CHAP, xvir INFLUENCE OF PHYSICAL CONDITIONS OF SOIL 69 



The dominating influence of physical characters of soil. 



The protagonist for the dominating importance of physical relation- 

 ships was Jules Thurmann (1849). His doctrine can be summarized as 

 follows : 



It is the physical structure of soil that regulates the distribution of 

 species ; 



Upon this structure depend the amount of water and the thermal 

 conditions in soil. 



The same species can grow on very different kinds of soil, if it encounters 

 the same conditions of moisture. 



Thurmann discusses the different weathering properties of kinds of 

 rock under the action of air, water, and heat (both frost and warmth), 

 As a result he di\'ides rocks into eugeogenous and dysgeogenous. 



Some kinds of rock are easily weathered and rapidly produce loose 

 masses (grit, sand, and similar detritus) ; these soft types are the eugeo- 

 genous, and in accordance with the fineness of their products of weather- 

 ing they are pelogenous when the particles are very fine and powdery, 

 especially clay and marl soils ; or psammogcnous, when the particles are 

 coarser sand. According as soil is more or less pelogenous or psammo- 

 gcnous, Thurmann employs the prefixes ' per ', ' hemi ', and ' oligo ', 

 to denote sub-divisions, or speaks of pelopsammitic soil. 



In opposition to types of rock that are easily weathered, are the hard, 

 resistant types dysgeogenous ; they give rise to scanty or no products 

 of weathering. 



Finely comminuted soil absorbs more water than does slightly 

 weathered rock.^ Eugeogenous rocks therefore bring into existence a 

 moist, cold soil ; dysgeogenous, a dry, warm soil. 



To plants that exploit moist soil and eugeogenous land Thurmann 

 applies the term hygrophilous ; plants that exploit drier soil and dysgeo- 

 genous rock he terms xerophilous. His hygrophilous species correspond 

 approximately to the silicicolous plants of Unger and others, and his 

 xerophilous species to their calcicolous ones. Indifferent species of 

 plants occurring on all kinds of soil Thurmann designates uhiquists. 



According to Thurmann, the obvious distinction between the floras on 

 calcareous and siliceous soils is caused, not by the preference of species for 

 lime or silica, but by the circumstance that calcareous rocks weather 

 with difficulty, and permit water to flow away rapidly through clefts and 

 fissures ; they produce a dry warm soil of slight depth, whereas quartz 

 and felspar produce a loose, deep, moist, and cold soil. When species 

 of rock, with identical chemical composition, in some cases are hard and 

 resistant but in others become easily disintegrated, then calcicolous 

 plants are found on the former soil even when it is sihceous, and silici- 

 colous plants on the latter soil even when it is calcareous. Furthermore, 

 a single species of plant in a definite climate may require a definite soil 

 on account of the physical characters of the latter ; for instance, in a 

 moist chmate it may choose a warm dry soil like lime ; but in a different 

 climate it may prefer an entirely different soil ; for instance, in a warm 

 dry chmate a moist, cold, sihceous soil. A favourable soil may facilit;ite 

 the existence of a plant in an unfavourable climate. According to Blylt, 



* See Chapter XII, p. 47. 



