ACID ACCUMULATION AND DESTRUCTION IN 

 LARGE SUCCULENTS 



ESMOND R. LONG 



The Desert Laboratory, Tucson, Ariz. 



The diurnal variation in the acidity of the sap of succulents 

 has been under investigation for a number of years by Richards. 1 

 In a large number of observations he has found that the acidity 

 of the expressed juice of cacti is considerably higher in the early 

 morning than at the close of the day, reaching its maximum just 

 before sunrise, and gradually decreasing throughout the day until 

 the minimum is reached approximately two hours before sunset. 

 He has come to the conclusion in view of his experiments and the 

 observations of Spoehr 2 on the photolysis of acids that in the de- 

 struction of the acids which accumulate during the night through 

 insufficient oxidation in the catabolism of carbohydrates, both 

 light and temperature play a part. The steps in the photolysis 

 of malic acid, which appears to be the chief acid constituent, other 

 than oxalic acid, in the sap of cacti, have been described by 

 Spoehr, 3 the process being apparently one occurring only in the 

 presence of certain salts and not to be attributed to the action 

 of an enzyme. 



In the following experiments the investigations of Richards, in 

 which Opuntia versicolor and a number of the flat-jointed opuntias 

 served, have been extended to two of the larger succulent cacti, 

 Echinocactus ivislizeni (bisnaga) and Carnegiea gigantea (sahuaro). 

 The object was not merely to measure the diurnal variation in 

 acidity in these plants, but it was thought that the large diameter 

 of the bisnaga and sahuaro would furnish an admirable opportunity 

 for studying the light and temperature effects upon acidity in dif- 



1 See Annual Report, Carnegie Inst., Wash., 1911, 1912 and 1913. 



2 Annual Report, Carnegie Inst., Wash., 1913. 



3 Ibid, and Biochem. Zeitschr. 57: (1913), Bd. 1, H. 2, 95-111 . 



261 



THE PLANT WORLD. VOL. 18, NO. 10, 1915 



