6 STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



proteins of plant and animal origin are very similar in composition. 

 It is, nevertheless, an open question as to what degree we are justified, 

 on the basis of this analogy, in drawing conclusions as to the function 

 of the proteins in the two types of organisms. 



The extraction of proteins from leaves is associated with consider- 

 able difficulty. Winterstein^ demonstrated this by the use of dif- 

 ferent methods on a variety of plants. These difficulties arise from 

 the fact that the proteinaceous material is quite indiffusible and 

 clings tenaciously to the structural elements of the plant, and that 

 any methods of extraction or expression yield complex mixtures 

 from which the protein can be freed only by means of a variety of 

 substances which in some degree also affect the proteins. Very 

 little systematic work has been done on the proteins with a view to 

 determining which are present in green leaves. A good beginning 

 has been made by Chibnall and Schryver,^ who attempted to work 

 out methods for isolating and identifying the proteins in the leaves 

 of some of the higher plants. Osborne, Wakeman, and Leavenworth^ 

 have also outlined a procedure for the separation of various protein 

 constituents from the alfalfa plant. 



Mature leaves are relatively high in proteins. The mesophyll, 

 freed from leaf-veins, often shows values for protein of over 30 per 

 cent of the dry material. Lakon,^ by means of the Molish reaction, 

 was able to demonstrate decided differences in the amount of protein 

 in variegated leaves. The chlorophyllous portions gave a very 

 intense protein reaction, while the albescent portions were poor in 

 proteins. This seems to confirm the opinion that in leaves the prin- 

 cipal masses of protein are located in or about the chloroplasts. 

 In fact, Meyer^ found that as leaves become older the reduction of 

 the green color runs parallel with a reduction in the protein-content 

 of the leaves. According to these investigations the leaf proteins 

 are located chiefly in the chloroplasts, while the nucleus and cyto- 

 plasm seem but very slightly affected by variations in protein- 

 content. Thus the size of the chloroplasts is greatly influenced by 

 the amount of protein in the leaf, in that, as the proteins disappear 

 when the leaf is kept in the dark, the chloroplasts become smaller 

 and contain less chlorophyll, while with an increase in proteins the 



> WiNTERSTEiN, E. Ueber die Sticksto£fhaltigen Bestandtheile greuner Blaetter. Ber. d. deutsch. 



hot. Ges., 19, 326-330 (1901). 

 Also Hamilton, T. S.. W. B. Nevens, and H. S. Grindley. The quantitative determination 



of amino-acids of feeds. Jour. Biol. Chem., 48, 249-272 (1921). 

 s Chibnall, A. C, and S. B. Schryver. Investigations on the nitrogenous metabolism of 



higher plants. Biochem. Jour., 15, 60-75 (1921). 

 » Osborne, T. B., A. J. Wakeman, and C. S. Leavenworth. The proteins of the alfalfa plant. 



Jour. Biol. Chem., 49, 63-91 (1921). 

 * Lakon, G. Biochem. Zeitschr., 78, 145-154 (1916). 

 6 Meyer, A. Eiweisstoffwechsel und Vergilbung der Laubblaetter von Tropaeolum majua. 



Flora, 11, 85-127 (1918). 

 Cf. also CaAPEK, F. Biochemie der Pflamen, 2d ed., 3, 293. Jena (1920). 



