Of almost equal importance to temperature is the amount and seasonal distribution of precipitation either in the form of rain or snow or as dew and fog. In regions of equiv alent temperature forests will in general occupy the areas of highest precipitation, deserts the areas of lowest precipitation, and grasslands the areas of intermediate humidity. In the United States, for instance, the mean annual rainfall in the desert region ranges from o-3oin., in the prairies from 2o-3oin., and in the regions of forest from 3o-9oin. In any estimate of the influence of rainfall, however, the humidity of the air is necessarily a con siderable factor and the efficacy of a given precipitation will be largely influenced by the temperature changes, upon which the rate of water-loss from the surface of the plant and from the soil in which it grows so largely depends. Further, this water-loss will be greatly augmented by wind,, and so the velocity and fre quency of air movements have an important influence on the limits of species and plant-communities. The frequent incidence of high winds inhibits the growth of trees near the summits of mountains, both by their drying action and mechanical effect, whilst the strong winds and low temperature together determine the distribution of arctic Tundra.
Since all green plants manufacture their or ganic food by means of the radiant energy absorbed from sun light, the intensity and duration of this illumination naturally affects the distribution of species. Many woodland plants are so constituted as to function best in this respect when growing in comparatively weak illumination, whilst others, such as many lit toral species, attain their optimum rate of food production in full sunlight. Even the weak light which reaches the floor of a wood land is sufficient for the requirements of some mosses, but when the shade is too intense the vegetation is confined to parasites and saprophytes and so the rich fungus flora of a dense beechwood or pinewood becomes one of its salient characteristics. It has, moreover, been found that the length of the period of daily illu mination may be an important factor in influencing reproduction. Some species appear to be indifferent to the length of the period of daylight, except in so far as the amount of organic food they manufacture is dependent on the total radiant energy they receive. Other species respond to a shorter daily illumination by earlier flowering and fruiting, whilst with still other species the effect is a retardation in the reproductive process.
Nevertheless, though the individual climatic factors play each their separate part, it is mainly upon their interaction that dis tribution of species depends. Thus it is that some are particularly
associated with oceanic climates characterized by small amplitude of temperature and high humidity, whilst others are found in the drier and more extreme conditions of continental areas. Examples of the former are furnished by the petty whin (Genista anglica) and the wild hyacinth (Scala nutans), features of the west of Europe, whilst the latter are represented by various species of Eryngiurn and Artemisia of the Russian steppes.
The character of the soil is chiefly important as to its physical structure, except where the chemical constitution exhibits extreme conditions. For upon the size of the constituent mineral particles and the proportion of organic material which the soil contains depends in large part its capacity to retain the water which reaches it in the form of precipitation or by capillarity. It is the feeble power of such retention that renders the conditions on a sand dune or a shingle beach comparable to those of a desert even in regions of high precipitation and humidity. It is in great, measure differences in water capacity, as this power of water re tention is termed, that are responsible for the different types of vegetation on clay, sand, and peat, within an area of uniform cli matic conditions. Nevertheless, other soil factors, such as the chemical nature, the reaction, the nature of the soil atmosphere and the constitution of the teeming population of micro-organ isms, etc., all play a part in determining the relative suitability of soils for one species or another. The soil conditions in un touched vegetation, however, present a very different distribution from the comparative uniformity of agricultural land, a definitely stratified organization, as a consequence of which species growing intermingled but with root systems of differing depths of pene tration may be occupying very different types of soil both as regards chemical and physical constitution. The direct influence of the chemical nature of the soil is of ten difficult to separate from its indirect influence on water supply, absorption of soluble salts, etc. This is well seen in the vegetation of calcareous and non calcareous soils. The so-called calcicole species which frequent the former may prefer calcareous soils either because of their physical qualities, because of their neutral or slightly alkaline re action, or because of their chemical properties. Further, such association may be obligatory or merely preferential. Preference for a particular soil type is sometimes a feature of an entire genus, as that of Clematis for calcareous soils, whilst in other groups closely allied species may have very different requirements.