ii8 THE LIFE OF THE PLANT 



and other complicated nitrogenous compounds : x as the 

 result of this transformation fresh quantities of salt- 

 petre will enter the plant, will again be transformed 

 into the substance of the plant, and this process will 

 continue indefinitely. It is different in the case of the 

 common salt. According to the laws of diffusion it 

 will pass into the plant until a solution of equal strength 

 be formed within and without the plant ; then its 

 further absorption will cease. If there should by any 

 chance happen to be more of it inside the plant than 

 outside, according to the same laws of diffusion, the 

 superfluity will pass back again from within the plant 

 to the solution. Now it is clear why just those sub- 

 stances which are transformed and assimilated by the 

 plant, being necessary to it (as is saltpetre in our 

 experiment), are extracted from the solution, whereas 

 those which are useless to the plant (as is the common 

 salt in our experiment) remain untouched in the solu- 

 tion, or, to be more accurate, almost untouched. 



It is therefore quite unnecessary to presuppose any 

 rational will, habits, tastes or instinct in order to 

 explain the discriminating property of the root simple 

 laws of physics are sufficient for the purpose. 



We must leave the root here and pass in our next 

 chapter to another organ, the leaf. It is obviously 

 impossible to exhaust such a rich subject in one short 

 chapter ; but I think that what we have now learned 

 is sufficient to give us a general idea of the life of the 

 root, which in such a limited space covers a course of 

 many miles, and sucks, eats into, and breaks up the soil 

 with its million hairs, and absorbs from it the soluble 

 mineral substances so sparsely scattered in it nitrogen 

 and the elements of its ash, those eight elements without 

 which the very existence of the plant is impossible. 



1 We are entitled to make this assertion because we can grow a plant 

 by furnishing it with saltpetre as the sole source of nitrogen. 



