The purine bases, other than the three mentioned in the preceding paragraph, are undoubtedly intermediate products in protein metabolism. In animals, they constitute a large proportion 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 relationships are illustrated by the following typical formulas: N==C-H H-N--C=O | | | | H-C C-H O=C C-H ║ ║ | ║ N--C-H H-N--C-H Pyrimidine Uracil C_{4}H_{4}N_{2} C_{4}H_{4}N_{2}O_{2} 2,6-dioxypyrimidine N==C-NH_{2} H-N--C=O | | | | O=C C-H O=C C-CH_{3} | ║ | ║ H-N--C-H H-N--C-H Cytosine Thymine C_{4}H_{3}N_{2}ONH_{2} C_ {4}H_{3}N_{2}O_{2}CH_{3} 2,oxy-6-amino-pyrimidine 2,6-dioxy-5-methyl-pyrimidine THE NUCLEIC ACIDS The nuclei of cells are composed almost wholly of complex organic salts, in which _proteins_ constitute the basic part and _nucleic acids_ the acid part. These salts, or esters, are known under the general name "nucleoproteins." The composition of the proteins is discussed in detail in the following chapter, and it seems desirable to present a brief discussion of the constitution of the nucleic acids here; although they are essentially acids rather than vegetable bases. The nucleic acids are complex compounds consisting of a carbohydrate, phosphoric acid, two purine bases, and two pyrimidine bases. So far as is known, all animal nucleic acids are identical and all plant nucleic acids are identical; but those of plant origin differ from those found in animal cells in the character of the carbohydrate and that of one of the pyrimidine bases which are present in the molecule, as shown in the following tabulation of their composition: