SPECIFIC ROLES OF MINERAL ELEMENTS IN PLANTS 423 



proportion of potassium in seeds is also low. Internal redistributions of this 

 element occur readily and more or less continuously during the life history 

 of a plant. Older leaves and other organs frequently lose potassium which 

 is transported to growing regions. 



The exact role of potassium in plants is obscure. Since it apparently 

 is not used in the construction of any vitally necessary cell constituents its 

 role is undoubtedly to be interpreted as chiefly a regulatory or catalytic one. 

 It may exert many of its effects by influencing enzymatic activity. Certain 

 results of a deficiency of potassium upon plant metabolism have long been 

 recognized. While the evidence for none of these effects can be regarded as 

 incontrovertible, potassium appears to be necessary for the normal maintenance 

 of the following processes : ( i ) the synthesis of simple sugars and starch, 

 (2) translocation of carbohydrates, (3) reduction of nitrates, (4) synthesis 

 of proteins, particularly in meristems, and (5) normal cell division. This 

 last effect is probably closely connected with the role of this element in the 

 synthesis of proteins. In the absence of potassium the cells elongate, but do 

 not divide. 



The potassium ion is usually the most abundant univalent cation in plant 

 cells, and undoubtedly exerts important effects upon such phenomena as the 

 permeability of the cytoplasmic membranes, hydration of the protoplasm, etc. 



Iron. — A deficiency of available iron in soils is seldom a limiting factor 

 in plant development, although occasional exceptions to this statement are 

 encountered. Deficiency of iron in soils is usually due to its insolubility 

 rather than to its actual absence. In general a larger proportion of the iron 

 is in a soluble state in relatively acid soils than in approximately neutral or 

 alkaline soils. 



Iron is indispensable for the synthesis of chlorophyll in green plants. 

 Deficiency of this element results in the development of a characteristic 

 chlorosis (Table 41). Iron does not, however, enter into the constitution 

 of the chlorophyll molecule. The state of the iron in plant tissues is also 

 often a factor determining its influence in chlorophyll synthesis. Chlorosis 

 due to iron deficiency is sometimes found in plants which contain considerable 

 quantities of iron (Rogers and Shive, 1932). In such plants the iron is 

 present in a precipitated or otherwise unavailable form. 



Iron is also supposed to act as a catalyst or oxygen-carrier in oxidation- 

 reduction processes occurring in living cells. Living protoplasm contains traces 

 of organically bound iron which may function in this way. 



Iron salts in any appreciable concentration are toxic to plants; for equal 

 concentrations ferrous salts are generally more toxic than ferric salts. While 

 toxic effects of iron can readily be demonstrated in solution cultures, it is 



