THE ABSORPTION OF MINERAL ELEMENTS 69 



ism. The reason that this artificial grouping is retained in text- 

 books is not due to their close relationship from a physiological 

 point of view, but rather to the similarity of their origin. All of 

 them are obtained from the mineral constituents of the soil. 

 That is the reason for their being called mineral substances of the 

 plant. 



The major ash elements may be divided into two distinct 

 groups: the metalloids, such as sulphur and phosphorus, and the 

 metals — potassium, calcium, magnesium, and iron. Sulphur and 

 phosphorus are found combined with the most important organic 

 substances of the protoplasm in the proteins and nucleoproteins. 

 As they possess a number of similar properties, S and P may be 

 compared with nitrogen, which is also indispensable for the building 

 of the protein molecule. The metals, on the contrary, are present 

 in the plant as free ions. This is especially true with regard to 

 potassium, up to 90 to 98 per cent of which is found in the plant 

 in ionic form and may be extracted from the dry-plant substance 

 by water, combustion not being necessary. Metallic elements 

 play the role of regulators of vital processes. 



This, however, is not the only function of the metals. Some 

 metallorganic compounds are present in the most important parts 

 of the cell, where they have a very vital function. Magnesium, for 

 instance, is found in the chlorophyll molecule. Hence at least 

 some of the metals may be considered as nutrient substances. 

 Phosphorus, as phosphoric acid, on the other hand, may be found 

 also in inorganic form. In leaves, up to 50 per cent of the phos- 

 phorus is found in the form of free phosphoric acid, and during the 

 germination of the seeds an almost complete disintegration of all 

 organic phosphorus compounds takes place. In general, then, the 

 separation of the ash elements into two main groups, as nutrient 

 substances and as regulators of vital processes, indicates more 

 clearly the function they may assume in the life of the plant. It 

 must not be forgotten that the same element in some combina- 

 tions may be a part of one group; in other combinations, it may 

 be in another group. 



24. Sulphur and Phosphorus. Their Function in the Plant 

 and Their Cycle in Nature. — Sulphur and phosphorus are just 

 as important to the plant as is nitrogen. Both of these elements 

 enter into the composition of protein substances, sulphur being 

 found in all proteins, phosphorus only in the more complex ones — 



