810 THE ROLE OF NITROGEN, ETC., I., 



Two distinct stages are therefore to be noted in the synthesis, 

 (1) the conversion of nitrate, ammonia, or other simple nitrogen 

 source into amino-acids; (2) the synthesis of amino-acids to 

 proteins. Abderhalden has proved that Aspergillus, whether 

 supplied with nitrate, gtycocoll, or glutaminic acid, etc., builds up 

 its proteins always in the same way; and that the proteins so 

 formed have the same constant composition, as shown by their 

 yielding identical cleavage-products. He therefore concludes 

 that Aspergillus must start from the amino-group in constructing 

 its proteins. 



The amino-acids and amides were proved present in consider- 

 able quantities in green leaves by Kellner,(25) in 1879. Emmer- 

 ling,(26) in 1884, discussed in detail the importance of these 

 compounds in protein-building, and collected much experimental 

 proof. Most of this earlier work depended on microchemical 

 methods which in many cases have proved unreliable, but of the 

 more recent work founded on exact chemical estimations, the 

 following may be cited. Suzuki, (27) in 1898, found that plants 

 fed on sugar-solution and nitrates were able to form protein in 

 the dark, and that the same occurred in plants which contained 

 a large amount of sugar. Hansteen,(28) in 1899, sjmthesised 

 protein from ammonia and glucose in the dark; this was also 

 quantitatively confirmed by Maliniak(29) in 1900. Zaleski,(30) in 

 1905, proved the formation of protein from amino-acids and 

 amides in the dark by exact quantitative analysis, and in the 

 case of bulbs and tubers, without any change in the amount of 

 nitrogen. Since Pasteur, in 1851, first observed the tendency of 

 plants to accumulate amides when grown in the dark or in air 

 free from carbon dioxide, there is now considerable evidence to 

 show that the synthesis of protein is greatly retarded under these 

 conditions; and further, that when such plants are afterwards 

 grown in the light, still without carbon dioxide. (31) they show no 

 diminution of the amides, (32) but when fed on carbohydrates in 

 the dark, or brought into the sunlight and air, soon show an 

 increase in the amount of protein. 



