38 



ACIDITY AND GAS INTERCHANGE IN CACTI. 



TABLE 8. Diurnal range of acidity of Opuntia versicolor. 



The diurnal range of acidity of Opuntia versicolor, taken at, 2-hour intervals over a period 

 of 26 hours, from 9 a. m., March 25, to 11 a. m., March 26, 1911, at Tucson, Arizona. The 

 day was an average one for the season, with temperature range of about 12 C., from 24.5 

 at 4 p. m. on March 25 to 12.2 at 8 a. m. on March 26. Except for a slight shower and 

 some cloudiness until 11 a. m. March 25 the sky was clear. 



Three simultaneous series were run, the material being freshly collected for each determi- 

 nation. The acidity is expressed in terms of cubic centimeters N/10 alkali for the total 

 acidity of a 50-gram sample. The acidity per gram fresh weight is also given. 



TABLE 9. Diurnal periodicity of acidity, in comparison with temperature within the plant 

 and also external temperature, Opuntia versicolor. 



On April 1 the external temperature was maximum at 1 p. m. 30.8 C. and minimum at 

 6 a. m. 14.4 C. The temperature within the cactus joint was maximum at 12 noon, 38.3 

 C., and minimum at 5 a. m., 12.8 C. All of the determinations were made with material 

 fresh from the plant and were from mature turgid joints. The acidity is given in terms of 

 cubic centimeters of N /10 KOH for the whole sample of 50 grams fresh weight and also 

 per gram. 



