52 THE SYNTHESIS OF PROTEINS ] 



resulted in an increase of protein. In darkness, on the other 

 hand, a less vigorous development obtained and only those 

 plants supplied with potassium salts showed protein formation. 



It would therefore appear that the action of potassium is 

 partly indirect, and only in darkness does it play an important 

 role in the production of proteins. Since potassium is mildly 

 radioactive, it may possibly serve as a source of energy in 

 promoting the analysis of carbohydrate.* 



These observations introduce the problems connected with 

 the light factor. Earlier opinions were that light was an 

 important and a direct factor in the synthesis of proteins : 

 radiant energy is certainly of great importance in that it 

 is a necessity for the green plant in the making of carbo- 

 hydrate, which is in its turn a requisite in the formation of 

 proteins. 



The obvious essentiality of nitrogen in the building of 

 proteins and the fact that in general the element is absorbed 

 by the plant in the form of nitrate has been remarked upon : 

 but nitrate as such is a relatively inert substance and does 

 not readily lend itself to chemical change; nitrite, on the 

 other hand, is a more labile substance. 



That the plant is able to convert nitrate into nitrite was 

 first observed by Laurent,f and later Irving and Hankinson,J 

 working on Sagittaria> came to the conclusion that nitrite 

 must be an intermediate compound in the metabolism of 

 nitrates. 



With regard to the occurrence of nitrites in the plant, Aso 

 has established their occurrence in etiolated potato shoots. 

 In this connexion it is of interest to note that so long ago 

 as 1888 Schimper,|| experimenting with cut leaves of Sambucus 

 and with potted plants of Pelargonium zonale, found that 

 nitrates were destroyed in green leaves exposed to daylight 

 but were not so destroyed if the leaves were kept in the dark, 

 and in agreement with this, shade leaves were found to be 

 richer in nitrates than sun leaves. Furthermore, no destruc- 

 tion of nitrate occurs in etiolated leaves exposed to sunlight. 



* See Stoklasa: " Biochem. Zeitsch.," 1916, 73, 107. 

 tLaurent: "Ann. Inst. Pasteur," 1890, 4, No. n. 

 % Irving and Hankinson: " Biochem. Journ.," 1908, 3, 87. 

 Aso : " Beih. Bot. Zentr.," 1903, 15, 208, and 1914, 32, 146. 

 HSchimper: " Bot. Zeit.," 1888,46, 128. 



