134 ADAPTATIONS. OECOLOGICAL CLASSIFICATION sect, hi 



extremely elementary condition,' and 'We have no exact understand- 

 ing whatsoever of the sum of physical factors which we term climatic' 



Communities of xerophytes must be set into different groups 

 according as they are due to nature of soil or to nature of climate. The 

 distinctions in this respect are recounted in the succeeding paragraphs. 



Soil may, to employ Schimper's ^ terms, be physically or physiologically 



Physical Drought. Soil is physically dry when it contains very little 

 free water ; this is the case with 



1. The surface of rocks or stones occupied by plants which compose 

 the lithophilous formations. 



2. Sandy soil which lies so high above permanent subterranean water 

 that this does not affect it, and which is very parched during dry 

 seasons owing to rapid drainage and desiccation ; upon it are psammo- 

 philous formations, which are allied to those on rubble, where soil is 

 formed of gravel and stones. 



Here too must be placed epiphytes, which nearly all have definite 

 adaptations for securing water (see p. 8y). 



Physiological Drought. Soil is physiologically dry when it contains a 

 considerable amount of water which, nevertheless, is available to the plant 

 only to a slight extent or can be absorbed only with difficulty, either 

 because the soil holds firmly to a large quantity of water or because the 

 osmotic force of the root is inadequate to overcome that of the concen- 

 trated salt solution in the soil. This may be the case when 



1. Soil is rich in free humous acids, or in chemical bodies that by their 

 peculiar action on the plant evoke xerophily ^ ; there result those forma- 

 tions that grow on sour (acid) soil. 



2. Soil rich in soluble salts, usually common salt, which brings into 

 existence that form of xerophily whicli we see in halophilous formations. 

 A halophyte is in fact a special form of xerophyte, as Clements repeatedly 

 urges, and Wiesner ^ and Schimper * recognized. 



In addition to the xerophytic formations thus grouped according to char- 

 acters of the soil, which is dry, or dries frequently or rapidly even in a 

 moist climate, there is another series of formations to which the physical 

 and chemical qualities of the soil are of subordinate import in comparison 

 with the extreme climate. The soil is neither too acid, too saline, nor 

 too poor in nutriment, and may be sufficiently moist to sustain luxuriant 

 vegetation, yet the climate is so extreme that the soil is either too cold 

 (as in the case of formations on subglacial tracts) or periodically so dry 

 for a long season that only xerophilous formations can thrive on it, 

 excepting in situations, such as marshes or river-banks, where the soil 

 contains sufficient moisture throughout the year ; hence in this case, also, 

 topographical features play a part. The vegetation of savannahs 

 {campos) in the interior of Brazil is a formation evoked by a dry season ; 

 yet it is everywhere confined to the higher ground in the hilly country, 

 while forest always occurs along the watercourses and on the mountains, 

 where greater humidity prevails in the soil ; there can be no doubt 

 that were the chmate moist throughout the year the campos would be 



^ Schimper, 1898. * See Livingston, 1904. 



' Wiesner, 1889. * Schimper, 1891, 1898, 



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