CHAPTER VII.

GEOGRAPHICAL DISTRIBUTION OF PLANTS.

The living herbs, profusely wild,

O'er all the deep green earth, beyond the power

Of botanists to number up their tribes.-THOMSON.

MULTITUDES of different species of plants are spread over the surface of the earth; not indiscriminately, and presenting the appearance of having been scattered at random, but each and all being especially adapted for their natural abodes, or, in other terms, their natural stations and habitations.

The natural station of a plant is determined by the nature of the surface, or soil. Thus, some species confine themselves entirely to water, these being again subdivided into those of the marsh, the lake, the river, and the ocean; other species belong to the land, and of these we find some peculiar to valleys, others to plains, and others to mountains, the latter being distinguished by the designation of alpine plants. Some, again, affect a clayey soil, some a chalky, and some a sandy soil; whilst others will thrive only in soils impregnated with soda and muriatic salt. Some plants are parasitical, taking root in the stems and branches of other plants. We also find that some plants flourish only when exposed to the action of strong light, whilst others prefer the shade, and others again spring up even when altogether excluded from light, being formed for tenanting caverns, and other dark recesses of the earth.

The natural habitation of a plant is dependent on the climate which is best adapted to its perfect developement. Thus we find that some species require the hottest climates, others delight in mild and temperate ones, others thrive only in the midst of ice and frost. A large proportion require a constantly humid atmosphere; several succeed in a dry air; though the greater number are equally averse to the extremes of dryness and moisture. Excess of heat, cold, or

drought, are the only obstacles to the growth of plants. The result of this remarkable adaptation of plants to particular climates, situations, and soils, is, not only that nearly the whole earth is clothed with vegetable life, but also that an almost endless variety exists among these productions, and we are almost tempted to exclaim with the poet,

The earth was made so various, that the mind

Of desultory man, studious of change

And pleased with novelty, might be indulged.

But although some plants, such for instance as the common heath, or ling, affect a sandy soil and temperate climate, we by no means find that the species is met with in all similarly circumstanced localities; on the contrary, it usually happens that most species, and some genera, are confined to certain districts, more or less extended. Some species, indeed, are limited to the narrowest bounds; thus, a species of marjoram (Origanum Tournefortii), which was discovered by Tournefort, in the year 1700, in the little island of Amorgos, growing upon one rock only, was observed eighty years afterwards on the self-same rock, but has never been met with in any other place. This, it is true, is an extreme case, and most species of plants have a wider range, but we find that almost every country, or rather perhaps, every natural division of the globe, possesses a flora, or assemblage of indigenous plants, peculiar to itself. A considerable difference is even observable between the native plants of the eastern and western districts of Great Britain; and although many species are common to these islands and to some parts of Europe, the number peculiar to our shores is by no means inconsiderable: we shall therefore readily suppose that the plants in distant regions will differ greatly; and accordingly we find that the Old and New World contain a very small number of identical species.

It must be understood that we are speaking of species, and not of genera; for although it is true that some genera, or kinds of plants, may be peculiar to each great division of the globe, this is not generally the case. We, however,

frequently find a preponderance of some particular kinds in particular regions. Thus, the saxifrages and mosses are more abundant in the Arctic regions than any other genera, and also more so in those districts, than in any other portion of the globe, Mexico, and the adjacent countries of the New Continent, are characterized by the preponderance of cacti; Southern Africa is marked by that of plants of the mesembryanthemum tribe, &c.

But though it thus appears that plants have their native regions, from whence they have been more or less spread by natural means, or by the agency of man, the geographical distribution of plants over the earth's surface may be considered as mainly dependent on temperature, modified by the moisture or dryness of the atmosphere.

The great influence temperature exercises on vegetation is shown in the different character of the floras of cold, temperate, and hot climates. Vegetation within the tropics fills the European traveller with amazement, by the majesty and vigour of its aspect. The towering height of the trees, the circumference of their stems, the richness and variety of their foliage, as well as the bright and finely contrasted colours of their blossom, all combine to give splendour to the scene; whilst the graceful foliage, and the columnar stems of the palms and arborescent ferns, give it a peculiar and striking character: and the aromatic plants, the far greater number of which are natives of these regions, fill the air with their perfumes. There, also, gigantic climbing plants tower to the top of the loftiest trees; and magnificent herbs grow to the height of the trees of our orchards, with flowers and foliage not less pre-eminent in their dimensions. As an instance of the gigantic vegetation of tropical regions we may mention the corypha umbraculifera, or fan palm, an East Indian species, which has leaves in the form of an umbrella, eighteen feet across. The aristolochia, a plant growing on the River Magdalena, bears flowers of such large dimensions, that, according to De Humboldt, they serve the children for hats.