The Organic Acids of Rhubarb (rheum Hybridum)* Iii. the Behavior of the Organic Acids during Culture of Excised Leaves

The organic acids of the rhubarb plant have been repeatedly investigated and brief reviews of the literature are to be found in the papers of Culpepper and Caldwell (1) and of Allsopp (2). The latter investigator has recently studied the quantities and distribution of the acids of the plant throughout the year, and has clearly shown that these substances play an active part in the general metabolism, increasing rapidly in both leaves and rhizome during the growing season and undergoing marked changes in the rhizome during the winter. Our interest in the acids of rhubarb leaves arose from observations upon the behavior of the acids of excised tobacco leaves during culture in water. Although little change takes place when these leaves are exposed to light, in darkness there is a prompt and very extensive loss of malic acid and a corresponding increase in citric acid. Considerable probability was shown to attach to the view that malic acid is transformed into citric acid under these conditions (3). The tobacco plant is a typical representative of the group of neutral plants, as classified by Ruhland and Wetzel; it seemed desirable therefore to study the behavior of the acids in one of the species classified by these investigators as an acid plant (4), especially as the metabolism of the amides and ammonia in the two groups is held to be different, and the or-