GAS INTERCHANGE. 73 



there in the course of the acidity. This is, indeed, the case, and in the subse- 

 quent analysis of the data from those experiments where the course of the acidity 

 was known, the relation of the rise or fall of acid-content to the proportional 

 evolution of carbon dioxide and absorption of oxygen will be discussed. The 

 greatest range was found, as might be expected, in the young joints, which are 

 far more active than the older joints, even when the latter are fully turgid. 



GAS RATIO AND RATE OF GAS INTERCHANGE. 



The actual rate of carbon-dioxide evolution and oxygen absorption shows a 

 tendency to increase as the ratio rises; or rather, the more rapid gas inter- 

 change usually results in a higher ratio. This, indeed, is what might be antici- 

 pated, for, as table 49 shows, the acidities also rise with the ratios and the 

 higher the acidity the greater the rapidity of gas interchange, since the break- 

 ing down of. the acid is so intimately connected with these processes. There 

 is in addition, of course, the influence of temperature, concerning which there 

 has already been discussion. Nevertheless, in table 49, the averages now 

 under consideration, where the arrangement of the data has been according to 

 ratios, even the general averages of experiments at all temperatures show an 

 increase of gas interchange with the ratio. In the first group an average of 

 17 experiments with a gas ratio of 0.46 gave a per gram-hour evolution of car- 

 bon dioxide of 0.089 c.c. fresh weight or 0.404 dry weight. Set against this, 

 at the other end of the table, is the average of 14 experiments with a gas ratio 

 of nearly 1.00 and an amount of evolved carbon dioxide about double that 

 found in the previous case. It is more instructive, however, to take a single 

 temperature group in this table and to follow it through the various ratios. 

 Each temperature group shows a pretty constant increase in gas interchange, 

 with the exception of that at 28 to 30 C., where there is irregularity, par- 

 ticularly with the higher ratios. This temperature group is compiled from the 

 smallest number of experiments, and it happened, moreover, that the material 

 giving the high gas ratios was exceptionally inactive. 



The experiments made at between 31 and 34 C. were the most numerous, 

 and that temperature is, indeed, nearly normal for these cacti. These show a 

 consistent rise in activity with the rise in ratio. At the ratio of 0.82 there is 

 nearly three times the amount of carbon dioxide given off as there is when the 

 ratio is 0.45; while at the still higher ratio of 0.93 the increase approximates 

 four times the amount of the original rate. All of the other temperature 



groups are sufficiently consistent to warrant the general statement that the 



CO 



greater the amount of gas interchange the more nearly the -Q-^ quotient 



approaches unity. Indeed, it may surpass it. This of course means that 

 while the evolution of carbon dioxide increases, the absorption of oxygen also 

 increases, but not so rapidly. 



It will be seen that along with the increasing ratios there is also increasing 

 acidity, both of the juice and of the total quantity in the tissues. The general 

 statement may be further supplemented by including this fact, and we may 

 say that high acidities, high ratios, and high rate of gas interchange are all 

 associated. When the acidity is high, there is, other things being equal, 

 a marked inclination for it to fall if the slightest stimulus which favors the 

 breaking down of the acids is present. Such conditions are realized in increase 



