EXPERIMENTAL DATA ON ACIDITY DETERMINATION. 



41 



TABLE 13. Diurnal periodicity of acidity of young, turgid, newly formed joints 

 from irrigated plants of Opuntia versicolor. 



The joints all showed some leaf development. The material was treated exactly as in 

 the series shown in Table 11 with mature joints. The acidity is given also in the same 

 terms. At Tucson, July 29, 1913. 



TABLE 14. Effect of direct sunlight upon acidity. Opuntia versicolor. 



The material used included both young and old turgid joints and was collected when the 

 acidity was relatively high. The experiments were set up at once on the collection of the 

 specimens. By immersion of the ventilated glass container under ice-water the temperature 

 was kept low and fairly constant. For comparison the acidity of control duplicate material 

 exposed in the open air until about 4 p. m. is also given. The acidity is expressed in terms 

 of centimeters N /10 alkali per cubic centimeter pure juice. The first set of observations, 

 taken between August 1 and 4, 1912, shows the effect of the increased intensity of the sun- 

 light at that period of the year. The days were brilliantly clear and in each case showed 

 a rise of about 10 C., from 32 at 10 a. m. to 42 at 4 p. m. The second set of observations 

 was taken in March and April 1911 ; days clear, temperature 26 at 10 a. m. to 32 at 4 p. m. 

 All the observations were made at Tucson. 



