160 PLANT SOCIOLOGY 



determined alpine plants (lime constant, lime preferring, archaean 

 constant, archaean preferring), linger went so far as to make the 

 chemical composition of the soil responsible for the origin of many 

 plant species. He claimed that they had segregated from the original 

 types under the influence of "very different soils." Closely related 

 species found upon lime and clay slates he called substitute or vicarious 

 forms. 



Though it may not be necessary to agree with linger 's statements in 

 every detail, the essence of his views is even now accepted. Limestone 

 formations and calcareous soils are very widely distributed in nature. 

 The dependence of plant distribution and plant grouping upon lime is 

 therefore easily determined. Thus it becomes clear why the behavior 

 of plants in regard to the selective action of lime was used as the pri- 

 mary basis of the grouping of vegetation according to the chemical 

 composition of the soil. 



While the importance of soil chemistry for the occurrence and 

 distribution of plants and plant communities is beyond all question, 

 purely chemical investigations alone are incapable of solving the soil 

 problem. 



Physical Soil Theory. — Credit must be accorded to the Swiss 

 Jules Thurmann for the first clear recognition of the influence of the 

 physical properties of soil. In his "Essai de Phytostatique" (1849) 

 he gives a summary of the soil theories commonly held and sets over 

 against the chemical soil theory of Unger his own physical theory. 

 The soils are divided by Thurmann according to their physical char- 

 acter into pelitic and psammitic soils. Pelitic soils are fine-grained 

 clay soils of a high degree of dispersity. They are products of the 

 weathering of pelogenous rocks (clay slates, marls, limestones). 



Psammitic soils are sandy soils with coarse particles. They owe 

 their existence to psammitic rocks which are rich in quartz, especially 

 granites, syenites, gneisses, and certain granular dolomites. 



The native rock is, according to Thurmann, either easily weathered 

 (eugeogenous) or weathered with difficulty (dysgeogenous). Siliceous 

 rocks, such as gneisses, granites, etc., are mostly eugeogenous; lime- 

 stone rocks are mostly dysgeogenous. According to Thurmann, these 

 fundamental physical differences of rocks and soils, and not chemical 

 conditions, determine the composition of vegetation. 



By reason of an enormous, although somewhat one-sided, mass of 

 data, the views of Thurmann were promptly accepted, but they never 

 wholly displaced the chemical soil theory. 



With Thurmann and with Unger alternately seemed to rest the 

 victory in the resulting strife that for several decades divided plant 



