80 



ACIDITY AND GAS INTERCHANGE IN CACTI. 



experiment having been begun at 4 p. m. It has already been cited as having 

 given the lowest gas-interchange ratio that was obtained in any case. In 

 this connection it is to be noted that the rise in acidity during the course of 

 the experiment was relatively very great, all of which is in keeping with the 

 statement just made regarding the relation of the course of the acidity and 

 the proportion of carbon dioxide evolved to oxygen absorbed. However 

 striking the result of this individual experiment may be, it would, of course, 

 not have important significance did it not show correlation with the others in 

 the series. All of the other ratio groups are averages of several experiments, 

 as indicated in table 52, and consequently carry more weight, especially since 

 they form a consistent series. This table of averages shows an acidity greater 

 at the end than at the beginning, up to the ratio group 0.71 to 0.80, where the 

 total dry-weight acidity is 2.13 at the beginning and 2.23 at the end. Above 

 that, in the two highest ratio groups, the acidity, which is also high, is falling. 



TABLE 52. General average of 51 gas-interchange determinations during summer of 1913, 

 arranged according to COa/C^ ratios and showing average acidity at beginning and at end 

 of experiments. 



If we take the gas ratio of 0.30 as the low point and 1.00 as the high, which 

 correspond very closely to the figures in table 52, the middle point for the 

 ratio would be 0.65. This will be found to correspond approximately to the 

 average middle point of the dry-weight acidity, which may be reckoned in a 



similar way from the figures in the table. So estimated, the dry-weight total 



CO 

 acidity would be about 2.00. In table 52 the average -Q- 2 ratio of 0.67 has 



the average dry- weight acidity of 1.79 to 2.00, which is as close a correspond- 

 ence to the predicted value as could be expected. The ratio, then, rises with 

 the acidity, and when the latter is at a point where it begins to fall the highest 

 ratios are realized. 



The analysis of individual experiments further corroborates this generali- 

 zation. Experiments 3 and 4 (table 57) are comparable, the material being 

 from similar plants and being carried on simultaneously. In the former, 

 where a fall in acidity is indicated, the ratio is higher than in the latter where 

 the acidity is stationary. In experiments 7 and 8 in the same table, which 

 are also comparable, the acidity of the first rose slightly, while that of the 



