PLANT COMMUNITIKS. 



87 



inhabit shady situations. Thus, in woods the leaves of the trees inter- 

 cept a large proportion of the sunlight, while in the less brightly 

 illuminated region below is an undergrowth of bushes, below which, 

 again, are herbaceous plants, and in the deep shade close to the ground 

 grow Mosses and Liverworts, and other lowly plants. Sometimes the 

 shading is so dense, as in a thick Beech or Pine forest, that scarcely any- 

 thing will grow in the deep gloom near the ground. 



Wind has a drying action on soil and vegetation, and so reduces the 

 available water supply and increases transpiration. When blowing more 

 or less constantly and strongly in the same direction, as along our coasts, 

 it has a considerable influence on the growth of trees and shrubs exposed 

 to its violence. They are more or less stunted and dwarfed in stature, 

 with their trunks and branches inclined to leeward. 



Using the water factor as the basis of classification, we may group 

 most of the principal plant communities under three heads, viz. — 



I. Hydrophytes, or water-plants, which live wholly or partially 



immersed in water. 



II. Xerophytcs : plants which are exposed, constantly or periodically, 



to a very dry soil and dry atmosphere. 



III. Mesophytes. — This is by far the largest group, and comprises 

 plants growing under conditions of medium water supply. 



To these may be added a fourth group, comprising plants growing on 

 soil containing much salt, as on the sea-coast and the salt steppes of 

 Asia. These are termed — 



IV. Halophytes. 



The boundaries of these groups are obviously not sharply defined, but 

 merge imperceptibly into one another. 



Hydrophytes, from the uniformity in their conditions of life, exhibit 

 considerable similarity in general habit and structure. They are not 

 subject to such extremes of temperature as land plants, nor is their 

 growth so liable to be checked by drought. Moreover, the conditions 

 leading to nutrition are especially favourable, particularly in plants which 

 are entirely submerged, since all their food constituents, as well as the 

 oxygen required for respiration, are present in solution in the surrounding 

 water. These conditions, and also the diminution of the light in passing 

 through water, all favour rapid and luxuriant growth. 



Among the structural features common to most completely hydro- 

 phytic plants are a thin- walled epidermis devoid of cuticle and with few 

 or no stomata, the great reduction of the root system and of water- 

 conducting and mechanical tissues, and the enormous development of 

 air spaces in the cortical region. The leaves are usually either long and 

 ribbon-shaped or cut into numerous fine segments, or awl-shaped ; but 

 when floating leaves are present they are usually large and undivided, 

 of oval or rounded form and leathery texture, and with stomata on the 

 upper side only. Frequently much-divided submerged leaves and entire 

 floating leaves occur on the same plant. Reproduction is largely 

 vegetative, and is effected by detachable buds, tubers, or even branches 



