ORGANIC SUBSTANCES AS PLANT FOOD 221 



2. Diamino acids: 



rf-Lysine, CH 2 NH 2 • CH 2 • CH 2 • CH 2 • CHNH 2 • COOH 



NH, X 

 d-Arginine, ^CNH • CH 2 • CH 2 • CH 2 • CHNH 2 • COOH 



NH^ 



3. Oxyamino acids: 



/-Serine, CH 2 OH-CHNH 2 -COOH 



4. Thioamino acids: 



/-Cystine, COOH • CHNH 2 • CH 2 S • SCH 2 • CHNH 2 • COOH 



B. Aromatic series: 



1. /-Phenyl alanine, C 8 H 6 -CH 2 -CHNH 2 -COOH 



2. /-Tyrosine, C 6 H 4 OH-CH 2 -CHNH 2 -COOH 



C. Heterocyclic compounds: 



1. /-Proline, (C 4 H 8 N)-COOH 



2. /-Tryptophane, (C 8 NH 6 )-CH 2 -CHNH r COOH 



3. Z-Histidine, (C 3 N 2 H 3 )-CH 2 .CHNH 2 -COOH 



A detailed study of proteins and their products of hydrolysis 

 would be out of place here. Further discussion of this subject can 

 be found in text books of organic or physiological chemistry. It 

 should be pointed out, however, that it is the aromatic nucleus of 

 tyrosine which gives the xanthoproteic and Millon's reactions. 

 Cystine is the typical compound in which form sulphur enters into 

 the composition of the protein molecule, and from it is obtained 

 hydrogen sulphide, which is always formed during the complete 

 decomposition of proteins in the process of putrefaction. Among 

 the products of hydrolysis of proteins, besides amino acids, a 

 small amount of ammonia, and often carbohydrates also are 

 found. 



The percentage relationship of various amino acids varies in 

 different proteins, and it is exactly these differences in the relation- 

 ship of amino acids which are the basic cause of the enormous 

 variety of protein substances. Thus, for example, the reserve 

 proteins from the seeds of some agricultural plants contain the 

 following quantities in percentage of the most important amino 

 acids and ammonia : 



