n lecithins from
different sources, just as they are different in fats from different
sources. Both oleic acid and a solid fatty acid have been found in the
hydrolysis products of lecithin from leguminous seeds. In certain lupine
seeds, the fatty acids present in the lecithin appear to be palmitic and
stearic.

OTHER PLANT PHOSPHATIDES

Phosphatides other than lecithin are common in plants. In these, various
sugars replace part or all of the glycerol as the alcoholic part of the
ester. Percentages of sugar varying from mere traces up to 17 per cent of
the weight of material taken, have been found in the products of hydrolysis
of phosphatides prepared from vetch seeds, potato tubers, plant pollens,
and whole wheat meal.

Furthermore, betaine

CH_{2}
/ \
(tri-methyl glycocoll, OC N[trb](CH_{3})_{3})
\ /
O

and perhaps other vegetable amines (see Chapter XII) sometimes replace
choline as the basic group in the phosphatides.

PLANT CEREBROSIDES

Bodies similar to the animal cerebrosides seem to occur in many plant
tissues, since plant lipoids which yield no phosphorus when hydrolyzed have
often been isolated. The sugar which constitutes the alcoholic portion of
their structure appears to be galactose in every case which has been
reported. Beyond this, little is known of the structure of these plant
cerebrosides, as they are very difficult to prepare in pure form and not
easily hydrolyzed.

PHYSIOLOGICAL USES OF LIPOIDS

Lipoids are so universally present in plant and animal tissues and so
commonly found in those parts of the organism in which vital phenomena are
most pronounced (brain, heart, embryo of egg, embryo of seeds, etc.), that
it is evident that they must play some important role in the activity of
living protoplasm. There is, as yet, however, no definite and certain
knowledge of what this role is. Various theories concerning the matter have
been put forward in recent years. For example, Overton, in 1901, presented
the idea that every living cell is surrounded by a semi-permeable membrane
consisting of lipoid material, which regulates the passage into and out of
the cell of substances necessary to its metabolism and growth. Recent
investigati