46 



ACIDITY AND GAS INTERCHANGE IN CACTI. 



TABLE 24. Acidity of Opuntia leptocaulis, 0. discata, and O. blakeana at various 



hours of the day. 



The amount of acid is given in cubic centimeters of N/10 KOH per gram fresh weight. 

 Owing to the large amount of glutinous substance expressed from the tissues of these species 

 in extracting the juice, the determinations as made by the method employed can not be con- 

 sidered more than approximate; nevertheless they show the characteristic decrease in 

 acidity as the day advances. The experiments were conducted at Tucson, Arizona, in 

 March and April 1911. 



TABLE 25. Diurnal decrease in acidity in Carnegiea gigantea (Sahuaro). 



The acidity is given in cubic centimeters N /10 KOH per gram fresh weight and per cubic 

 centimeter pure juice. The material came from a single plant and was in every case from 

 the cortical tissue. Care was taken that the different samples came from somewhat sepa- 

 rated regions to avoid possible traumatic effects. This experiment was made at Tucson, 

 Arizona, in April 1911. 



