VEGETATION distribution of vegetation over the surface of the earth is primarily determined by the nature of the climate and the character of the soil. The climatic factors—especially humidity and temperature—are the more important. The amount of water in the soil and in the atmosphere, the periods of the year in which precipitation takes place, and the conditions under which moisture is absorbed by plants, affect their structure in a marked degree. Those which grow in a region where the humidity is high differ in many respects from those which grow where it is low. But an important distinction between physical and physiological humidity must at once be made. Certain conditions may prevent the absorp tion of moisture by plants even when it is present in abundance. For example, when the temperature is low, water cannot be so easily absorbed by vegetation as when it is high, and for the purposes of plant life, therefore, the colder parts of the world are relatively dry. Again, the presence of humus acids renders the absorption of water by plants difficult, and districts where these acids occur in the soil are physiologically dry. Those plants which flourish in regions physiologically humid are known as hygrophytes, and they are fitted both by their internal structure and their external parts, such as their leaves which are broad and relatively thin, to get rid of excess moisture. Xerophytes, on the other hand, grow in regions which are physiologically dry, and their structure is such as to retain what moisture there is. Their transpiring surface is generally limited, and their leaves are, in some cases, thick and leathery, in others, needle-shaped, and in yet others, so adjusted as to lie parallel to the rays of the sun. Certain plants known as tropophytes are alternately hygrophilous and xerophilous. At the beginning of the physiologically dry season they drop their xerophilous parts, such as their leaves, to assume them later when climatic conditions become more favourable.
If humidity is the most important factor in determining the structure of plants it is heat to which they owe their growth.
Every plant lives between two temperatures, above the upper of which or below the lower, it cannot continue to exist for long. Each function of the plant, moreover, such as germinating, flowering, and seeding, has its own upper and lower zero points, between which that function can alone take place.
The effect of water upon plant life is therefore very different from that of heat. Upon humidity depends the type of vegetation —whether it be woodland or grassland, or whether desert conditions prevail. The amount of moisture necessary for woodland is, as a general rule, greater than that required for grassland and the seasonal distribution is different ; while the amount required for either woodland or grassland is greater in tropical than in temperate regions. According to Schimper, the essential characteristics of a good woodland climate are " a warm vegetative season, a continuously moist subsoil, and calm air, especially in winter." For grassland, on the other hand, the nature of the subsoil is of little importance, but it is absolutely necessary that there should be " frequent, even if weak atmospheric precipitation during the vegetative season, so that the superficial soil is kept in a moist condition " ; and further, a moderate degree of heat during the same period. Where the best woodland conditions prevail, the
trees are hygrophilous ; in a less favourable environment they tend to be xerophilous.
The part played by the soil is generally of secondary importance ; but, as its water-holding capacity is affected by its physical struc ture, that, along with its chemical composition, enables it to incline the balance towards woodland or towards grassland when the other factors are evenly matched.
These two great types of vegetation—woodland and grassland— are each capable of considerable subdivision. In those tropical regions where the rain falls at all seasons of the year the forest is evergreen, and is noted, not only for its luxuriance, but for its great wealth of shrubs, mosses, Hanes, and epiphytes. When there is a well-marked dry season, as in the monsoon area, the vegetation, though dense, is tropophilous, the trees shedding the most of their leaves at the beginning of the dry period. Savanna forests, in which the trees are xerophilous and interspersed with grass, occupy considerable areas within the tropics where the rainfall is relatively low ; while, under the least favourable conditions for tree growth in tropical regions, thorn-forests, such as the Caatinga of Brazil, cover the ground. In the warm temperate belts, an evergreen hygro philous forest of considerable density is found in those regions which have rain at all seasons of the year ; when the summers are moist and the winters dry, this passes into savanna forest and thorn forest. On the other hand, when the winters are moist and the summers dry, as round the Mediterranean, a sclerophyllous wood land with thick leathery leaves prevails. In the cool temperate belts there are two great types of woodland, the summer green forest, which is tropophilous, and the evergreen coniferous forest, the structure of which is xerophilous. In tropical regions and in the warm temperature belts, grassland is generally of the savanna type; the grasses are tall and grow in tufts, while the landscape is broken by trees and shrubs. Under unfavourable conditions savanna passes into steppe. In the cool temperate belts grass land varies from meadow to steppe. In meadow the grasses are hygrophilous and grow in close formation ; in steppe they are xerophilous and the formation is open.
In all these cases local conditions of soil and climate may cause variations in the prevailing type of vegetation.
It is difficult to correlate the distribution of economic plants with natural types of vegetation because the former have frequently been introduced into, and acclimatised in, regions to which they are not indigenous. Frequently, also, their original character has undergone great change as a result of their new environment and their long cultivation by man. But it will be seen from what follows, that the climate and soil which suit certain natural types of vegetation are likewise adapted to the growth of certain plants of great economic value, though it must not be supposed that their limits are coterminous.