TRACE ELEMENTS IN PLANTS 103 



assimilation of whole roots but not of pulp, and even this 

 assimilation by whole roots is ascribed by Burstrom to its effect 

 on respiration and ion uptake. His general conclusion is there- 

 fore that manganese, and not iron, directly catalyses nitrate 

 assimilation. 



It will be observed that the observations of both Lundegardh 

 and Burstrom emphasize the contrasting effects of manganese 

 and iron. A number of other workers have called attention to 

 this, and indeed there appears to be considerable reason to 

 suppose that the function of manganese is to be found in its 

 relation to the oxidation and reduction effected by iron salts. 

 Thus Hopkins (1930), from observations on the growth of the 

 unicellular green alga Chlorella, held that manganese brings 

 about the reoxidation of iron after its reduction in the plant to 

 the ferrous state ; if the amount of manganese in the plant is 

 deficient there results too high a proportion of ferrous iron, 

 while if manganese is present in excess the reduction of ferric 

 iron is prevented and the concentration of ferric iron is too high. 

 In either condition the oxidation-reduction processes of the cell 

 involving iron are disturbed. 



On this view it is to be expected that the ratio of manganese 

 to iron in the plant is of more importance than the absolute con- 

 centration of manganese. In this connexion it may be noted that 

 Bertrand (1912 b, c) found the ratio of manganese to iron + zinc 

 determined the development of conidia in Aspergillus niger, the 

 capacity to produce these reproductive bodies being inhibited 

 if the ratio Mn/Fe + Zn was too low. With higher plants the im- 

 portance of the ratio of manganese to iron has been emphasized 

 by a number of workers. Thus Pugliese (1913) and also Totting - 

 ham and Beck (1916) write of an antagonism between iron and 

 manganese in the growth of wheat, and the former gives the 

 optimum ratio of the two elements in the culture solution as 

 1/2-5. Scharrer and Schropp (1934) found that with maize in 

 water culture the growth of the roots was at a maximum when 

 the ratio of Mn/Fe in the culture solution was 1/7. That chlorosis 

 might be induced by manganese when iron is deficient was 

 reported by Gile in 1916 in the pineapple and again by Scholz 

 in 1934 in the blue lupin. 



The relation between manganese and iron in the plant has 



