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Geographical Distribution of Plants

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GEOGRAPHICAL DISTRIBUTION OF PLANTS, also called GEOGRAPHICAL BOTANY, and PHYTOGEOGRAPFIY, is that branch of botany which treats of the geographic distri bution of plants, and connects botany with physical geography. A knowledge of facts belonging to it has been gradually accumulating ever since the science of botany began to be studied, but its importance was little understood until very recent times. Hum boldt may be said to have elevated it to the rank which it now holds as a distinct branch of science. It was indeed impossible for botany to be studied without attention being arrested by the great diversity of the productions of different countries, and even of those not very dissimilar in climate. But it was long ere important generalizations were attempted; and a large accumulation of particular facts was in the first place necessary. Even to this day, the deficiency of information concerning the botany of wide regions Is painfully felt.

Every climate has plants particularly adapted to it. The plants of the tropics will not grow in frigid, nor generally even in temperate regions; as little will arctic or sub arctic plants endure the heat of the torrid zone. And as the climate changes with the elevation above the level of the sea, the mountains of tropical countries have a flora analogous to that'of the temperate, and even of the frigid zones. The vegetation of every place bears a relation to its mean annual Aemperature. But owing to the peculi arities of different plants, it bears also important relations to the mean temperatures of the summer and winter months; and thus great diversities are found not only in the indigenous vegetation of countries very similar in their mean annual temperature, but even in their suitableness for plants which may be introduced into them by man. or is temperature the only thing of importance in relations of climate to vegetation. Moisture must be ranked next to it. Some plants flourish only in a dry, and some only in a humid atmosphere. The flora of the very dry regions of Africa and of Australia is almost as notably different from that of moist countries in similar latitudes as that of the temperature from that of the torrid zone. Nor is the difference merely in the species of plants produced, but in the whole character of the vegetation, which very much consists either of succulent plants with thick epidermis, or of with hard and dry foliage.iffifit -'tirialized by Microsoft 6 Much depends also on soil. Sandy soils have their peculiar vegetation; peat is also favorable to the growth of many plants which are seldom or never to be found in ally other soil. The chemical constitution of soils determines to some extent the character of their flora; and therefore certain plants are almost exclusively to be found in districts where certain rocks prevail, and a relation is established between botany and geology. i.imestone districts, for example, have a flora differing to a certain extent from other districts even of the same vicinity. Some British plants are almost entirely limited to the chalk districts. The other physical qualities of the soil are not unimportant. Light soils are suitable to plants with fine roots divided into many delicate fibrils, as heaths, which will scarcely grow in stiff clay.

Some groups of plants are almost entirely limited to peculiar situations, as the alga and other smaller groups of aquatic plants. Some are exclusively tropical; others are only found iu the colder parts of the world; and if any of the group occur within the tropics, it is on mountains of considerable elevation. But besides all this, and apart

from all obvious differences of climate, soil, etc., some groups of plants, and these often containing many species, are only or chiefly found in certain parts of the Thus the cactacem are exclusively American; whilst of the numerous species of heath (erica), not one is indigenous to America, although many other plants of the heath family (ericea) are so. Sometimes the plants which chiefly abound in one part of the world seem to be replaced by other but similar species, sometimes by those of another group, in another part of the world, with similar physical characteristics. Thus mesembryacca and crassulacea seem in some countries to occupy the place of the American cactacem, the black-fruited crow-berry (empetrum) of the northern parts of the world finds a representative in a red-fruited species, extremely similar, in the southern parts of South America. Of many groups which chiefly belong to certain climates or certain parts of the world, there are yet species which wander, as it were, into very different climates or remote parts of the world; these species being often, however, unknown where the other species of the group abound. Thus the common periwinkle is a northern wanderer of a family mostly tropical. Some groups,are common to parts of the world widely remote, and their prevalende is characteristic of these parts, as rhodo d,enclrons and magnoliacea of North America and of the mountainous districts of the East Indies, although the American and the Asiatic species are not the same. Some species are believed to exist only within a very narrow range; others are very widely diffused. A few are found in the colder parts both of the northern and southern hemi spheres, and also on the intervening tropical mountains. Some groups also, containing many species, are confined to particular regions, as the important cinchcmce to a dis trict of the Andes, and the calceolaria to higher parts of the same mountain chain.— Marine vegetation, like terrestrial vegetation, has species and groups that are very generally diffused, and others confined to particular regions.

The geographical limits of species have no doubt been in many instances uninten tionally modified by man, and the extent of this modification it is extremely difficult to ascertain. There is enough; however, in the known facts of botanical geography, evidently independent of such agency, to afford foundation for interesting and impor taut speculations, of which some notice will be taken under the head SPECIES.

Many of the principal facts of botanical geography will be found stated in the articles EUROPE., ASIA, AMERICA, and AUSTRALIA, and in articles on natural orders and genera of plants. Schouw and Meyen are among the chief authorities on this subject; and the former has endeavored to divide the earth into 25 botanical regions, characterized by the prevalence of particular forms of vegetation. The reader will find much informa tion on botanical geography, collected in a very accessible form, in the Physical Atlas of Johnston and Berghaus.—Henfrey's Vegetation of Europe (Van Voorst, London, 1852) may be consulted with advantage; and the Cybele Britannica, and Geography of British Plants, of Mr. II. treating of the geographic distribution of plants in the British isles, are unrivaled among works of their kind.