8 



STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



The relation of protein nitrogen to non-protein nitrogen can be 

 seen from Pigorini's* results of the analysis of Morus leaves (table 2). 



The true function of the leaf proteins is a question on which there 

 exists very little definite information, although it has been considered 

 one of the most vital problems in plant physiology. In the nitrogen 

 economy of animals it is apparent that a portion of the assimilated 

 proteins serves to replace the 'Svear and tear" on the protoplasmic 

 machinery. For this Rubner introduced the conception ''repair 

 quota" of protein. Also a "growth quota" of protein is necessary 

 to supply material for the development of protoplasm in new cells 

 in growing organisms. When protein is available in such amounts 

 beyond the requirements of "repair quota" and "growth quota," 

 it can be converted into glucose and fatty acids and thus serve as 

 fuel, very much as though these substances had been ingested as 

 food. To such material Rubner gave the term "dynamic quota." 

 Under conditions of copious protein-supply the animal can to a degree 

 store or deposit the protein in the tissue cells. It seems to be an 

 undecided question whether such protein becomes a part of the 

 living tissue or is stored by the cells very much as glycogen is. The 



Table 2. 



animal organism, unlike the plant, is constantly losing a certain 

 (luantity of nitrogen, and it appears that if this quantity is not re- 

 ])laced by ingested protein-food the proteinaceous body-tissue is 

 drawn upon. 



In the autotrophic plant the condition seems to be quite different. 

 Here also, however, it is first of all essential that, in the discussion, 

 well-defined conditions be established and that generaUzations be 

 not made too inclusive. This has been an unfortunate feature of the 

 contributions to protein metaboUsm in plants. It should hardly 

 need emphasis that results with the fungi may not find immediate 

 application to autotrophic plants, nor that the behavior of germinat- 

 ing seeds corresponds to mature chlorophyllous leaves. In general, 

 it appears as though plants were exceedingly economical with their 



' C:zAPKK, F. Biochemie der Pfianzen, vol. ii, 294; 2d ed., 1920. 



