102 THE FUNCTIONS OF 



this with the conclusion of Emerson and Lewis on the part they 

 play in the utilization of carbon dioxide in green plants. In- 

 cidentally, Steinberg calls attention to the significance of this 

 similarity in regard to the question of the validity of the sugges- 

 tion of Ruben and Kamen that carbon dioxide utilization by 

 micro-organisms is essentially the same process as the dark or 

 Blackman reaction in photosynthesis. 



So far the functions of the trace elements in general have been 

 considered. With regard to the specific functions of the in- 

 dividual trace elements little, if anything, of any definite value 

 can be said regarding copper, molybdenum and the less well- 

 established trace elements, but contributions of considerable 

 interest have been made regarding the functions of manganese, 

 zinc and boron. A consideration of these follows. 



Manganese. The view has been very generally held that 



manganese is related to oxidation in the plant. Bertrand (1897), 



as we have seen, first called attention to the importance of 



manganese when he found it essential for the action of the 



oxidizing enzyme laccase. Later he maintained that it was 



essential for the action of oxidizing enzymes in general. In 



recent years Lundegardh (1939) has produced more direct 



evidence that manganese is concerned in respiration. Thus he 



found that the oxygen intake by wheat roots was increased by 



155 to 470 per cent by the addition of 0-00005 M manganese 



chloride. Contrasted with this, addition of ferric chloride or 



ferric citrate generally brought about a decrease in oxygen 



intake, the average reduction with 0-00005 M ferric citrate being 



about 21 per cent. From these observations it is concluded that 



manganese, but not iron, catalyses aerobic respiration. 



About the same time Burstrom (1939) examined the assimila- 

 tion of nitrate both by whole wheat roots and by wheat-root 

 pulp in presence of small quantities of iron and manganese, and 

 came to the conclusion that, whereas without iron and man- 

 ganese nitrogen assimilation does not take place, in presence of 

 a small concentration of manganese assimilation takes place 

 both with whole roots and root pulp, the optimum effect being 

 produced with about 4 nig. manganese per litre with whole 

 roots and about 12-3 mg. per litre with pulp. Without added 

 manganese the addition of iron brought about feeble nitrogen 



