The conventional criterion employed in testing the ability of an aerobic organism to utilize cellulose is growth on filter paper. As ordinarily carried out, this test provides little information as to the activity of the culture. Moreover, there are grounds for believing that it may be too exclusive a criterion, because filter paper is hardly representative of cellulose as it occurs in the plant. The structural cellulosic framework of most plant tissues is not solely built up from the glucose

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... lysaccharide, typified by cotton, that the chemist recognizes as pure cellulose. Other polysaccharides are normally present in the most intimate association with the "true" cellulose. These associated polysaccharides, conveniently termed cellulosans, are not impurities, but form an integral part of the cellulosic fabric through which thev are distributed. In chain length they are considerably shorter and perhaps more variable than the true cellulose component. The cellulosan molecules are oriented and retained by lateral forces similar to those operative between cellulose chains. They are, however, removable by extraction with alkali, or by hydrolysis with dilute acids. Complete removal is a matter of great difficulty. In most plant celluloses the cellulosan fraction is predominantly, but not exclusively, xylan. The effect of the presence of these cellulosans on the availability of the cellulosic fabric containing them has not hitherto been investigated. Quantitative information was therefore sought as to the comparative activity of a number of aerobic cellulose bacteria on filterpaper and on cornstalk cellulose preparations, high and low in cellulosan. EXPERIMENTAL Cellulose preparations (a) Cornstalk cellulose. The cellulose was isolated from finely ground cornstalks by a modification of the procedure described by Norman and Jenkins (1933) . All hypochlorite treatments were acidified. The cellulose was finally washed free from sulphite by repeated suspension in water and filtration, and was then dried at 600. (b) Extracted cornstalk cellulose low in cellulosan. Prior to the isolation of cellulose by the method just mentioned, the cornstalks were extracted for one hour in flowing steam with an excess of one per cent NaOH. The cellulose prepared from this residue was further extracted by treatment, first with four per 1