104 ACIDITY AND GAS INTERCHANGE IN CACTI. 



acid is determined in toto by titration, no decrease in acidity would show. 

 To determine if this be true, it would be necessary to investigate the precise 

 nature of the acids present when the conditions are normal and to make a 

 comparison with those found in the absence of oxygen. 



When the oxygen-supply is increased rather than diminished the question 

 arises what may be the relation of the interchange of oxygen and carbon 

 dioxide. The actual amount of both is measurably increased, but the latter 

 more than the former, so that the ratio naturally rises also. As we have seen, 

 increased oxygen supply hastens deacidification and it may very well be that 

 the major portion of the accelerated carbon dioxide production is due simply 

 to the splitting of the acid and is not a proper respiratory reaction. It has 

 been shown in other plants that increased oxygen supply is not accompanied 

 by a correspondingly large increase in carbon dioxide output, a fact which 

 supports the above opinion. Furthermore this interpretation is in keeping 

 with all the experiments recorded here, in which falling acidity is concerned, 

 where the CO 2 /O 2 ratio has been shown usually to be high. 



That a high CO 2 /0 2 ratio is accompanied by a high acidity which is falling, 

 and that the reverse is true with low acidity that is rising, are no more than would 

 be expected, since we know that the breaking down of malic and other related 

 acids is attended by the evolution of carbon dioxide without a corresponding 

 absorption of oxygen, whereas their formation is accompanied by the absorp- 

 tion of oxygen without the evolution of any gaseous waste product. There 

 is strong support for the interpretation that exhaled carbon dioxide may 

 arise from sources other than that of a true respiratory process in the case 

 of falling acidity, for even at normal temperatures the organic acids which are 

 concerned can be split to simpler forms with the evolution of this gas without 

 any activity of living protoplasm. In deacidification the initial acid-content 

 of the tissues bears a relation both to the amount of carbon dioxide produced 

 and to the gas interchange ratio. With the greatest amount of acid there is 

 also the greatest production of carbon dioxide, which, as it is not accompanied 

 by a similar increase in the absorption of oxygen, leads to the highest CO 2 /Oj 

 ratio; and as the acidity diminishes there is commonly a corresponding 

 decrease in the evolution of gas and in the ratio. 



If the amount of water contained in the tissues is large, the juice itself may 

 be relatively weak, although the total acid-content is great. Increase of juice 

 concentration, which is not always, though usually, accompanied by a pro- 

 portional increase in total acidity, is likewise correlated with high ratios, 

 though the effect is not so well marked. In other words, while deacidification 

 seems to take place rather more rapidly when the juice concentration is high 

 than when it is low, the actual amount of total acid present is more important 

 in determining the rate of carbon dioxide production and the gas ratio. 



The phenomena connected with rising acidity are also in accord with what 

 has been said above. The oxygen which is absorbed is used largely in the 

 partial oxidations which lead to the formation of malic acid, though there is 

 also a residual carbon dioxide evolution that is always present. The major 

 portion, then, of the energy release which results from the aerobic respiration 

 would seem to be connected with the formation of this acid. The question 

 of the possible source of the residual carbon dioxide has already been discussed. 

 It seems gratuitous to maintain, as does Nathansohn, that the breaking-down 

 of the acids is simply a continuation of the ordinary respiration processes, 



