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Influence of Temperature on Vegetation - Vegetation varies with the Latitude and Height above the Sea- - Geographical Distribution of Land Plants Distribution of Marine Plants Corallines, Shell-fish, Reptiles, Insects, Birds, and Quadrupeds — Varieties of Mankind, yet identity of Species.
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 light and 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 150 or 200 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 vine and olive; 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 shrubs, till a verdant carpet of mosses and lichens, enamelled with flowers, exhibits the last sign of vegetable life during the short but fervid summers at the polar regions. Such is the effect of cold and diminished light on the vegetable kingdom, that the number of species growing under the equator and in the northern latitudes of 45° and 68° are in the proportion of the numbers 12, 4, and 1. Notwithstanding the remarkable difference between a tropical and polar flora, light and moisture seem to be almost the only requisites for vegetation, since neither heat, cold, nor even comparative darkness, absolutely 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 almost void of light, though generally blanched and feeble. The ocean teems with vegetation. The snow itself not only produces a red lichen, 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 these inhospitable climes against the piercing winds that sweep over fields of everlasting ice. Those undaunted mariners narrate that under this cold defence plants spring up, dissolve the snow a few inches round, and the part above, being again quickly frozen into a transparent sheet of ice, admits the sun's rays, which warm and cherish the plants in this natural hothouse, till the returning summer renders such protection unnecessary.
The chemical action of light is, however, absolutely requisite for the growth of plants which derive their principal nourishment from the atmosphere. They consume the carbonic acid gas, nitrogen, aqueous vapour, and ammonia it contains; but it is the chemical agency of light that enables them to absorb, decompose, and consolidate these substances into wood, leaves, flowers, and fruit. The atmosphere would soon be deprived of these elements of vegetable life were they not perpetually supplied by the animal creation; while, in return, plants decompose the moisture they imbibe, and, having assimilated the carbonic acid gas, they exhale oxygen for the maintenance of the animated creation, and thus preserve a just equilibrium. Hence it is the combined and powerful influences of the whole solar beams that give such brilliancy to the tropical forests, while, with their decreasing energy in the higher latitudes, vegetation becomes less vigorous. On that account it is vain to expect that the fruit and flowers raised in our hothouses can ever have the flavour, perfume, or colouring equal to that which they acquire from the vivid light of their native skies.
By far the greater number of the known species of plants are indigenous in equinoctial America; Europe contains about half the number; Asia, with its islands, somewhat less than Europe; Australia, with the islands in the Pacific, still less; and in Africa there are fewer known vegetable productions than in any
part of the globe of equal extent, for that rich and luxuriant region discovered by Dr. Livingstone has yet to be explored botanically. 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. Some exceptions to this, however, are to be met with in the jungles of the Deccan, &c. 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 heat 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 60th degree 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 exuberance of vegetation; nor will it ripen generally above the height of 12,000 feet; in Tibet it ripens at a still greater elevation. Colonel Sykes states that in the Deccan wheat thrives as low as 1800 feet above the sea. The best wines are produced between the 30th and 45th degrees of north latitude. With regard to the vegetable kingdom, elevation is equivalent to latitude as far as temperature is concerned. ascending the mountains of the torrid zone, the richness of the tropical vegetation diminishes with the height; a succession of plants similar to, though not identical with, those found in latitudes of corresponding mean temperature takes place; the lofty forests by degrees lose their splendour; stunted shrubs succeed; till at last the progress of the lichen is checked by perpetual snow. On the volcano of Teneriffe there are five successive zones, each producing distinct families of plants. The first is the region of vines, the next that of laurels; these are followed by the region of pines, of Ericas or heaths, of grass; the whole covering the declivity of the peak through an extent of 11,200 feet of perpendicular height.
Near the equator oaks flourish at the height of 9200 feet above the sea; and, on the lofty range of the Himalaya, the primula, the convallaria, and the veronica flower, but not the primrose, the lily of the valley, or the veronica, which adorn our meadows; for, although the herbarium collected by Moorcroft, on his route from Neetee to Daba and Gartope in Chinese Tar
tary, at elevations as high or even higher than Mont Blanc, abound 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 difference of 25o 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 peculiarly its own. The whole globe has been divided by physical geographers into various 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 the continents almost unite; but, in approaching the south, the floras of these three great division's of the globe differ more and more even in the same parallels of latitude, which shows that temperature alone is not the cause of the almost complete diversity of species that everywhere prevails. The floras of China, Siberia, Tartary, of the European district including central Europe and the coast of the Mediterranean, and the Oriental region comprising the countries round the Black and Caspian Seas, all differ in specific character. Only twenty-four species were found by MM. Humboldt and Bonpland in Equinoctial America identical with those of the Old World; and Dr. Robert Brown not only found that a peculiar vegetation exists in Australia between the 33rd and 35th parallels of south latitude, but that at the eastern and western extremities of these parallels not one species is common to both, and that certain genera also are almost entirely confined to these spots. The number of species common to Australia and Europe are only 166 out of 4100, and probably some of these have been conveyed thither by the colonists; but the greater part of that continent is still unexplored. However, this proportion exceeds what has
hitherto been observed in southern Africa, and, from what has been already stated, the proportion of European species in Equinoctial America is still less.
Islands partake of the vegetation of the nearest continents; but, when very remote from land, their floras are altogether peculiar. The Aleutian Islands, extending between Asia and America, partake of the vegetation of the northern parts of both continents, and may have served as a chain of communication. In Madeira and Teneriffe, the plants of Portugal, Spain, the Azores, and of the northern coast of Africa, are found; and the Canaries contain a great number of plants belonging to the African coast. But each of these islands possesses a flora that exists nowhere else; and St. Helena, standing alone in the midst of the Atlantic Ocean, produces only two or three species of plants recognised as belonging to any other part of the world.
It appears from the investigations of M. de Humboldt that between the tropics the plants, such as grasses and palms, which have only one seed-lobe, are to the tribe which have two seedlobes, like most of the European species, in the proportion of one to four; in the temperate zones they are as one to six; and in the Arctic regions, where mosses and lichens, which form the lowest order of the vegetable creation, abound, the proportion is as one to two. Annuals with one and two seed-lobes, in the temperate zones, amount to one-sixth of the whole, omitting the cryptogamia (N. 218); in the torrid zone they scarcely form one-twentieth, and in Lapland one-thirtieth part. In approaching the equator the ligneous exceed the number of herbaceous plants; in America there are 120 different species of forest trees, whereas in the same latitudes in Europe only 34 are to be found.
Similar laws regulate the distribution of marine plants. Groups of algæ, or marine plants, affect particular temperatures or zones of latitude and different depths, though some few genera prevail throughout the ocean. The polar Atlantic basin to the 40th degree of north latitude presents a well-defined vegetation. The West India seas, including the Gulf of Mexico, the eastern coast of South America, the Indian Ocean and its gulfs, the shores of New Holland, and the neighbouring islands, have each their distinct species. The Mediterranean possesses a vegetation peculiar to itself, extending to the Black Sea; and the species of marine plants on the coast of Syria and in the port of Alexandria differ