248 PLANT PHYSIOLOGY 



All vital processes of the cell, such as the movement of proto- 

 plasm, its increase in mass during nutrition, and the growth of 

 the cell, are dependent on changes in the stability of the colloidal 

 system. Cell colloids may change from sols to gels and vice 

 versa, as for instance during the growth of cell walls, the forma- 

 tion of starch grains, or the division of chromosomes. Thus the 

 electrolytes may be considered most important regulators of 

 many functions of the cell. The rate and even the direction of 

 various processes of the cell may be influenced by the composition 

 and concentration of the salts. Since the effect of the ions on 

 the colloids depends on their electric charge and specific prop- 

 erties, one should not be surprised that the presence of an intricate 

 system of anions and cations is necessary for the complex col- 

 loidal-chemical phenomena that take place in the cell. 



The requirements of the cell for a definite combination of 

 cations make it very difficult to study the specific role of each 

 of them. If one element or another is excluded from the nutrient 

 solution, it can only be ascertained that the development of the 

 plant has been inhibited. In an experiment with buckwheat, 

 for instance, the following results were obtained: In a complete 

 nutrient solution, the weight of the plant was 13.8 times greater 

 than that of the seed; in a solution without K, 9.2 times greater; 

 in one without Ca, 1.3 times; without Mg, 5.1 times; and without 

 Fe, 7.3 times. It would be a mistake to conclude from these 

 results that the elements, the exclusion of which gave a small 

 decrease in yield (K, Fe), are less necessary than the others. 

 Two contributing causes may have influenced the visible effects. 

 There may have been a relatively greater storage of these ele- 

 ments in the seed, or else their absence may have caused less 

 evident disorders in the organism. In general, it must be 

 admitted that very little is definitely known of the specific 

 functions of the different cations and of the various disturbances 

 caused by their absence. The only fact established with cer- 

 tainty is that each of the cations has its role, which cannot be 

 taken by another, even one closely related in the periodic system. 

 Thus potassium cannot be replaced either by lithium or by 

 sodium, and only a few fungi allow a partial substitution of 

 rubidium for potassium. Calcium, likewise, cannot be replaced 

 by any of the alkaline earth elements, and iron cannot be changed 

 for either nickel or cobalt. 



