62 PLANT PHYSIOLOGY 



which decomposes sugar into alcohol and carbon-dioxide, a body we shall 

 have to consider in the next lecture. It need only be noted here that, as 

 already said, probably no alcohol is formed in normal respiration. This com- 

 pels us to assume the existence of another enzyme related to but not identical 

 with zymase. Further statements, a propos of this subject, rest on too hypo- 

 thetical a basis to justify the space their discussion would occupy. On the 

 other hand, oxidizing enzymes in great numbers have been often demonstrated 

 in the plant since the time of SCHONBEIN, and have been more recently very 

 carefully studied. They ought not to be compared in their activity with 

 ' active oxygen ', for this latter must, if present in the cell, attack all oxidizable 

 bodies it could not oxidize the sugar and leave the protoplasm and cell-wall 

 untouched. The oxidizing enzymes^ on the contrary, have specific actions. 

 Most of them that have been thoroughly studied oxidize bodies which do not 

 concern us here (comp. CZAPEK, 1907, 523), while the hypothetical oxidases 

 of carbohydrates, &c., have as yet been but little studied. 



Another conception of respiration has been suggested by BACH and CHODAT 

 (1903). They liken it to the oxidation of self-oxidizing substances outside 

 the organism, which apparently takes place by the taking up of molecular 

 oxygen by the self-oxidizing bodies, with the formation of a peroxide. Per- 

 oxides (e.g. H 2 O 2 ), however, readily give off ' active oxygen ' in the transition 

 to simple oxides, and this active oxygen is able on its part to oxidize other 

 bodies not easily oxidizable. In the plant, therefore, there would be one 

 enzyme known as oxygenase, which plays the part of the self-oxidizing body, 

 becoming a peroxide under the influence of atmospheric oxygen. This peroxide 

 would then be similarly stimulated by a second enzyme (peroxydase) to a 

 more vigorous evolution of oxygen, just as H 2 O 2 is by salts of iron, and the 

 oxygen atoms split off would bring about the combustion of the carbohydrate. 

 The advantage of this conception is that it links respiration with the well- 

 known processes of inorganic oxidation, but its disadvantage lies, as it appears 

 to us, in failing to unite ordinary and intra-molecular respiration. It is to be 

 noted that the existence of yet a third enzyme has been demonstrated in the 

 plant, viz. a catalase. This enzyme splits off O 2 from peroxide, and destroys any 

 excess without giving origin to any ' active oxygen ' (comp. BACH and CHODAT). 



We note, in the first place, that both these conceptions are purely hypo- 

 thetical ; both have this in common that they attribute respiration to enzyme 

 action, and all that the protoplasm does is to construct these enzymes. Pre- 

 viously one had been usually accustomed to ascribe to the protoplasm another 

 and much more important function. According to a view expressed by PFLUGER, 

 and emphasized from the botanical point of view by DETMER especially (1883), 

 the protoplasm itself ought to be regarded as constantly undergoing oxidation, 

 and as constantly being rebuilt. The disappearance of sugar in respiration 

 would, according to this theory, be due not to its direct oxidation, but to its 

 employment in the reconstruction of the oxidized plasma. This conception 

 has never been proved, and it requires no express refutation. We need only 

 remark that the phenomena discussed in Lecture XVI the respiration of 

 inorganic substances may serve as a standing criticism of PFLUGER'S idea. 

 Thus the question whether it is always proteid (protoplasm) that must be 

 respired must be answered in the negative. 



205, 11. 21-42, for in our next lecture . . . plant economy read As to special 

 functions we may refer to Lecture XXIV, and only remark here that life itself 

 is endangered by continuous withdrawal of oxygen. According to DUDE'S 

 (1903) experiments, Aspergillus lived only 4-4^ hours when it was supplied 

 with sugar, and it died in an hour or even 40 min. when supplied with glycerine 

 or tartaric acid substances, that is to say, which are less suitable for support- 



