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perature of the ethereal regions, yet the summer heat in these high latitudes is insupportable.
THE gradual decrease of temperature in the air and in the earth, from the equator to the poles, is clearly indicated by its influence on vegetation. In the valleys of the torrid zone, where the mean annual temperature is very high, and where there is abundance of moisture, nature adorns the soil with all the luxuriance of perpetual summer. The palm, the bombax ceiba, and a variety of magnificent trees, tower to the height of a hundred and fifty or two hundred feet above the banana, the bamboo, the arborescent fern, and numberless other tropical productions, so interlaced by creeping and parasitical plants, as often to present an impenetrable barrier. But the richness of vegetation gradually diminishes with the temperature; the splendour of the tropical forest is succeeded by the regions of the olive and vine; these again yield to the verdant meadows of more temperate climes; then follow the birch and the pine, which probably owe their existence in very high latitudes more to the warmth of the soil than to that of the air; but even these enduring plants become dwarfish, stunted shrubs, till a verdant carpet of mosses and lichens, enamelled with flowers, ex
hibits the last signs of vegetable life during the short but fervent summers at the polar regions. Such is the effect of cold on the vegetable kingdom, that the numbers of species growing under the line and in the northern latitudes of 45° and 68°, are in the proportion of the numbers 12, 4, and 1. But notwithstanding the remarkable difference between a tropical and polar Flora, moisture seems to be almost the only requisite for vegetation, since neither heat, cold, nor even darkness destroy the fertility of nature; in salt plains and sandy deserts alone hopeless barrenness prevails. Plants grow on the borders of hot springs -they form the oases, wherever moisture exists, among the burning sands of Africa-they are found in caverns void of light, though generally blanched and feeble-the ocean teems with vegetation-the snow itself not only produces a red alga, discovered by Saussure in the frozen declivities of the Alps, found in abundance by the author crossing the Col de Bonhomme from Savoy to Piedmont, and by the polar navigators in the arctic regions, but it affords shelter to the productions of those inhospitable climes, against the piercing winds that sweep over fields of everlasting ice. Those interesting mariners narrate that, under this cold defence, plants spring up, dissolve the snow a few inches round, and
that the part above, being again quickly frozen into a transparent sheet of ice, admits the sun's rays, which warm and cherish the plant in this natural hot-house, till the returning summer renders such protection unnecessary.
By far the greater part of the hundred and ten thousand known species of plants are indigenous in equinoctial America; Europe contains about half the number; Asia with its islands somewhat less than Europe; New Holland, with the islands in the Pacific, still less; and in Africa there are fewer vegetable productions than in any part of the globe, of equal extent. Very few social plants, such as grasses and heaths that cover large tracts of land, are to be found between the tropics, except on the sea coasts, and elevated plains. In the equatorial regions, where the heat is always great, the distribution of plants depends upon the mean annual temperature; whereas in temperate zones the distribution is regulated in some degree by the summer heat. Some plants require a gentle warmth of long continuance, others flourish most where the extremes of heat and cold are greater. The range of wheat is very great: it may be cultivated as far north as the 60° of latitude, but in the torrid zone, it will seldom form an ear below an elevation of 4500 feet above the level of the sea from the exuberance of vegetation; nor will it ripen
above the height of 10800 feet, though much depends upon local circumstances. The best wines are produced between the 30° and 45° of north latitude. But with regard to the vegetable kingdom, elevation is equivalent to latitude, as far as temperature is concerned. In ascending the mountains of the torrid zone, the richness of the tropical vegetation diminishes with the height; a succession of plants similar, though not identical with those found in latitudes of corresponding mean temperature takes place; the lofty forests lose by degrees their splendour, stunted shrubs succeed, till at last the progress of the lichen is checked by eternal snow. On the volcano of Teneriffe, there are five successive zones, each producing a distinct race of plants. The first is the region of vines, the next that of laurels, these are followed by the districts of pines, of mountain broom, and of grass; the whole covering the declivity of the peak through an extent of 11200 feet of perpendicular height.
Near the equator the oak flourishes at the height of 9200 feet above the level of the sea, and on the lofty range of the Hymalaya the primula, the convallaria, and the veronica blossom, but not the primrose, the lily of the valley, or the veronica which adorn our meadows; for although the herbarium collected by Mr. Moorcroft on his route
from Neetee to Daba and Garlope in Chinese Tartary, at elevations as high or even higher than Montblanc, abounds in Alpine and European genera, the species are universally different, with the single exception of the rhodiola rosea, which is identical with the species that blooms in Scotland. It is not in this instance alone that similarity of climate obtains without identity of productions; throughout the whole globe, a certain analogy both of structure and appearance is frequently discovered between plants under corresponding circumstances, which are yet specifically different. It is even said, that a distance of 25° of latitude occasions a total change not only of vegetable productions, but of organised beings. Certain it is, that each separate region both of land and water, from the frozen shores of the polar circles, to the burning regions of the torrid zone, possesses a flora of species peculiarly its own. The whole globe has been divided by botanical geographers into twentyseven botanical districts, differing almost entirely in their specific vegetable productions; the limits of which are most decided when they are separated by a wide expanse of ocean, mountain chains, sandy deserts, salt plains, or internal seas. A considerable number of plants are common to the northern regions of Asia, Europe, and America, where these continents almost unite; but in ap