30}H_{61}OH) esters of cerotic acid (C_{25}H_{51}.COOH) and carnaubic
acid (C_{23}H_{47}.COOH). It is the best known vegetable wax. Poppy wax is
composed chiefly of the ceryl ester of palmitic acid (C_{17}H_{35}.COOH).

Since waxes contain no glycerol, they give no odor of acrolein when heated
with dehydrating agents, do not become rancid, and are less easily
hydrolyzed than the fats. They are soluble in the same solvents as the
fats, but generally to a less degree.

The facts that waxes are impervious to water and usually occur on the
surfaces of plant tissues have led to the conclusion that their chief
function is to provide against the too-rapid loss of water by evaporation
from these tissues. This seems to be borne out by the common experience
that many fresh fruits and vegetables will keep longer without shriveling
if their waxy coating is undisturbed. No other function than that of
regulation of water losses has been suggested for the plant waxes.

C. THE LIPOIDS

The lipoids, or "lipins," as some authors prefer to call them, are
substances of a fat-like nature which are found in small quantities in
nearly all plant and animal tissues and in considerable proportions in
nerve and brain substance, in egg yolk, etc., and in the seeds of plants.
When hydrolyzed, they yield fatty acids or derivatives of fatty acids and
some other group containing either nitrogen only or both nitrogen and
phosphorus. The facts that they are extracted from tissues by the same
solvents which extract fats and that they yield fatty acids when hydrolyzed
account for the name "lipoid," which comes from the Greek word meaning fat.
Some writers, who object to the word "lipoid" as a group name, prefer to
call these substances the "fat-like bodies."

The first group of lipoids to be studied were those which occur in the
brain; and the name _cerebroside_ was given to those lipoids which, when
hydrolyzed, yield fatty acids, a carbohydrate and a nitrogen-containing
compound but no phosphoric acid; while those lipoids which contain both
nitrogen and phosphorus were called _phosphatides_. Substances which
correspond in composition to both these types are found in plant tissues
and the same class names are applied in a general way to lipoids of either
plant or animal origin.

Plant lipoids have not been studied to nearly the same extent as have those
which occur in the animal body; and certain observers believe that