Japan; and the third, Stichneuron, in the East Indies.
Dicotyledons. There are approximately 80,000 species of dicotyledons, about equally divided between the two great groups Archichlamydeæ and Sympetalæ. These must be considered separately, since they have developed independently and are quite distinct in the general features of the geographical distribution.
Archichlamydeæ. The most conspicuous facts in connection with the distribution of the Archichlamydeæ are as follows: In the representation of tropical families, there is a notable pairing of continents, the American tropics usually being one member of the pair, and Asia or Africa the other. For example, the great alliance to which buttercups, mustards, poppies, laurels, etc., belong is represented by tropical forms chiefly in America and Asia, the omission of Africa being notable; the alliance to which geraniums, balsams, flaxes, rues, etc., belong has its tropical forms chiefly in America and Africa; while the melastomas are massed in Brazil, and the myrtles are common to South America and Australia. Thus the predominance of America appears in the display of tropical Archichlamydeæ. This becomes all the more clear from the statement that in America almost all the tropical and subtropical families are represented, and two very large families, the cactus and melastoma families, are found as natives exclusively in America. The Archichlamydeæ are also peculiar in not containing any terrestrial family of world-wide distribution, as do the monocotyledons and Sympetalæ; nor is there any distinctly boreal family as among the Sympetalæ. There have been developed among them certain very characteristic north temperate families, the smartweeds, pinks, crowfoots, mustards, saxifrages, roses, evening primroses, and umbellifers, all of which, however, have their representatives in tropical and boreal regions; but the display of these forms is not to be compared with the massing of the Sympetalæ in the temperate regions. While there is a much larger display of Archichlamydeæ in the north temperate than in the south temperate regions, two very large and characteristic families have been developed in the Southern Hemisphere, viz. the Proteaceæ, trees that belong to the same general alliance as do the oaks, elms, hickories, etc., of the Northern Hemisphere; and the Thymelæaceæ, characteristic of Australia and the Cape region of Africa, and represented in our American flora by the leatherwood (Dirca). The Archichlamydeæ contain most of the hard-wood trees, and it is interesting to note that the trees of the tropics and the temperate zones belong to different alliances. For example, the oaks, hickories, elms, beeches, birches, etc., belong to a great alliance characteristic of north temperate regions, while the laurel, belonging to an entirely different alliance, is the characteristic forest type of the tropics. By far the greatest family of the Archichlamydeæ, in point of numbers and general success, is the pea family (Leguminosæ), represented by about 7000 species. Although it seems to be abundantly displayed in the temperate regions, it is far more extensively represented in the tropics, being the greatest tropical family of the group. The lianas or climbers are chiefly represented among Archichlamydeæ by such forms as the grape-vines and their allies. Naturally, they are chiefly displayed in tropical forests and are most largely developed in the moist forests of South America, as are the epiphytic orchids and bromelias among the monocotyledons. Their next largest display is in Africa.
Sympetalæ. The Sympetalæ are the ranking plants of the plant kingdom and are also those most recently evolved. While they include a number of shrubs and trees in the tropics, they are by no means so extensively represented in temperate regions by shrubs and trees as are the Archichlamydeæ. The prominent facts of their distribution are as follows: the earlier groups of angiosperms (Archichlamydeæ and monocotyledons) were massed in the tropics, the temperate and boreal regions being relatively poor in true flowering plants. The Sympetalæ, therefore, while largely represented in the tropics, found the temperate and boreal regions comparatively free for occupation, and it is in these regions that they have become peculiarly dominant. This does not mean that there are not more Sympetalæ in the tropics than in the temperate regions, for the proportion of species in favor of the former is about three to two; but it does mean that this is about the normal ratio of tropical increase, which is far exceeded by the other groups. Accordingly, of the Sympetalæ alone can it be said that their distribution throughout tropical and temperate regions is about uniform. The families of Sympetalæ are not only better defined structurally than those of the Archichlamydeæ, but their geographical distribution is much more definite. This follows the fact that they are more highly specialized and more definitely related to environment than are the Archichlamydeæ. Among the Sympetalæ, three families, the plantains, mints, and composites, have a world-wide distribution. The Compositæ, as the ranking family of the plant kingdom, and also the youngest and largest, deserves special mention. It includes at least 12,000 species, which, although found everywhere, are most numerous in the temperate regions, where they constitute from one-tenth to one-seventh of all of the seed-bearing plants of every flora. They are especially characteristic of temperate America, where asters, goldenrods, sunflowers, etc., abound. Two great north temperate families, the figworts and the borages, have also been developed; and the heaths form a family characteristic of boreal conditions.
