INTRODUCTION 19 



districts is frequently poor in mineral salts, or is 

 charged with too great a percentage in the case of 

 bogs and moors, where there is a superficial layer 

 of peat, of humous acids. In this respect there 

 may be "water, water everywhere, but not a drop 

 to drink," or, in a word, physiological drought : 

 that is to say, the root-hairs may become clogged 

 with the superabundant solutes, and unable to absorb. 

 Since transpiration must go on the plant must 

 suffer. To prevent this such plants have to adapt 

 themselves to meet this kind of drought, in the 

 same way as when physical drought on dry soils 

 occurs. 



In the valleys and other areas where the water is 

 rich in mineral salts in solution plants that require 

 such a rich supply can flourish. Root-hairs require 

 a small proportion of salts, i to 3 per cent., alkaline 

 as a rule, and sour acid solutions are detrimental to 

 all but bog and moor plants, and even to these in 

 excess. 



The soil, if clogged with water, prevents the access 

 of oxygen to the roots, and this again is a cause of 

 wilting. 



The permeability of the soil determines the amount 

 of water presen , or rather whether it is retained or 

 not. Thus, a clay soil retains water and absorbs it, 

 but a sandy soil is non-retentive. The structure of 

 the soil, loose or fine, the size of the particles, the 

 texture, and density regulate the amount of water 

 present. 



