Water as a Factor.— The simplest plants grow in water, and are in every way dependent upon it. Terrestrial plants — for example, prac tically all flowering plants and ferns—have adapted themselves to two media, air and water, and their dependence upon water is not so marked. The active root-hairs are still really aauatic, and must always be in contact with an adequate supply of water. The stems and leaves are aerial, but their behavior and form are largely determined by the water in the air, that is, the humidity. The water-supplv is used by the root-hairs, while water-loss is the result of evaporation from the surface of the leaves. The excess of supply over loss will determine the form of the plant; it is evident that plants can notgrow where the loss exceeds the supply. The balance between these is so nice that plants grow well only where it is maintained. The most luxuriant vegetation is the forest, where both supply and loss of water are great. An excess of supply over loss is almost as cer tain to produce stunting and dwarfing, as seen in the plants of ponds and marshes, as is an excessive water-loss, which is the condition typical of deserts and high mountain peaks. The total amount of water present in the soil will vary with the rainfall, and with the behavior of surface and underground streams. Much of the rainfall runs off the surface, while a part of it sinks below the roots and is carried off by underground drainage. What is left remains in contact with the soil-particles as a thin film, and it is this which is absorbed by the root-hairs. The pull exerted by the absorptive power of the hair upon the water film is greater than the attraction of the soil particle, and the latter loses its water. If this continues, however, the ratio lessens, and in soils that are drying out the particles hold the water-films with increasing tenacity. In consequence, plants will wilt and die in soils that still contain water. Loose soils, such as sand and gravel, will give up all but 05 to 1 per cent of the water-content, while compact days retain as high as 12 to 30 per cent. An excess of salts in the soil, or a lack of air usu ally produces a similar effect. They decrease the absorptive power of the root-baira and lead to the production in marshes and bogs of plants showing the effects of an insufficient water supply.
Air Humidity.— The humidity of the air exerts a direct control upon the amount of water evaporated from the leaves. It is evident that the water-loss of the plant will be slight where the amount of moisture in the air is great, and that the evaporation will be great where the air is dry. This effect of humidity is so marked that plants which grow in moist climates often have structures designed to increase water-loss, while those living in desert-like places regularly protect themselves by thickening their epidermis, and decreasing the amount of surface exposed. The relative humidity of lowlands and sea coasts, especially in the tropics, is above 80 per cent; in deserts and upon high mountains it is rarely more than 30 per cent, and often falls below 15 per cent.
Influence of Light.—The amount of light present in a habitat influences directly the food making activities of the plant. The green col oring matter of plants, the chlorophyll, is formed readily and abundantly only in the light, and the combination of crude materials, water, carbon dioxide, and salts into foods available for the protoplasm can occur only in the pres ence of this pigment. Light thus bears a pe
culiar relation to the nutrition and growth of plants, and in a large degree determines their form and size. Sunlight• produces vigorous, stocky stems, and thick leaves, as a rule, while i plants grown in the shade have tall slender stems, and broad thin leaves. Plants that occur under ground, in caves, or grow within organisms, do not develop chlorophyll, and without excep tion belong to the flowerless forms known as fungi. The intensity of the light varies through out the day and year; it is greater in the tropics than at the poles, and on the tops of high peaks than at their bases. In forests and thickets the light is often very diffuse, varying from .01 to .003, summer sunlight being 1.
Temperature.— This is directly concerned with the nutrition and growth of plants. Heat is necessary for the germination of seeds, and for the sprouting of bulbs and tubers. It must be present in a considerable degree for the food making activities of plants, and upon it in a large part depends the size of individuals, and the luxuriance of vegetation. The growing period is the season during which temperatures favorable to plant growth prevail ; the length of this period determines in great measure the native vegetation of a country, and the culti vated plants which can be grown there. In directly, temperature exerts a pronounced effect upon die form of plants, by decreasing the moisture of the air, and thus increasing the water-loss. In tropical and subtropical deserts this indirect action of heat is a predominant factor. Soil temperatures are of much less importance, though they have much to do with germination and the activity of underground parts.
Wind Effects.— Wind influences plant life both directly and indirectly. Its mechanical action is marked in regions where forceful and constant winds prevail, notably seacoasts and high mountain peaks. Shrubs and trees become bent or prostrate, and their branches are de veloped almost wholly on the leeward side. Over open stretches where strong winds prevail, for example prairies and steppes, the plant forms are largely grasses and grass-like plants, which are not easily torn or whipped by the wind. As is well-known, winds play an ex tremely important part in carrying the pollen of trees and grasses, and in scattering seeds and spores. Like heat, wind decreases the humidity of the air, and correspondingly increases the evaporation from leaf-surfaces. It does this by removing the more or less saturated air in contact with the plant, replacing it with air con taining less moisture. This action is character istic of the dry southwest winds in the corn belt, which rapidly carry away the moisture of the leaves, causing the latter to curl, thus de creasing the surface and affording some measure of protection. The stunted forest vegetation of arctic and alpine timber-lines is largely due to the drying action of almost constant winds. The direction, force, and duration of the wind must all be taken into account in the study of vegetation.