The impress a peculiar region may have upon the general aspect of its vegetation, resulting in the assumption of the same general appearance by plants of the most diverse relationships, is illustrated by the fact that many sympetalous families of South America are characterized by a heath-like habit, the region being known as one of the heath-like plants. There is no such notable continental pairing as is shown by the Archichlamydeæ and the monocotyledons, due probably to the fact that extensive forests and boreal distribution permit free intermingling of continental flora. Among tropical Sympetalæ, however, the pairing is somewhat evident, and in tropical display America is distinctly dominant. It is also worthy of note that there is a remarkable paucity of aquatic plants among Sympetalæ, as compared with Archichlamydeæ and with monocotyledons. This seems to be related to the fact that the two latter groups had already become well established in fresh and brackish waters, while it was the temperate and boreal land surface that was especially free for occupation by the Sympetalæ.
Conclusion. Through the lapse of time since the advent of plant life upon the earth, the meteorological factors, wind, moisture, heat, and light, have been at work as plant distributors. At divers times and in many parts of the world their work has been interrupted, assisted, or rendered nugatory by geological agents — glaciers, especially of the ice age, which have driven vegetation before them and exiled many plants from their pristine homes; seismic upheavals or sinkings, the former interposing tall mountain ranges, the latter wide oceans, beyond which certain species could not migrate, and which to a greater or less degree isolated these species, sometimes genera, families, and even tribes. The efforts of these factors and agents have been and are assisted by man, by other animals, and by plants; the first either wittingly carrying useful or ornamental species from country to country, or more often unwittingly introducing useless or even noxious ones; the second in a more limited range transporting seed either in or on their bodies; the third breaking down and disintegrating rock, becoming incorporated with the detritus and thus affording a habitation to a succeeding plant society. Far from having ceased their labors, these meteorological factors and geological agents, working in conjunction or in conflict, still profoundly influence the ceaseless change in the distribution of plants.
Bibliography. Contejean: Géographie botanique (Paris, 1881); Drude, Handbuch der Pflanzengeographie (Stuttgart, 1890); id., Manuel de géographie botanique (Paris, 1897); De Candolle, Géographie botanique raisonnée (Paris, 1855); Engler, Versuch einer Entwicklungsgeschichte der extratropischen Florengebiete (Leipzig, 1879); Engler and Drude, Die Vegetation der Erde (first 4 vols., Leipzig, 1896-1901); Grisebach, Vegetation der Erde (Leipzig, 1884); Schimper, Pflanzengeographie auf physiologischer Grundlage (Jena, 1898); Wallace, Island Life (New York, 1895); Warming, Oekologische Pflanzengeographie (Berlin, 1896). The ecological aspects of distribution are touched upon under the following heads: Ecology; Alpine Plants; Arctic Plants; Benthos; Clay Plants; Cliff Plants; Cosmopolites; Desert; Dune Vegetation; Duration; Endemism; Forest; Grasslands; Halophytes; Heath; Hydrophytes; Meadow; Mesophytes; Prairie; Rock Plants; Ruderal Plants; Savannas; Swamp; Thicket; Tundra; Xerophytes. Details of floristic phytogeography may be found in the articles on various genera and species of plants mentioned above.