104 THE FUNCTIONS OF 



more recently been dealt with by Shive (1941). This writer calls 

 attention to two important points in this connexion, the first 

 being that the active functional iron in the tissues is in the 

 ferrous state, the second that the oxidizing potential of man- 

 ganese is higher than that of iron. Shive holds that if iron is 

 absorbed in the ferric state much of it is reduced in the plant by 

 powerful reducing systems unless this is prevented by a counter- 

 reactant. The manganese functions as such a counter-reactant, 

 oxidizing ferrous to ferric iron which is precipitated, probably 

 in organic complexes. Hence, if manganese is deficient in the 

 plant, there will be an excess of active ferrous iron which induces 

 chlorosis, a chlorosis due to iron toxicity. On the other hand, 

 if the concentration of manganese is high the concentration of 

 active ferrous iron is low, and if too low a chlorosis due to 

 iron shortage will result. Thus it is necessary for healthy growth 

 that the proportion of iron to manganese should lie within 

 certain limits, and Shive concluded that, for the species in- 

 vestigated by him, the ratio of active iron to active (soluble) 

 manganese in the plant should lie between 1-5 and 2-5. This 

 conclusion was derived from the results of a series of culture 

 experiments with soya bean described by Somers and Shive 

 (1942) in which the quantities of both iron and manganese in 

 the culture solutions were varied. They used in all eighteen 

 different combinations of iron and manganese in their culture 

 solutions. The iron content varied from 0-005 to 3-00 p. p.m. and 

 the manganese content from to 5-00 p. p.m.; the various 

 combinations are shown in Table IX. Actually complete absence 

 of manganese is never attained in the culture itself since there 

 will be some present in the seed used and no doubt a little will 

 also be introduced as impurity either from culture vessels or 

 other nutrient salts used. The culture solutions also contained 

 the usual major nutrients and boron. The concentrations of iron 

 and manganese and a pK of 4-6-4-8 (to prevent precipitation of 

 iron) were maintained approximately constant by the use of a 

 technique in which a continuous flow of solution passed through 

 the culture vessels, and the solutions were completely changed 

 every other day. 



Approximate determinations of both soluble and insoluble 

 iron and manganese in the tissues were also made. To separate 



