200 THE BIOLOGY OF FLOWERING PLANTS 



by first defining xerophytes as plants possessing means of 

 aiding absorption and depressing transpiration, and then 

 by stating that the xerophytic vegetation corresponds to 

 physiological dryness. The conception of a xerophyte has 

 undoubtedly had its uses in stimulating research ; its weak- 

 ness lies in the fact that, arguing from a resemblance in the 

 structural features of their vegetation to that of undoubtedly 

 arid regions, Schimper tended to fit other habitats, such as 

 the moor, to the Procrustean bed of " physiological dryness," 

 without, as we have seen, sufficient experimental evidence. 

 Xerophyte was first used to denote those plants inhabiting 

 arid regions and showing markedly features such as might 

 naturally be expected to diminish transpiration ; plants 

 growing in much less markedly dry stations, or in stations 

 not obviously dry at all, showed similar characters, and were 

 included in the class ; various reasons were then advanced 

 for believing that the stations inhabited by these " pseudo- 

 xerophytes " were xerophytic — or, as Schimper put it, 

 physiologically dry. 



We must retain the word " xerophyte," because it is in 

 universal use and because it indicates conveniently a group, 

 very large and very diverse, of structural peculiarities. 

 Xerophyte is a useful word to contrast with mesophyte, the 

 ordinary land plant of our climate, or hydrophyte, the aquatic. 

 But it does not indicate a natural biological group of plants, 

 and should never be taken as a sufficient definition of a 

 biological type. Only a beginning has been made with the 

 analysis of the different xerophytic types and their habitats, 

 as in the case of the succulents and the moorland plants. 

 We may summarise the main factors to which xerophytism 

 in the widest sense is related. They are : (i) High evapo- 

 rating power in the atmosphere, either alone or combined 

 with (2) difficulty in water absorption, which may be due 

 to diverse causes. (3) Periodic low soil temperature. 

 (4) Structural " defects," such as insufficient conducting 

 power of the wood. (5) A direct reaction of the chemistry 

 of the plant cell to water shortage. (6) Phylogenetic 

 causes. 



