ons by Osterhout and others indicate, however, that Overton's
hypothetical lipoid membrane is not essential to a proper explanation of
the migration into and out of the cell protoplasm of nutritive materials,
etc. Other investigators have cited results which appear to indicate that
lipoids play an important, but as yet unknown, part in the process of fat
metabolism. Others go even further than this, and argue that since the
extraordinary rapidity of the chemical changes which take place in plant
protoplasm indicates the necessity of the presence there of exceedingly
labile substances, and since both fats and proteins are relatively stable
compounds, it is possible that the lipoids, which contain both nitrogenous
and fatty acid groups, play an exceedingly important part in the metabolism
processes. Bang, in particular, has pointed out (in 1911) that the lipoids
are probably the most labile of all the components which constitute the
colloidal system known as plant protoplasm. The importance of such
considerations will be more apparent after the relation of colloidal
phenomena to the activities of plant cell contents has been more fully
discussed (see Chapter XVI).

Experimental studies of the physiological uses of lipoids have thus far
been devoted almost exclusively to those of animal tissues. They have been
seriously hampered by the difficulty of securing properly purified extracts
of lecithin and similar lipoids. The same labile character which apparently
makes them so important in the chemical changes in the cell makes them
equally unstable compounds to work with in attempting to secure pure
preparations for the purposes of experimental study. On this account, there
is, as yet, no certain knowledge concerning their actual physiological
uses. It is evident, however, that they have some really important role to
play, which opens up a promising field for further study.

REFERENCES

ABDERHALDEN, E.--"Biochemisches Handlexikon, Band 3, Fette, Wachse,
Phosphatide, Cerebroside, ..." 340 pages, Berlin, 1911.

HOPKINS, E.--"The Oil-Chemist's Handbook," 72 pages, New York, 1902.

LEATHES, J. B.--"The Fats," 138 pages, _Monographs_ on Biochemistry,
London, 1913.

LEWKOWITSCH, J.--"Chemical Technology and Analysis of Oils, Fats, and
Waxes," Vol. I, 542 pages, 54 figs.; Vol. II, 816 pages, 20 figs.; and Vol.
III, 406 pages, 28 figs., London, 1909.

MACLEAN, H.--"Lecithin and Allied Substances