TRACE ELEMENTS IN PLANTS 101 



emphasized, however, that if the parts the various elements play 

 are similar, they are not identical, for they cannot replace one 

 another in the organism. Reference has already been made to 

 Thatcher's view that manganese and iron form a pair of mutually 

 coordinating catalysts for oxidation-reduction processes in- 

 volving the addition and removal of oxygen, while zinc and 

 copper form another pair concerned in similar processes in- 

 volving the addition and removal of hydrogen. 



Since deficiencies of manganese, zinc and copper character- 

 istically induce chlorosis, it is understandable that they have 

 been held to be concerned in chlorophyll formation (cf. McHargue. 

 1923, 1926 a; Bishop, 1928). They have also been thought to act 

 as catalytic agents in photosynthesis (see e.g. Stoklasa, 1911; 

 McHargue, 1923; Dufrenoy and Reed, 1934). Recent support 

 for this view is forthcoming from work by Emerson and Lewis 

 (1939), who found that when the trace elements of Arnon's 

 groups A 4 and B7 (see p. 9) were added to the culture medium 

 of Chlorella pyrenoidosa not only was the rate of growth of the 

 alga increased but the amount of photosynthesis per unit 

 quantity of light absorbed was also increased. The trace elements 

 were added in two groups, those of Arnon's A 4 group + molyb- 

 denum, now denoted by the symbol A 5, and those of Arnon's 

 B7 group without molybdenum, this group now being denoted 

 B6. The addition of the A 5 group was more effective than the 

 B6 group, and the addition of A5 + B6 more effective than 

 either alone. It would thus appear that the trace elements play 

 some part in the photosynthetic process, but what this is can at 

 present be only a matter of conjecture. 



Some observations of Steinberg (1942) may be mentioned here. 

 Starting from the observation that when nitrogen is supplied 

 only as nitrate the growth of the fungus Aspergillus niger is 

 lessened in air deprived of carbon dioxide, he examined the 

 effect of removal of carbon dioxide on the growth of this fungus 

 in absence of various micro-nutrients. Under these conditions 

 omission of zinc, copper or manganese from the culture medium 

 reduced the growth of the fungus proportionately more when 

 carbon dioxide was absent than when it was present. Steinberg 

 concludes that the trace elements probably play a specific part 

 in the utilization of carbon dioxide by A. niger, and compares 



