14 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Plants cannot exist without water, though different kinds require 

 it in very different amounts. Our ideal plant must be well adapted 

 in its water economy to the prevailing conditions ; thus if water is 

 scarce, it must have some means of keeping down the loss which 

 takes place continuously from its aerial parts in the process known 

 as transpiration. This economy may be effected by such devices 

 as a thick and impervious bark or waxy or hairy covering, by pro- 

 tection of the ' breathing pores ', or by reduction of the total surface. 

 Often more than one method is employed by a plant, which at the 

 same time will have to be adapted to other factors of the environment. 

 Through long processes of evolution, involving among other things 

 the elimination of unsuitable features, different kinds of plants have 

 become adapted to different environments, and this, as we shall see 

 for example in Chapter III, is one of the most fundamental bases 

 of their distribution and consequently of plant geography. 



Plant Sociology 



Although opinions vary as to what constitutes a species (broadly 

 speaking, a kind), we all have some conception of how similar 

 individuals, whether plants or animals, make up such an entity. 

 The numerous individuals comprising a particular species, while by 

 no means all exactly identical, nevertheless are closely comparable 

 in most respects, and normally have the appearance of belonging 

 to the same kind. We have already observed that different plant 

 species and other entities, often of many and various groups, become 

 associated together in nature to compose what we term vegetation. 

 This is made up of more or less definite plant communities, related 

 at least in part to local conditions. Each of these communities is 

 characterized by its own particular form (physiognomy), and in 

 most cases also by one or more predominant species. 



Plant sociology, also called phytosociology, is, strictly speaking, 

 the study of the plant communities that make up vegetation — 

 including their inception and formation, their structure, and, above 

 all, their composition. Accordingly some parts of this subject, and 

 particularly the composition of plant communities, are of vital 

 interest to the plant geographer, even as the distribution of these 

 communities forms an important part of his study. But in spite 

 of a wide overlap of material, students of the two disciplines 

 approach their problems and subjects from different angles of 

 interest, and so it is not proposed to consider plant sociology 



