128 THE ROLK OF NITROGKN IN PLANT-MKTABOLISM, iv., 



and peptones. Then ammonia is always set free as the 

 result of the cleavage of protein. 



There is invariably formed in the leaves of plants, by a 

 secondary action, a group of substances not found in artificial 

 hydrolysis, viz. — the amides, glutamin and asparagin. They 

 are formed from ammonia and carbohydrates, and hence the 

 ammonia never accumulates in the plant under normal con- 

 ditions. This cleavage of protein is the work of the protease 

 ferments, which have been shown to have a wide distribu- 

 tion in plants. 



The soluble cleavage-products are carried to the growiag 

 seed, where the protein is regenerated, probably by the 

 reverse action of ferments, since both protease and erepsin 

 have been found in unripe seeds* ; and it has been shown 

 that t>he same ferments are able both to hydrolyse and 

 synthesise proteins under suitable conditions.! Emmerling]: 

 and others were led, by their results, to the view that the 

 amides were the principal material for the synthesis. 



Now the only amides yet discovered in plants are those 

 of aspartic acid and glutaminic acid, and these are only 

 two out of about twenty known hydrolytic products of plant- 

 proteins. It is, therefore, incomprehensible how the entire 

 molecule of the protein can be synthesised chemically from 

 amides. We must rather look for an interpretation based 

 on our present knowledge of the protein-chemistry ; and, 

 first, when we consider the manner in which the hydrolysis 

 of proteins is brought about by enzymes /// vitro, we find a 

 large proportion of those fii'st cleavage-products, meta- 

 proteins, albumoses, peptones, and the larger polypeptides, 

 with only small amounts of amino-acids. Then we should 

 expect to find these substances in the translocating material 

 of the living plant. 



* Zalkski— Ber. Bot. Ge.s. 23. 

 t Taylor— Jourii. Biol. Chem. 3, 1907, p. 87. 



Brailsford Robkrtson— i7«d. p.95. 

 X Landw. Versuchs-Stat. B.34, 1; B.54, 215. 



