26 



ACIDITY AND GAS INTERCHANGE IN CACTI. 



In other words, while the actual amount of acid contained in the tissues is not 

 very different at the two seasons the juice itself is less concentrated during 

 the summer rainy season and consequently there must be more water held in 

 the plant. 



One very interesting fact indicated in table 6 is that, with increasing acidity, 

 there is a very distinct tendency for the dry-weight acidity to increase more 

 rapidly than that of the juice. Table 6 gives the averages of the dry and 

 fresh weight total acidity and also the juice acidity, the basis of the arrange- 

 ment being the dry weight. The higher acidities, above 4 c.c. decinormal 

 alkali per gram dry substance, show a sharp increase of the total acid-content 

 over the concentration of the juice, so that the ratio of the former to the latter, 

 instead of being less than 3 to 1, is more than 4 to 1, a point that is most con- 



TABLE 6. Average acidity of 103 determinations, arranged in groups according to 



dry-weight acidity. 



sistently shown in the averages of the acidity of young joints, though it is also 

 indicated in those of the mature-turgid ones. In the group which includes the 

 examples of highest acidity (over 6.00 per gram dry weight) with both the 

 young and mature-turgid joints the ratio is 4.3 to 1. 



It is true that with the mature material the ratio of the next preceding group 

 falls considerably below that which is required, owing presumably to the fact 

 that 2 of the 5 examples averaged had an unusually high juice acidity. If 

 these are omitted the ratio is similar to that found with young joints of equal 

 acidity. The variations in the acidity of the flaccid joints are much less than 

 with the others, so that on this basis of classification into acidity groups the 

 determinations fall into only two divisions and the vast majority of the exam- 

 ples come in the group with the lowest acidity. Nevertheless, as far as the 

 ratio of dry weight to juice acidity is concerned, we find a difference between 

 the two groups similar to that already described. The same relation of acidi- 



