The agents which exercise a decided influence on vegetation are light, heat, water, and earth.

The concurrent influence of all these agents, in a greater or less degree, is absolutely required for the perfection of vegetable products; and according as some of them exist in excess, or in diminished quantity, is the functional operation of the vegetable organs injured or destroyed. Different plants require these agents in very different degrees, and hence the distribution of plants over the face of the globe; some flourishing near the snow line of the mountains, or near the limits of perpetual snow, north or south, while others can exist only under the influence of tropical heat. Some grow amidst the sandy deserts, others only immersed in water. Some grow in the caverns of the earth while others must have the direct rays of the sun. Some require a rich soil, others grow suspended in the air. From this adaptation of vegetation to every variety of influence, the earth is covered with verdure; from the perpetual snows of the mountains or the arctics to the equator, each position giving existence to its appropriate flora.