THE ABSORPTION OF MINERAL ELEMENTS 73 



surprised that the presence of an intricate system of anions and 

 cations is necessary for the complex colloidal phenomena which 

 are part of the life of the cell. 



The requirements of the cell of a definite combination of 

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

 of them. By excluding from the nutrient solution one or another 

 of the elements we can only ascertain 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 138 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 might have been a greater storage of 

 these elements 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 certainty is that each of the cations has its specific 

 role and cannot be substituted by another, even one closely related 

 in the periodic system. Thus potassium cannot be substituted 

 either by lithium or by sodium, and only a few fungi allow a partial 

 substitution of potassium by rubidium. Calcium, likewise, cannot 

 be replaced by any of the alkali elements, and iron cannot be 

 substituted either by cobalt or nickel. The role of the various 

 metals in the life of the plant will be discussed only in general. 



Potassium is closely connected with the vital activity of proto- 

 plasm, since most of it is found in the meristem of the young organs, 

 the cells of which are rich in protoplasm. An important role 

 is ascribed to potassium in the general metabolism of the cell, 

 especially in the formation of the carbohydrates and proteins. In 

 the active regions of the plant as much as 50 per cent of K2O may 

 be found in the ash. Of all the necessary cations, potassium is the 

 only radioactive element. Thus it may have also some other 

 significance for the plant as yet unexplained. 



The magnesium content in the ash is considerably less than 



