Notes and Comment. 199 



'The behavior of other species. Many halophytes thrive in 

 cultivation on soils that are salt. They do not require the salt; 

 they merely endure it better than other species, occuring in nature 

 in salt spots, on or the sea shoree, etc., where the latter cannot 

 compete with them. (2) Influence of other soil factors. The 

 soil presents a trying composite of physical, chemical and or- 

 ganic factors which may mutually influence and replace each 

 other, so that the dependence of plants on one of them is often 

 influenced by the action of others. Many plants that shun 

 limestone, for example, will endure large quantities of lime in 

 the soil if they are provided with a sufficient supply of potash. 



(3) Influence of climate. Climatic factors may to a degree 

 replace soil factors; in a moist climate the same plants may 

 thrive on a dry soil which would avoid this in a dry climate. 



With optimum climatic conditions a species is often in- 

 different to soil in the center of its range, while at the periphery 

 of its area of distribution, where it is less capable of resisting 

 unfavorable climatic conditions, it becomes established only 

 on soils best suited to it, and it becomes there a soil indicator. 



(4) Physiological varieties. There is no doubt that certain 

 kinds of soil influence many plants in such a way that "soil 

 varieties "arise which in cultivation on other soils may return 

 to their normal form. Examples of these are the well-know 

 forms of fern on serpentine and of violet on zinc soils. Such 

 "soil varieties" occur in nature only on the soils to which they 

 belong. 



Physiological varieties, not distinguishable by external 

 characters, may arise on granite or limestone for instance which 

 deport themselves quite difi'erently under difl'erent external 

 conditions. In regions where other conditions are better suited 

 to the "limestone form" this may become dominant and ap- 

 pear to be an indicator of limestone, in others the "granite 

 form" thrives best, and in a third region both thrive and the 

 plants appear as if indifferent to the kind of soil. 



Plants as soil indicators are considered in their relations 

 to chemical constituents of the soil, humus content, and moisture. 

 Only the first and second are discussed, the former at length 

 and with special reference to plants belonging to limestone and 

 other rocks as studied by the author in Switzerland. The pains- 



