422 UTILIZATION OF MINERAL SALTS 



interferes with the translocation of carbohj^drates and amino acids. Calcium 

 is of widespread occurrence in plants in the form of calcium soaps and is 

 believed by some investigators to occur in plant cells as calcium proteinates. 

 Calcium is of frequent occurrence in plant cells in the form of insoluble crys- 

 tals of calcium oxalate, and also forms salts by reactions with other organic 

 acids. Some investigators consider that calcium performs an important func- 

 tion in combining with the organic acids which are by-products of protein 

 synthesis, thus preventing the accumulation of organic acids within the cells. 



Magnesium. — This element is the one and only mineral constituent of 

 the chlorophyll molecule. A large proportion of the magnesium present in 

 plants is therefore in the chlorophyll-bearing organs, although seeds are also 

 relatively rich in this element. Magnesium generally occurs in soils in suf- 

 ficient abundance to supply the needs of plants, although occasional exceptions 

 to this statement are found. Deficiency of magnesium results in the develop- 

 ment of a characteristic chlorosis (Table 41). Redistribution of magnesium 

 from older to younger organs of plants occurs readily. 



Magnesium is believed by some workers to be intimately related to oil 

 formation and the synthesis of nucleo-proteins in plant cells. It has been 

 suggested that the role of magnesium in these processes is that of a carrier of 

 phosphates. Phosphates are used in the synthesis of the nucleic acids, one of 

 the constituents of the nucleo-proteins. Phosphates also enter into the com- 

 position of lecithi?i. The formation of oil appears to be preceded or accom- 

 panied by the synthesis of this compound. Filaments of the alga Vaucheria 

 fail to develop oil droplets when growing in media in which magnesium is 

 lacking. Magnesium is much more abundant in oily than in non-oily seeds. 



Excess quantities of magnesium may prove toxic in solution cultures, an 

 effect which may be offset by the presence of sufficient quantities of calcium 

 (antagonism!). It is doubtful, however, if magnesium ever exerts such toxic 

 effects in soils. 



Potassium. — The bulk of the potassium absorbed by a plant ordinarily 

 moves into it in the earlier stages of its life history. Unlike all of the other 

 ash constituents required by plants in appreciable amounts potassium is not 

 definitely known to be built into organic compounds of fundamental physio- 

 logical significance. It occurs in plants almost solely as soluble inorganic 

 salts. Potassium salts of organic acids also occur in plant cells. In spite 

 of this fact potassium is an indispensable element and cannot be completely 

 replaced even by such chemically similar elements as sodium or lithium. The 

 young and active regions of plants, especially buds, young leaves, root tips, 

 etc. are always rich in potassium. Older tissues in which relatively few 

 living cells remain — such as wood — contain relatively little potassium. The 



