42 



ACIDITY AND GAS INTERCHANGE IN CACTI. 



TABLE 15. Effect of increased temperatures on acidity. Opuntia versicolor. 



The material used included both young and old turgid joints and in one case young fruit. 

 It was collected either in the morning, when the acidity was relatively high, or in the after- 

 noon, when it was low, to test the effect on both the decrease and the increase of acidity. 

 The experiments were set up at Tucson in July and August 1912 as soon as possible after 

 gathering the material and were carried on in darkness. The acidity is expressed in terms 

 of cubic centimeters N /10 per cubic centimeter pure juice. 



TABLE 16. Accumulation of acid at various temperatures. Opuntia versicolor. 



To show the effect of sustained exposure to various temperatures of material of relatively 

 low acidity. The figures given are in each case the averages of two parallel determinations. 

 The separate lots of material were kept in the dark at the temperatures given, which main- 

 tained a fair degree of constancy. The total acidity refers to that of a 50-gram sample and 

 is expressed in cubic centimeters of N /10 alkali. 



SERIES I. 



[Material collected at 3 h 30 p.m., March 28, 1911. Series begun at 4 p.m. Acidity at beginning: 



Total, 31.20; per gram, 0.60.) 



