TRACE ELEMENTS IN PLANTS 113 



Catechol or polyphenol oxidase catalyses the oxidation of 

 compounds with the orthodihydroxyphenol grouping such as 

 catechol and pyrogallol. In the presence of the enzyme and a 

 low concentration of catechol a number of other substances such 

 as guaiacum, benzidine and ascorbic acid are also oxidized 

 which are not oxidized by the pure enzyme alone. These further 

 oxidations may require the presence of some substance such as 

 orthoquinone, produced by the oxidation of catechol in the 

 primary reaction, so the presence in plant tissues of a substance 

 either of the catechol or orthoquinone type along with the 

 oxidase would make possible the oxidation of a wide range of 

 phenolic compounds. 



Enzymes of this type are widely distributed in plants, but 

 whether they are all copper compounds it is not yet possible to 

 say. That the enzymes from plants so different from one another 

 as the potato and mushroom both contain copper suggests that 

 this is a possibility, and if this should prove to be so, the func- 

 tion of copper as a catalyst in vital oxidations can be regarded 

 as established. 



Reference has already been made to Thatcher's opinion that 

 zinc and copper form a pair of mutually co-ordinating catalysts 

 for oxidation-reduction reactions. It is therefore interesting to. 

 find that Reed and Dufrenoy connect the action of zinc in plants 

 with the distribution of the copper-containing polyphenol 

 oxidase. 



Boron. Various functions have been ascribed to boron in 

 higher plants. The effects attributed to the action of this element 

 include an influence on the water relations of the protoplasm, 

 a favourable influence on the absorption of cations and a 

 retarding influence on the absorption of anions, a favourable 

 influence on the absorption of calcium, a part in the formation 

 of pectic substances in the cell wall, and an essential part in 

 carbohydrate and nitrogen metabolism. 



As regards the supposed effect of boron on the water relations 

 of the protoplasm, it has already been pointed out that a very 

 general symptom of boron deficiency, and often one of the 

 earliest internal symptoms, is an enlargement of thin-walled 

 cells. In this connexion some observations made by Schmucker 

 (1933. 1935) on the germination of the pollen grains of tropical 



