ic acids, or their derivatives, which have
pronounced odors or flavors, in the flesh surrounding the seeds of fruits,
in the endosperm of vegetable seeds, or in the tubers, etc., of perennial
plants, thus making them attractive as food for animals and men,
undoubtedly serves to insure a wider distribution of the reproductive
organs of these plants; a fact which has unquestionably had a marked
influence upon the survival of species in the competitive struggle for
existence during past eras and in the development and cultivation of
different species by man. Indirect evidence that the proportion of these
attractive compounds present in certain species may have been considerably
increased by the processes of "natural selection" in the past is furnished
by the many successful attempts to increase the percentage of such
desirable constituents in fruits or vegetables by means of artificial
selection of parent stocks by skillful plant breeders.

CHAPTER X

FATS AND OILS, WAXES, AND LIPOIDS

Included in this group are several different kinds of compounds which have
similar physical properties, and which, in general, belong to the type of
organic compounds known as esters, i.e., alcoholic salts of organic acids.
The terms "oil," "fat," and "wax," are generally applied more or less
indiscriminately to any substance which has a greasy feeling to the touch
and which does not mix with, but floats on, water. There are many oils
which are of mineral origin which are entirely different in composition
from natural fats. These have no relation to plant life and will not be
considered here.

The natural fats, vegetable oils, and plant waxes are all esters. There is
no essential difference between a fat and an oil, the latter term being
usually applied to a fat which is liquid at ordinary temperatures. The
waxes, however, are different in chemical composition from the fats and
oils, being esters of monohydric alcohols of high molecular weight, such as
cetyl alcohol, C_{16}H_{33}OH, myristic alcohol, C_{30}H_{61}OH, and
cholesterol, C_{27}H_{45}OH; whereas the fats and oils are all esters of
the trihydric alcohol glycerol, C_{3}H_{5}(OH)_{3}. Lipoids are much more
complex esters, having some nitrogenous, or phosphorus-containing, group
and sometimes a sugar in combination with the fatty acids and glycerol
which make up the characteristic part of their structure.

In general,