30 PLANT PHYSIOLOGY 



quantity and remain in the same state in which they entered the 

 cell. In the bulb of the common onion, for instance, considerable 

 amounts of glucose are stored. In pigweed and other plants, 

 much potassium nitrate accumulates, and its concentration in 

 the root cells may exceed by far its concentration in the soil 

 solution. Attempts have been made to explain this occurrence 

 by the fact that sugar and potassium nitrate are present in these 

 cells in special unstable combinations with some substances of 

 the cell sap. All efforts at isolating such combinations have 

 failed, however. 



An example of this is the abundant accumulation of free 

 inorganic ions in the cells of submerged water plants that are 

 directly surrounded by water with a comparatively low salt 

 concentration. By analyzing the cell sap of the fresh water 

 alga Nitella, Hoagland observed the following amounts of the 

 more important ions: chlorine, 106 milliequivalents per liter, 

 sodium, 79; potassium, 58. The water of the pond where these 

 algae grew contained: chlorine, 1 milUequivalent per liter; 

 sodium, 1.2; potassium, 0.05. Thus the concentration of the 

 ions of chlorine in the cell sap of the alga was 100 times, sodium 

 66 times, and potassium 1,160 times greater than in the sur- 

 rounding water. Nevertheless, these ions remained in an entirely 

 uncombined state. Analogous relations were observed by 

 another investigator, Osterhout, in the cells of marine algae. 

 As is well known, their cells accumulate large amounts of bromine 

 and iodine, the content of which in sea water is quite insignificant. 



Very often the explanation of these phenomena is sought in 

 the membrane equihbrium explained by Donnan. It is not 

 possible to give a detailed account of this equilibrium, which 

 may be found in courses on physical and colloidal chemistry. 

 Only the chief consideration on which Donnan's theory is based 

 need be considered here. If a membrane impermeable to 

 colloids and containing an easily hydrolyzable salt, one of whose 

 ions is of a colloidal character such as sodium proteinate, is 

 immersed in water, the sodium ions will not diffuse from the 

 sac. They will be retained inside by the electrostatic attraction 

 of the protein micelles. Now if some easily penetrating salt, 

 e.g., potassium chloride, is added to the water, then after some 

 time equilibrium will be established between the four ions. 

 At equihbrium, the ions charged oppositely to the colloid ion 



