THE VEGETABLE BASES 161 



seeds and are found also in cacao beans and kola nuts. The use 

 of these three compounds in the metabolism of the plants 

 which elaborate them is wholly unknown. They are not so 

 directly related to protein metabolism as are the other purine 

 bases. 



The purine bases, other than the three mentioned in the pre- 

 ceding paragraph, are undoubtedly intermediate products in 

 protein metabolism. In animals, they constitute a large propor- 

 tion of the waste-products from the use of proteins in the body. 

 It is not clear that there are similar waste-products in plant 

 metabolism, however. In both plants and animals, the purine 

 bases which are a part of the nucleic acids undoubtedly play an 

 important and essential part in growth, since they form the major 

 proportion of the nucleus, from which all cell-division proceeds. 



THE PYRIMIDINE BASES 



These compounds do not occur free in plants; but since they 

 are constituent groups in the plant nucleic acids (see below), a 

 brief explanation of their composition is desirable. They are 

 nitrogenous bases, similar to, but somewhat simpler than, the 

 purine bases. Their general composition and structural relation- 

 ships are illustrated by the following typical formulas : 



N=C H H N C=0 



C H O=C 



H C C H O=C C H 



H N C H 



Fyrimidine Uracil 



C<H 4 N 2 O 2 

 2, 6-dioxypyrimidine 



II II 



N C ] 



N=C NH 2 H N C=0 



HI I I 



O=C C H O=C C CH 3 



H N C H H N C H 



Cytosine Thymine 



2, oxy-6-amino- 2, 6-dioxy-5-methyl- 



pyrimidine pyrimidine 



