General Physiology of the Plant, 193 



constituent of the substance nuclein, of which the cell- nucleus 

 is mainly composed, and of various proteids or derivatives 

 of them, it also plays an important part in the metabolism 

 of protoplasm ; for no albuminoids can be formed if it be 

 absent. It also enters into the composition of chlorophyll 

 (P- 40). 



POTASSIUM. 



Origin. Potassium is obtained from a variety of salts 

 in the soil, of which the chief are the sulphate, phosphate, 

 and chloride. 



Importance. Potassium, like phosphorus, is necessary 

 for the metabolic processes in the plant. While phosphorus 

 assists in the formation of proteids, potassium exerts a like 

 influence in the formation of carbohydrates. 



CALCIUM. 



Origin. Salts of lime in the soil, such as the sulphate, 

 phosphate, nitrate, and carbonate. 



Importance. The use of calcium in the plant economy 

 is still a matter of some doubt. Probably one of its uses is 

 to form a medium whereby the excess of organic acids may 

 be got rid of (see under sulphur). 



MAGNESIUM. 



Origin. From the same salts as those of calcium. 

 The chlorides of magnesium and of calcium are, however, 

 unfavourable to the growth of the plant. 



Importance. The value of magnesium to the plant is 

 involved in even greater doubt than is that of calcium. It 

 has been found to be a constituent of the globoid associated 

 with grains of aleurone where these occur (p. 157). 



IRON. 



Origin. Many compounds of iron are employed to give 

 the very small amount of that metal required by the plant. 



o 



