Chap. I] WATER 3 



necessary to place in a third category all plants whose conditions of life are, 

 according to the season of the year, alternately those of hygrophytes or of 

 xerophytes. All such plants, including, for instance, the great majority 

 of the plants composing the Central European flora, should be termed 

 tropophytes. The structure of their perennial parts is xcrophilons, and that 

 of their parts that are present only in the wet season is hygrophilons. 



The classification of plants as hygrophytes, tropophytes, and xerophytes 

 is the first step towards the physiological comprehension of the earth's 

 vegetation and its components, the formations. Extensive districts, for 

 instance a large portion of the tropical coasts and mountain ranges, are 

 marked by the prevalence of hygrophytes ; others, such as steppes, deserts, 

 and polar zones, of xerophytes ; and others, again, for instance the greater 

 part of the north temperate zone, of tropophytes. There are hygrophytic, 

 xerophytic, and tropophytic climates. Every climatic district exhibits, besides 

 the corresponding oecological type of vegetation, one of the two other 

 types in certain localities, because the properties of certain kinds of soil 

 weaken, or strengthen, the influence of the climate. The influence of the 

 soil may be termed edapJiic l . There are climatic and edaphic hygrophytes, 

 xerophytes, and tropophytes. 



Characteristics occasioned by physiological humidity or drought deter- 

 mine the physiognomic, or rather, oecological aspect of the vegetation of 

 the districts 2 and of the separate stations within them. Systematic phyto- 

 geography must therefore reckon these differences amongst the most 

 important, for there are also hygrophilous, tropophilous, and xerophilous 

 species. There are, further, some species — and this fact is as important 

 to the systematist as to the physiologist — which adapt themselves to the 

 varying conditions of humidity so completely that their extreme forms 

 appear to belong to different species, but these by a change in the supply 

 of moisture may pass over into one another. 



ii. XEROPHYTES. 



Physiological drought is caused by external factors which either reduce 

 absorption or which favour transpiration, or, and this the most frequently, 

 there is a combination of these influences 3 . 



Factors reducing Absorption. 



1. Scarcity of free water in the soil, that is to say, of water that is less 

 attracted by the particles of soil than it is by roots. According to their 

 physical nature different soils exhibit very unequal degrees of physiological 

 dryness i . 



1 to e'8a<f)os, ' the soil.' 



" The Zones depending on heat are subdivided into Districts depending on the quantity 

 of atmospheric precipitation. See Part III, Introduction. 

 3 Schimper, I. 4 See Part I, Chap. V, The Soil. 



B 1 



