ABSORPTION OF NUTRITIVE SALTS 413 



solution ; (2) the extent to which the exudations from the 

 root-hairs — whether these be carbon dioxide, salts of 

 mineral acids, or organic acids — act on the soil particles. 

 . The first of these is a function of the vital energy of the 

 plant and its ability to utilize sunshine and carbon dioxide 

 to produce organic matter. It may be compared to. the 

 property which enables one animal to do more work than 

 another animal of the same weight on a similar ration. 



The removal from the ascending water current in the 

 plant of substances derived from the soil is accomplished 

 in the leaves. By the dissociation of these substances, 

 ions are constantly furnished for metabolism into materials 

 that may be built into the tissues of the plant. The re- 

 maining ions are kept in the solution. There is a con- 

 stant tendency to bring the composition and density of 

 the solution into equilibrium, by diffusion and diosmosis, 

 with the solution between the soil particle and the root- 

 hair. The rapidity with which the metabolic process 

 removes a substance from the solution in the plant, there- 

 fore, determines the rate at which it is removed from a 

 solution of given composition and density in the soil. 

 Plants making a rapid growth remove more nutrients in 

 a given time than those making a slower growth, when 

 the nutrient solution is of a given composition and density. 



Another factor that affects the rate of absorption of 

 salts from the soil is the solvent influence of exudates from 

 the root-hairs. This subject has already been treated (pars. 

 321-326), and it only remains to be said that this action 

 apparently varies with different kinds of plants, and 

 probably accounts in no small measure for the difference 

 in the ability of different plants to withdraw salts from 

 the soil. 40^ 



These several factors, which, when combined, deter- 



