The Organic Acids of the Leaves of Bryophyllum Calycinum

It has been known since the work of Mayer (1) in 1875 that the thick succulent leaves of many plants of the family Crassulaceae become enriched in titratable organic acids during the night and diminish in acid content when the leaves are illuminated. An analogous diurnal variation in acidity has been observed in a number of other succulents, and occasionally in species of different growth habit (2). This behavior of the organic acids is usually referred to as the crassulacean type of met,abolism, and for many years has attracted a great deal of attention. As a preliminary to a chemical study of the phenomenon, it was necessary to examine the organic acids of a typical species in some detail, particularly since modern methods for the isolation of organic acids have hitherto been applied in only one case (3) and, as will be made clear in another connection (4), there is reason to be dissatisfied with the published results. The organic acids have accordingly been isolated from the leaves of Bryophyllum calycinum, the species originally examined by Mayer, and separated by means of distillation of the ethyl esters. Indirect analysis of a sample of the leaves of this common greenhouse plant showed that, in addition to small proportions of oxalic and succinic acids, roughly 7 per cent of the dry weight consisted of I-malic acid, 2 per cent of citric acid, and 14 per cent of “unknown acids” (the total organic acidity minus the sum of the acidity due to malic, citric, and oxalic acids) calculated arbitrarily as citric acid. The present study has shown that by far the greater part of the (‘unknown acids” consists of isocitric acid. The bearing of this wholly unexpected observation upon the old and puzzling problem of the nature of the so called crassulacean malic acid is discussed in another paper (4). For the present, it is desirable to point out that isocitric acid, although synthesized many years ago by Fittig and