CHAPTER XXV 



UTILIZATION OF MINERAL SALTS 



A clear distinction should be drawn between the absorption of a salt and 

 the subsequent utilization of it or its component ions. The term utilization 

 is emplo3'ed in a loose sense to refer to the incorporation of mineral elements 

 into the relatively permanent constituents of the cell walls and protoplasm. 

 Absorption of the ions or molecules of salts does not necessarily mean that 

 they will be utilized. Many of the ions absorbed by a plant remain for more 

 or less indefinite periods in the ionic state in the cells. Sooner or later many 

 of these ions are usually incorporated either into the structure of more com- 

 plex but unassimilated molecules synthesized by the plant such as storage 

 proteins, calcium oxalate, glycosides, etc., or into the protoplasm or cell walls 

 proper. There may, therefore, be a considerable time lag between the absorp- 

 tion of an ion and its utilization, while some of the absorbed ions may remain 

 indefinitely as such within the cells. Furthermore, some mineral elements 

 may be utilized in one organ of a plant, subsequently released by disintegra- 

 tion of cell constituents, translocated to other organs of the plant, and there 

 reutilized. Redistribution of minerals which have accumulated in cells but 

 have not actually been utilized is also of common occurrence in plants. 



General Roles of Mineral Elements in Plants. — Considered as one 

 group or class of substances found in plants, the mineral elements function in 

 a number of distinct ways : 



I. Building Material from Which Parts of Protoplasm and Cell Walls 

 Are Constructed. — A number of the mineral elements become permanent parts 

 of molecules which are integral parts of the protoplasm and cell walls. As 

 examples we may cite the sulfur in proteins generally, the phosphorus in 

 nucleo-proteins and lecithins, the magnesium in chlorophyll, the calcium in 

 calcium pectate, etc. Uncritical discussions of mineral elements in plants 

 often consider this to be the sole role of these elements. Actually a very 

 large proportion of the mineral elements in plants act in some other way 

 than as material from which essential parts of plant cells are constructed, or 

 else are of no apparent consequence in the metabolism of the plant whatso- 

 ever. 



415 



