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 rôle 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," 206 pages, _Monographs_ on Biochemistry, London, 1913. SOUTHCOMBE, J. E.--"Chemistry of the Oil Industries," 204 pages, 13 figs., London, 1918. CHAPTER XI ESSENTIAL OILS AND RESINS Included in this group are all those substances to which the characteristic odors of plants are due, along with others similar in structure and possessing characteristic resinous properties. They have no such uniformity in composition as is exhibited by the oils which are included among the fats and waxes; but belong to several widely different chemical groups. Furthermore, there is no sharp dividing line between the essential oils and certain esters of organic acids on the one hand and the fats on the other. For example, if an aromatic fluid essence is a light fluid, non-viscid, and easily volatile, it is usually classed with the organic esters; denser liquid substances, of oily or waxy consistency, and with comparatively slight odor and taste are usually fats, while oils of similar physical properties but possessing strong characteristic odors are classed as essential oils, regardless of their chemical composition. Included in this general class are compounds having a great variety of chemical structures; e.g., hydrocarbons, alcohols, phenols, organic sulfides and sulfocyanides, etc. Many of these compounds are crystalline solids at ordinary temperatures, but melt to oily fluids at higher temperatures. The characteristic property which assigns any given plant extract to this group is that it has a strikingly characteristic odor or taste, often accompanied by some definite physiological effect, or medicinal property.