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 rôle in the activity of living protoplasm. There is, as yet, however, no definite and certain knowledge of what this rôle 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 investigations 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).