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Crassulacean plants in general undergo diurnal variation in the acidity of the leaf tissue. They share this behavior in greater or less degree with plants of a number of other families, but crassulacean plants are the classical material for demonstration of the phenomenon; Bryophyllum calycinum, in particular, has been repeatedly investigated. Most studies have dealt with the titratable acidity of extracts of the leaves and with attempts to account for the chemical interconversions that take place in terms of reactions in which carbohydrates share. BENNET-CLARK (1) reviewed the earlier literature in 1933 and pointed out that the physico-chemical properties of polybasic organic acids must be taken into consideration in the interpretation of the results of titration. He agreed with previous workers, however, who had claimed that malic acid is the component which increases during the night and decreases during illumination of the leaves. As a general working hypothesis to account for the observed behavior, BENNET-CLARK suggested that polysaccharides are converted to monosaccharides (sedoheptose, a 7-carbon sugar, in the case of Sedum praealtum) which, in turn, are oxidized to malic acid during the period when the leaves are darkened. The reverse reaction occurs during illumination. No evidence was obtained regarding the by-product of the conversion of monosaccharide to malic acid (respectively, compounds containing 6 and 4 carbon atoms) which was assumed to be a compound that contained either 1 or 2 carbon atoms (3 in the case of sedoheptose). The observations suggested, however, that carbon dioxide is not produced, and he expressed the view that, whatever the nature of the by-product, it was probably ultimately converted back into the polysaccharide. Accordingly, in its simplest form, BENNETCLARK'S speculation on the mechanisms that underlie crassulacean metabolism involves an equilibrium between malic acid and a polysaccharide; this equilibrium is upset in one direction or the other according to the conditions of illumination of the leaves. The most comprehensive investigations of crassulacean metabolism that have been made since the appearance of BENNET-CLARK's review are those of WOL.F (14, 15, 16, 17, 18). This investigator studied organic acid fractions isolated by extraction with ether and likewise observed a reciprocal relationship between the quantities of organic acids and carbohydrates, especially starch. He confirmed GUTHRIE'S (5) observation that citric acid 360

Studies in the Metabolism of Crassulacean Plants: The Diurnal Variation in Organic Acid and Starch Content of Bryophyllum calycinum