ABSORPTION OF MINERAL ELEMENTS BY PLANTS 251 



investigations by various authors have shown that the salt 

 solution almost never penetrates into the roots in the same con- 

 centration that is present in the nutrient solution. In very dilute 

 solutions, the plant absorbs salts in excess of water, and the 

 solution gradually decreases in concentration. Conversely, in 

 concentrated solutions the plant absorbs more water than salts, 

 and consequently the solution becomes more and more concen- 

 trated. The concentration of the nutrient solution has to be 

 selected according to the needs of the plants as well as in relation 

 to the conditions of water loss. 



The independence of the absorption of salts and water is most 

 convincingly proved by the well-established fact that various 

 salts are absorbed by plants at a different rate and that even the 

 anion and cation of one and the same salt penetrate in unequal 

 proportions. This phenomenon has already been encountered 

 in the discussion of the nutritive value of ammonium and 

 nitrates, when there were distinguished physiologically acid 

 salts, whose cations are absorbed more rapidly than the anions, 

 the solution consequently becoming acid, from the physiologically 

 alkaline salts, in which, conversely, the anion is absorbed more 

 readily than the cation. 



This difference in the absorption of separate anions and 

 cations leads to a gradual alteration of the nutrient solution 

 originally presented to the plant roots and very often in a direc- 

 tion unfavorable to the plant. In the growing of plants in water 

 cultures, it is necessary, therefore, to change the nutrient solution 

 from time to time, usually every 10 to 15 days. 



Knop's is one of the best nutrient solutions; in his solution, 

 these alterations in composition are small. Prianishnikov has 

 recently given much attention to this side of the question, and 

 the nutrient solution prepared by one of his coworkers, Cincadze, 

 is being widely used in experiments with water cultures. An 

 adequate composition of the nutrient solution is of special impor- 

 tance in the growing of plants in sand cultures, where it is not 

 so easy to change the solution completely. 



The absorption of mineral substances by the roots of plants 

 is a very complex process. Its first stage consists in the pene- 

 tration of the salts from the soil solution into the root hairs and 

 the surface cells of the root in general. This penetration is 

 controlled by the same laws that have already been discussed 



