ABSORPTION OF WATER 153 



sand cultures must be chosen, therefore, according to the require- 

 ments of the plant. If this is not taken into account, the plant 

 may absorb various substances in an unequal degree and the com- 

 position of the solution may change considerably with the growth 

 of the plant. Even with perfect adaptation to the character of 

 the plant, one does not succeed in preparing a solution that would 

 be absorbed unaltered. Sand cultures, therefore, are more diffi- 

 cult to manage than water cultures, where the solution may be 

 changed from time to time. 



Not only the composition, but also the general concentration 

 of the solution changes measurably as the plant develops. If it is 

 too dilute, the plant draws from it relatively more salts than water 

 and it gradually becomes more diluted. From strong solutions, on 

 the contrary, the plant absorbs proportionately more water than 

 salts, resulting in an increased concentration of the solution, which 

 finally leads to decreased growth. The absorption of water and of 

 salts, thus, to a certain degree is independent of each other. This 

 is shown by experiments in which transpiration is artificially 

 checked by placing the plants in a moist atmosphere or by shading 

 them. The total amount of the water lost thereby is considerably 

 reduced. The amount of accumulated salts, on the other hand, 

 remains almost unchanged. 



The most direct way of determining in what proportion water 

 and the substances dissolved in it are absorbed by the plant, is 

 by the analysis of the sap excreted by plants in bleeding; with the 

 exception of spring bleeding, when the exuding sap is extremely 

 high in reserve substances. Such determinations have been 

 carried out recently by Sabinin, who has shown that the compo- 

 sition of sap does not correspond to the liquid in which the root 

 system is immersed. If kept in very dilute solutions of K, P, Mg, 

 and other substances, the plant is able to concentrate consider- 

 ably these elements in its sap. On the contrary, when placed in 

 strong solutions, the concentration of the sap excreted in bleeding 

 is relatively low. There is no doubt that this condition is due to 

 the membrane equilibrium of Donnan, mentioned in Art. 38. 



Substances incapable of penetrating through the protoplasm or 

 unable to accumulate in the cells are absorbed by the roots from 

 the soil in rather insignificant quantities. An alien substance, for 

 instance a dye or an insecticide, cannot be introduced into the 

 conductive system of a plant by watering the soil with a solution 



