72 



ACIDITY AND GAS INTERCHANGE IN CACTI. 



of 6.78 per gram dry weight and of 2.00 per cubic centimeter juice, gave at the 

 beginning only a ratio of 0.83, rising as the acidity diminished to 0.91. Thus 

 it may be seen that the question is by no means a simple one and that a proper 

 understanding of the problem demands a closer analysis than is given by 

 these general averages. Nevertheless, it is impressive that despite great 

 variations, which, as will be shown later, are dependent upon the course of the 

 acidity at the time the determination was carried on, the high acidities and 

 high ratios go together. Later the conditioning circumstances of these various 

 experiments will be considered. 



It is the same with the low acidities and low ratios as with the higher ones. 

 Great individual variation is found in the various experiments, but here, as 

 in the other case, the age of the tissue and the course of the acidity are impor- 

 tant influences. In the lowest ratio group, that below 0.40, there were only 

 two experiments which agreed well. One was with some mature-flaccid joints 

 (experiment B', table 56) and the other was with young material (experiment 

 12, table 58). This latter gave the lowest gas ratio that was found, as follows : 

 ratio 0.29, acidity juice 0.38 per cubic centimeter, total acidity 0.72 per gram 

 dry weight. The acidity was rising. The former with a considerably lower 

 acidity gave a somewhat higher ratio. There were, it is true, a few experi- 

 ments in which little or no carbon dioxide was produced and a considerable 

 amount of oxygen was absorbed, but these departed so very widely from the 

 rule that they may be regarded as somewhat anomalous. The lowest acidity, 

 as far as the juice itself was concerned, was found in a young specimen (experi- 

 ment DD 3 , table 55) and was but 0.16, though the total acidity was not pro- 

 portionally low. This specimen gave the low gas ratio of 0.53, which is, how- 

 ever, considerably higher than either of the two mentioned above. Other 

 cases of as low or even lower ratios were found, but with considerably greater 

 acidity as indicated below. 



TABLE 48. 



The variation is considerable, but not more than might be anticipated. In 

 general, it is true that plants that have the lowest acidities give the lowest gas 

 ratios, especially where they are mature and flaccid. From a closer study of 



CO 

 the data, it becomes evident that the variation in the -^p ratio at any given 



acidity is very largely dependent upon whether the acidity happens to be rising 

 or falling. Where the acid-content is high it is naturally usually falling, and 

 rising where it is low. Consequently, the largest variation of gas ratio is to 

 be expected in the middle ranges of acidity, since there would be less uniformity 



