104 ' PLANT PHYSIOLOGY 



not oxidized by the molecular oxygen of the air. According 

 to Bach, these substances are oxygen acceptors and have been 

 termed ''oxygenases." As such, they possess little capacity 

 for oxidizing compounds that are difficult to oxidize; they must 

 first be subjected to the action of certain enzymes, ''peroxidases," 

 that split off an atom of oxygen. 



The theory of Bach served as a basis for the theory of respira- 

 tion suggested by Palladin. The latter author assumed that the 

 primary acceptors of oxygen were "respiratory chromogens," 

 substances of a phenolic nature, readily oxidized in the air and 

 giving "respiratory pigments" of a quinone-like structure and 

 easily demonstrated in plant juices owing to their dark color 

 after oxidation. 



The theory of Bach and Palladin created great interest but 

 could not entirely explain the chemistry of respiration; for it 

 gave a satisfactory explanation for the oxidation only of poly- 

 phenols, and attempts to connect this oxidation with the oxy- 

 dative decomposition of sugars remained of a purely theoretical 

 nature. The recent investigations of Warburg represent a very 

 important advance in the study of the process of respiration. 

 He made an attempt to apply the well-known oxidizing power 

 of iron salts, always present in the Hving cell, to explain the 

 process of respiration. 



Warburg interprets the activation of oxygen by the fact that 

 iron, one of the components of complex organic substances, 

 combines with oxygen from the atmosphere and then transmits 

 it to the oxidizable substance. In this process, iron in the ferric 

 condition (Fe+++) is reduced to the ferrous condition (Fe++), 

 which again may combine with oxygen. Proof of the essential 

 significance of iron in respiration Warburg finds in the fact that 

 hydrocyanic acid and hydrogen sulphide are extremely toxic 

 cell poisons that stop respiration. According to his opinion, 

 these poisons block the active atoms of iron, by forming stable 

 compounds with them, and thus they lose their capacity of 

 combining with oxygen. Warburg observed this phenomenon 

 by means of a model of respiration that he constructed using 

 carbonized hemin. This model possessed the ability to oxidize 

 amino acids, in particular cystine. These experiments brought 

 him to an understanding of the primary significance of iron in 

 the process of respiration. 



