86 THE FACTORS [Part I 



roots no longer absorb water. A soil that is rich in soluble salts even 

 when thoroughly soaked with water is therefore to a plant a completely 

 dry soil *. 



Nevertheless, by the absorption of salts from the substratum, plants 

 capable of enduring concentrated saline solutions in their cells acquire 

 a certain degree of accommodation, whereby they are enabled to satisfy 

 their demands for water from increasingly concentrated solutions. The 

 importance of this property to the oecology of plants is, however, under 

 natural conditions, less than might be supposed from the results of laboratory 

 experiments, since the concentration of salts in the soil is usually exposed 

 to considerable changes. Thus, for instance, the root-system of a littoral 

 plant, according to the alternations of sunshine and rain, storm and calm, 

 ebb and flow, is bathed in turns by fresh water, or by ordinary or even 

 concentrated sea-water. 



The soluble salts in the soil, not only during their absorption, but also, 

 at least so far as they are not consumed by the plant, during their whole 

 passage through it, exercise osmotic actions that greatly affect the pro- 

 cesses of development. Thus, merely moderately strong saline solutions 

 cause, as does drought, the closure of the stomata of many plants, especially 

 of those whose natural habitats are deficient in salt, and thereby power- 

 fully affect the assimilation of carbon 2 . The retarding influence on growth 

 of concentrated saline solutions has been frequently observed, and is in 

 all probability primarily traceable to this factor. 



The indispensability to vegetable organisms of certain mineral con- 

 stituents of the soil, especially nitric, phosphoric, and sulphuric acids, potash, 

 lime and magnesia, as well as iron, depends not on their physical, but on 

 their chemical properties. Some of their elements become constituents of 

 protoplasm, and others play a part in metabolism that is indeed secondary 

 but yet quite necessary. 



But it is not merely substances which are indispensable to plants that 

 influence their chemical relations. Even those that can always be dis- 

 pensed with set going, if they are absorbed, both physical and chemical 

 actions which influence vegetable organisms, sometimes favourably, some- 

 times injuriously, and sometimes in a manner that is quite recognizable but 

 is apparently indifferent oecologically. Above a certain degree of con- 

 centration all substances entering a plant in large quantities are poisonous, 

 if they are either not at once, or not at all, assimilated. The degree of 

 concentration at which a solution begins to be poisonous varies with its 

 chemical composition and with the species of the plant. The inequality 

 of the pozvers of resistance of different species is to a great extent responsible 

 for the differences in the floras of substrata that differ chemically from one 

 another. 



1 See p. 4. " Stahl, op. cit. 



