IMPORTANCE OF WATER 119 



protein and lipoid particles in the surface layer. With the accu- 

 mulation of the first, permeability increases, with the predomi- 

 nance of the latter, it decreases. 



38. Conditions of Accumulation of Soluble Substances in the 

 Cell. Importance of Chemical Alteration in Substances for Their 

 Accumulation. Donnan's Membrane Equilibrium.— According 

 to the laws of diffusion, such substances as salts and sugars, which 

 are soluble in water and are absorbed by the cell, must continue 

 to enter the cell until the concentration of each substance becomes 

 equal, both outside and inside the cell. As the plant usually 

 obtains rather dilute solutions of nutritive substances, it is evi- 

 dent that their entrance into and accumulation within the cell 

 require special conditions. 



A most important prerequisite is the chemical change of the 

 absorbed substances. For instance, when carbohydrates are 

 stored in the tubers of the potato, the sugar obtained by the cells 

 from the leaves is transformed directly into starch, which is insol- 

 uble in water. The concentration of sugar in the cells of the grow- 

 ing tuber is, therefore, extremely low and does not impede the dif- 

 fusion of new amounts of sugar. The same is observed in ripening 

 oil-bearing seeds. The only difference is that in this case fats are 

 accumulated at the expense of the soluble carbohydrates. Pro- 

 tein compounds are formed from amino acids, and so on. As a 

 general rule, the substances entering the cell are subject to chemical 

 transformation which assures their uninterrupted absorption. 



To demonstrate the dependence between the accumulation of 

 substances in the cell and their transformation, one may use a sac 

 of hardened collodion, representing a model of the plasma sac 

 lining the plant cell from the inside. Water diffuses readily 

 through the collodion membrane, salts penetrate with greater dif- 

 ficulty, while colloidal substances hardly pass at all. If such a sac 

 is filled with water and immersed in the solution of some salt, for 

 instance in ferric chloride (Fig. 46), the salt will penetrate into the 

 sac until the outer and inner concentrations become equal. No 

 accumulation of iron within the sac will take place. If instead of 

 pure water the sac is filled with a solution of tannin which produces 

 with iron a dark-colored compound of colloidal nature, the solution 

 will remain inside the sac and will bind the iron ions penetrating 

 into the sac. As a result, the concentration of the iron ions inside 

 the sac will be always considerably lower than on the outside, 



