56 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



2. The absorbent faculty of the soil, which has already been ex- 

 plained. 



3. The climate. 



Where but little rain falls, the soluble salts produced by weathering, 

 being incompletely washed out, accumulate and may crystallize out, 

 especially on clay surfaces.^ 



An inadequate supply of soluble salts is unfavourable to plant-growth, 

 but too large an amount of them is also fatal to most species, because 

 the osmotic absorption of water is thereby impeded. The same effect 

 is produced by an abundance of humous acids in the soil. Such types of 

 soil belong to those described by Schimper ^ as being physiologically dry. 

 Plants on physiologically dry soil are often identical with those on arid 

 {physically dry) soil, or are guarded from excessive transpiration by the 

 same protective devices. 



The quantity and quality of nutriment influence plant-form. Defective 

 nutriment (that is an inadequate supply of one or more substances) may 

 be the cause of dwarf-growth (nanism) ; this has been demonstrated by 

 many physiological investigations, and is shown in natural vegetation, 

 for instance, on sand-fields and other poor soils. Dwarf-shrub is a growth- 

 form characteristic of soil poor in nutriment, and particularly of heath. 

 The amount of a single substance may determine the issue. It is a 

 general rule that the size of a crop, in so far as it is dependent upon nutri- 

 ment, is determined by that nutritive substance which is available to 

 the species concerned in relatively the smallest quantity (Liebig's Law of 

 the Minimum). 



When a nutritive substance occurs in so small a quantity that the 

 crop is decreased for this reason, then, according to Atterberg's rule, the 

 substance in question is present in the plant in relatively smaller amount 

 than are those nutritive substances of which there is no deficiency ; it is 

 then easy to surmise that other, morphological distinctions, may also 

 thereby arise. 



The form of the root is adjusted to the characters of the soil. According 

 to the investigations of Sachs ^ the more concentrated the nutrient solution 

 the shorter are the roots. Roots are mostly long and feebly branched in 

 poor soil, for example in plants on sand, especially on dunes, in Central 

 Europe ; nevertheless the majority of heath-plants show the contrary. 

 Roots branch very copiously and form dense clumps in rich soil. If 

 roots encounter strata of soil with different quantities of nutriment, 

 the contrasts in the ramification within the different strata are striking. 

 " Roots search for food as if they possessed eyes.'* 



The chemical constitution of the nutritive substratum in certain cases 

 evokes formal differences. This is particularly true of one substance, com- 

 mon salt. It is recognized that all halophytes are distinguished by a special 

 configuration ; they have fleshy leaves, transparent tissue, and so forth .^ 

 The effects of calcium carbonate and other substances are less obvious. 



Distinctions in soil have probably led to the separation of new species : 



The calamine violet (Viola calaminaria) is presumably a form that 

 has arisen from Viola lutea by the action of zinc in the soil.^ 



' Hilgard, 1892. * Schimper, 1898. 



' Sachs, J. von, 1865, p. 177. * Liebig. 



* See Section VII. ' Schimper, 1898 (1903, p. 93). 



