48 



ACIDITY AND GAS INTERCHANGE IN CACTI. 



during the cooler periods, when the external conditions are much drier. As 

 this point is brought out better in the discussion of the gas-interchange experi- 

 ments, further comment will be deferred until they are reached. 



The various points referred to above are brought out in table 26, which 

 gives averages of the experiments, which are shown in full in table 28. 



TABLE 26. 



The acidities of the juice were also determined in connection with this 

 series, using parallel material at the end of each temperature period. There 

 is, of course, a fall in acidity with the rising temperature, which is greatest in 

 the young joints where there is the largest evolution of carbon dioxide. It 

 is least in the flaccid specimens where the acidity is already low. It does not 

 appear, however, that there is an exact relation between this fall of acidity 

 and the respiration-rate, for the disparity between the loss of acid in the turgid 

 and flaccid material is much greater than the difference of the amount of 

 carbon dioxide evolved. Nor, indeed, is the difference in the rate of acidity 

 decrease at 40 and 45 in correspondence with the increase in respiration. 

 Other evidence will be given to show that the breaking down of the acid 

 undoubtedly influences the quantity of carbon dioxide produced, but it seems 

 that it is not the whole course of the formation of that waste product. 



The averages of the acidity decrease may be compared with the rates of 

 carbon dioxide formation given above. These figures are in terms of cubic cen- 

 timeters of N/10 KOH per cubic centimeter of juice and are given in table 27. 



TABLE 27. 



That there is, however, some relation between acidity and the rate at which 

 carbon dioxide is evolved is shown by a series of experiments at approximately 

 the same temperature, but at widely differing acidities (tables 29 and 30). 

 The relation is not entirely constant, but points only in one direction. Taking 

 the averages of the three series with turgid joints (table 29), the forenoon 



