rally supposed that they are formed by the decomposition of proteins.
But they are developed in only a few particular species of plants and are
always present in these plants in fairly constant quantities. Hence, it
appears that, in these species, the production of alkaloids is in some way
definitely connected with protein metabolism; but it is certain that this
is not a common relationship, as it is manifested by such a limited number
of species of plants, and there is absolutely no knowledge as to its
character and functions. Some authorities prefer to regard the alkaloids as
waste-products of protein metabolism; but here, again, it is difficult to
understand why such products should result in certain species of plants and
not in others.

THE PURINE BASES

This is a group of compounds, widely distributed in both plant and
animal tissues, all of which are derivatives of the compound known as
_purine_, C_{5}H_{4}N_{4}. All of the naturally occurring compounds of
this group may be regarded as derived from purine, either by the
addition of oxygen atoms, or by the replacing of one or more of its
hydrogen atoms with a methyl (CH_{3}) group or an amino (NH_{2}) group.
The following structural formula represents the arrangement of the
purine nucleus, the numbers being used to designate the nitrogen or
carbon atoms to which the additional atoms, or groups, are attached in
the more complex compounds of the group. In purine itself, the four
hydrogen atoms are attached in the 2, 6, 7, and 8 positions.

6
1N==C--
| | 7
--2C 5C---N--
| | \
| | 8C--
| | //
3N--4C--9N

The double bonds, in each case except those between the 4 and 5 carbon
atoms, are easily broken apart and readjusted, so that other atoms or
groups can be attached to any atom in the nucleus except the 4 and 5 carbon
atoms. In all of the statements with reference to the structure of the
purine bases, the term "oxy" is used to mean an oxygen atom attached by
both its bonds to one of the carbons in the nucleus, instead of its
customary use to mean the monovalent OH group replacing a hydrogen, as in
the case of all other nomenclature of organic compounds. With this