112 THE FUNCTIONS OF 



Skoog also placed sections of stems from zinc-deficient and 

 control plants, from which auxin had previously been removed, 

 on agar blocks containing a known concentration of indole-3- 

 acetic acid, and found that always more of this growth hormone 

 was inactivated in the blocks in contact with tissue from zinc- 

 deficient plants than in the blocks in contact with control tissue. 

 This suggests that deficiency of zinc brings about excessive 

 destruction of auxin, an effect attributed to an increased oxida- 

 tive activity of the tissues, since these displayed increased 

 capacity to oxidize benzidine in presence of hydrogen peroxide. 

 That zinc functions as a catalyst in relation to oxidation- 

 reduction processes in the cell is thus again indicated, while its 

 relationship to auxin maintenance suggests why zinc deficiency 

 may lead to retardation or cessation of growth. 



As will be mentioned later, zinc enters into the composition 

 of the molecule of the enzyme carbonic anhydrase, which cat- 

 alyses the action H 2 C0 3 ^C0 2 + H 2 0. As far as the writer is 

 aware this enzyme has so far only been found in animals, but 

 if it should occur in plants, it would suggest that zinc might be 

 concerned in the excretion of carbon dioxide and possibly in 

 other processes about which, in the absence of any information, 

 it would be idle to speculate. It may, however, be pointed out 

 that deficiency of zinc would then mean a shortage of the enzyme 

 catalysing the release of carbon dioxide so that this end product 

 of the oxidation of carbohydrate would accumulate as carbonic 

 acid in the tissues. This would lead to a slowing down of the 

 oxidation process and conceivably to just such an interference 

 in the normal oxidation mechanism in the tissues as Reed and 

 Dufrenoy hypothesize. 



Copper. Precise information regarding the functions of 

 copper in the plant is forthcoming on one point: it has been 

 found that this metal enters into the composition of the oxidizing 

 enzyme or enzymes known as catechol oxidase or polyphenol 

 oxidase. This was shown by Kubowitz (1937) for the oxidase 

 present in potato tuber and by Keilin and Mann (1938) for the 

 similar oxidase present in the cultivated mushroom (Agaricvs 

 campestris). The enzyme, in fact, appears to be a copper- 

 protein compound containing not less than 0-30 per cent of 

 copper. 



