THE CHEMICAL PROCESSES IN PLANT RESPIRATION 1 37 



acids in the new edition of the basic handbook "Biochemie der 

 Pflanzen " by F. Czapek^ under the section on plant respiration. 

 From what follows it is clear that this statement can scarcely 

 be regarded as free from objections. 



A. Mayer- declared long ago that in plant respiration under 

 some conditions malic and oxalic acids could be formed in place 

 of carbon dioxide. That the views of A. Mayer depend on 

 observations too weak to hold is evident from the fact that he 

 also assumed the reverse process, i.e. a direct synthesis of sugars 

 from plant acids by light, whereby carbon dioxide was not even 

 temporarily formed. Mayer's theory was then perfected and 

 extended by Warburg^ and by Puriewitsch.'' These authors 

 assumed that sugar is generally oxidised by means of the 

 formation of plant acids containing more oxygen. Finally 

 oxalic acid is said to be formed as the preliminary to complete 

 combustion. Oxalic acid was then erroneously taken to be the 

 plant acid which accumulates in excess. In reality, malic acid 

 is the most important acid product of the metabolism of fleshy 

 fruits and succulents, as is shown by the following table:'' 



Sedum Azureum 



Oxalic Malic 



Acid Acid 



May 25 1.82% 7.62% 



June 17 0.48% 8.73% 



June 21... •. 2.07% 8.42% 



July 8 o. 74% 10. 13% 



July 29 0.35% 7-72% 



The malic acid content of succulents is subject to great 

 variations. It accumulates at night but during the day a 

 decrease in the amount of acid is to be noted. ^ This depends 

 on the oxidation of malic acid to CO2 in the light. In darkness 

 this process ceases. This explains the accumulation of acid at 

 night. 



' Czapek, F. Biochemie der Pflanzen. 2nd. ed. 3: 65-110. 1921. 



2 Mayer, A. Landwirtschaftl. Versuchs-Stationen. 34: 127. 1887. 



'Warburg, O. Untersuch, aus d. bot. Inst. Tubingen. 2: 53- 1886. 



* Puriewitsch, K. Aufbau und Abbau von organischen Sauren in Samenpflanzen. 1893. 



'Andre, G. Compt. rend. 140: 1708. 1905; cf. also Branhofer und Zellner. Z. f. 

 physiol. Chem. 109: 12. 1920. 



6 Kraus, G. Abh. d. Naturf. Halle. Vol. 16. 1884; Lange, P. Inaug. Diss. Halle. 1886; 

 Puriewitsch. loc. cil. [For a discussion of the acids in the cacti see Spoehr, H. A. The 

 carbohydrate economy of cacti. Carn. Inst. Wash. Publ. 287. 1919-! ^^ — 7~p\ ' — v- 



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