TRACE ELEMENTS IN PLANTS 107 



necrotic areas developed in the most chlorotic leaves, this being 

 accompanied by death of the stem apices. 



In both it was possible to correct the pathological symptoms 

 by altering either the iron or manganese concentration in such 

 a way as to bring the Fe/Mn ratio to a value between 1-5 

 and 2-5. A further indication of the importance of the Fe/Mn 

 ratio was obtained by Somers, Gilbert and Shive (1942) in the 

 respiration rate of soya beans in water cultures supplied with 

 different proportions of the two nutrients. Respiration rates 

 were always definitely lower when the Fe/Mn ratio was outside 

 the range 1-5-2-5 than when it was within it. 



In further support of the oxidation-reduction hypothesis out- 

 lined above, Somers and Shive mention a series of tests carried 

 out with maize seedlings in which cobalt was substituted for 

 manganese. The oxidation potential of cobalt is higher than 

 that of manganese, so it should, on the hypothesis, have a 

 greater tendency than manganese to lessen the metabolic 

 efficiency of iron by effecting the oxidation of the latter to 

 the insoluble ferric state, and this, Somers and Shive state, 

 was so. 



The work of Shive and his associates constitutes a very strong 

 argument in favour of the hypothesis they present regarding 

 the connexion between iron and manganese. An extension of 

 such experiments to other species is clearly very desirable. If 

 the connexion, which means that manganese deficiency is the 

 same thing as iron excess and vice versa, should prove to be 

 general, it must be admitted that it has not so far been realized 

 by those workers who have had much experience with the effect 

 of mineral deficiencies in the field. It would, however, afford an 

 explanation of why a characteristic manganese deficiency has 

 been observed in plants containing much manganese, while 

 in others the symptoms of this deficiency have not appeared 

 when the manganese content has been exceptionally low 

 (cf. p. 59). 



In a study of cation absorption by tobacco, Swanback (1939) 

 made some observations on the effect of manganese on the 

 absorption of potassium and calcium. The plants were grown in 

 nutrient solutions containing calcium in three different con- 

 centrations. In the absence of manganese the symptoms of 



