68 OECOLOGICAL FACTORS AND THEIR ACTION SECT, i 



calcareous soil by competition, and are compelled to select a soil containing 

 less calcium, without having any real preference for silica, which is a very 

 neutral substance ; thus Contejean interprets the matter. To the silici- 

 colous plants belong the majority of those growing on sand and moor in 

 north-temperate Europe. 



Nitrophilous plants (nitrophytes, ruderal plants). These thrive best 

 in soil where compounds of ammonium and nitric acid are abundant, 

 and therefore especially in the vicinity of human dwellings (dung-heaps, 

 highly-manured soil). They belong to certain special families (Chenopo- 

 diaceae, Cruciferae, Solanaceae, and others), and nitrates occur in their 

 cell-sap. Other species develop feebly on such soil, because they take 

 into their tissues more nitrate than they can endure. 1 Certain fungi and 

 mosses (Splachnaceae) flourish only on dung. 



The solfataras of Java, according to Holtermann's assertions, have a 

 peculiar flora that differs from those of others. 2 



Other substances also can act as poisons if they be supplied in large 

 quantities. 



If gypsum be scattered over a meadow certain ferns and grasses die 

 off, while clover becomes more luxuriant. 



Similarly iron (iron sulphate, ferrous oxide) may act injuriously if 

 present in quantity, though it is one of the absolutely indispensable 

 nutritive elements. 



Investigations conducted at Rothamsted in England have demon- 

 strated in a particularly clear manner the significance of the chemical 

 constitution of nutriment ; they showed that with nitrogenous manure, 

 especially with nitrates, grasses preponderated and expelled Leguminosae ; 

 whereas, on the contrary, potassic salts favoured Leguminosae. Experi- 

 mental manuring of high moors has, according to Weber, led to entirely 

 similar results ; certain species of grasses were expelled by others. But 

 one can hardly say that research has yielded any considerable sup- 

 port to the chemical theory we are discussing ; calcicolous and silici- 

 colous plants, the calamine-violet, and even halophytes, are perhaps 

 always capable of flourishing in a soil not containing the respective 

 substances they affect, or practically in any soil, in botanic gardens for 

 instance. On the other hand, the amount of nutriment in soil plays a 

 more prominent part. 



In course of seven years' wanderings, A. P. de Candolle found nearly 

 all species upon soils of varied chemical nature ; and Blytt came to the 

 conclusion that the very few species which in 1870 he had regarded as 

 restricted to definite soils in Norway must be further reduced in number 

 as a result of his later investigations. 



' Every distributional relationship may be due to either a physical 

 or a chemical cause, but the simultaneous presence of both prevents us 

 from clearly distinguishing the part played by either singly'. 3 This is 

 perfectly correct, and the history of botanical science shows that some 

 botanists, in opposition to those previously mentioned, ascribe greater 

 importance to physical than to chemical relations. 



1 Schimper, 1890-1. . Holtermann, 1907. 



* Vallot, 1831. 



