tein 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, a