THE CHEMICAL PROCESSES IN PLANT RESPIRATION II 7 



and make possible their oxidation by means of atmospheric 

 oxygen. In fact it must be pointed out that the peroxidase 

 preparations so far exhibited from plants, only oxidise the 

 hydrogen of phenols and are unable to break carbon chains and 

 in general to unite the oxygen of peroxide with carbon. Wie- 

 land's theory may be successfully applied to cases in which the 

 oxidation is wholly or partly due to the spHtting-off of hydrogen. 

 Then it serves well and provides the explanation of processes 

 which present difficulties for the Bach-Engler theory. An 

 example of these is the following oxidation described by Thun- 

 berg.^ Succinic acid is easily changed to fumaric acid in the 

 presence of muscle extract and methylene blue (M) as the 

 hydrogen acceptor: 



COOH— CHo— CHo— COOH + M = COOH— CH = 



CH— COOH + MHo. 



Furthermore, fumaric acid can change to malic by the fixa- 

 tion of water. 



COOH— CH=CH— COOH + HoO = COOH— CHo— 



CHOH— COOH. 



In this way it is possible to explain a whole series of bio- 

 chemical reactions which involve a transfer of hydrogen or 

 oxygen. It must be borne in mind, however, that there are 

 objections to Wieland's theory as an explanation of all the 

 processes of oxidation in plant cells. Wieland- himself expresses 

 the opinion that his theory possesses a general validity and that 

 the Bach-Engler theory should be abandoned as too weak to 

 hold.' 



1 Thunberg. Skand. Arch. f. Physiol. 30: 285. 1913. 



2 Wieland, H. Ber. d. chem. Ges. 55 : 3639. 1922. 



i A criticism of Wieland's theory is to be found in a paper by Batelli and Stern 

 (Compt. rend. soc. physiques et hist. nat. Geneve. 37: 65. 1920) while an 

 entirely new idea was recently advanced by Quastel (Biochem. Jour. 20: 166-194. 

 1926). This new theory was evolved primarily to explain the results of Quastel's 

 studies of dehydrogenations produced by resting bacteria but it offers interesting 

 suggestions for other oxidations and reductions. Space does not permit a full 

 summary but the basic idea is that of an activation of the substance to be 

 oxidised by reason of the action of an electrical field of force and the consequent 

 shifts in certain chemical bonds within the molecule. This theory is said to 



