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.