416 THE ECOLOGY OP PLANTS. 



Woody perennials (trees and shrubs) with deciduous leaves 

 show distinctly xerophilous characters in winter (the pro- 

 tected winter buds, cork-covering of stems and of leaf-scars, 

 cork-layer closing up the lenticels), while in summer they 

 bear thin leaves like those of mesophytes or hygrophytes. 

 It has been proposed to call such plants, which are 

 more or less hygrophilous in summer and xerophilous in 

 winter, Tropophytes (i.e. " changing plants "). This 

 applies only to deciduous plants ; evergreen land-plants 

 are more or less strongly xerophilous in character, their 

 leaves being usually tough, thick, and leathery (e.g. Holly), 

 or needle-like (e.g. Pine), with thick cuticle. 



It must be remembered that no hard-and-fast lines can be 

 drawn between these types. For instance, some aquatic 

 plants can, when the stream or pond dries up, continue to 

 live and grow, sometimes even more vigorously, i'i the air, 

 thus changing from hydrophytes to mesophytes. The same 

 species of plant may be found growing under hygrophilous 

 or xerophilous conditions in different localities. In each 

 case the structure of stem and leaf, as well as the form 

 of the leaves and the general habit of the plant, become 

 modified to suit these different modes of life. Coast-plants 

 can, in many cases, be grown in ordinary soil, and develop 

 thin instead of fleshy leaves. 



40O. Environment. In the study of Plant Biology you 

 should always be on the alert to ascertain whether the differ- 

 ences in form and structure presented by different plants can 

 be accounted for by differences in their mode of life and 

 habitat. This study requires a sound practical knowledge 

 of the morphology and physiology of plants. All the factors 

 that make up the environment must also be taken into 

 account. These factors fall into four main groups : (1) the 

 physiographic factors, including altitude, exposure, slope; 

 (2) the climatic factors, including temperature, rainfall, 

 light ; (3) the edaphic factors, including the physical and 

 chemical characters of the substratum, i.e. in most cases the 

 soil ; (4) the biological factors, including other plants, 

 animals, and man. 



In dealing with a comparatively small area, like that of 

 Britain, we find that the plant- societies are determined much 



