DEPARTMENT OF BOTANICAL RESEARCH. 89 



Gas Interchange and Acidity in Cacti, by H. M. Richards. 



The actual rate of carbon-dioxide evolution and oxygen absorption 

 is governed by more than one factor. Primarily, as one might expect, 

 the age and condition of the tissue exert a very considerable effect. 

 The mature joints which are in the flaccid condition that characterizes 

 them in the dry season show a relatively low rate of gas interchange, 

 while the turgid mature joints are half again as active, and in the 

 3^oung joints (that are just forming and are in their most rapid growth 

 phase) the rate is at least twice as great. But conditions may be 

 present which entirely mask these differences. 



Temperature, of course, is an active influence in determining the 

 rate at which the gases are given out or taken up, as can be seen by the 

 table on page 88, where the experiments are arranged in four tempera- 

 ture groups. In this tabulation, which represents the actual averages 

 of most of the gas-interchange experiments that have been undertaken 

 in this research, there are, indeed, a few discrepancies in this regard. 

 Thus at 35° to 37° C, under the ratio of 0.51 to 0.60, the rate appears 

 lower than at 31° to 34° C. This is simply because the number of 

 experiments averaged in that case is small and they happened to be 

 with relatively dormant material. The tabulation is exactly as the 

 experiments happened to fall. Under the ratio 0.81 to 0.90, which 

 represents the averages of the greatest number of experiments, there 

 is seen to be a consistent rise in gas-interchange rate to somewhat 

 more than double the initial rate, paralleling a rise in temperature 

 of about 12.5° C. 



Acidity is also a controlling factor, for while it is usually highest 

 at the lower temperatures it also rises with the ratio, as is shown most 

 strikingly in the column giving the cubic centimeters of acid per gram 



CO 



dry weight. It is evident from inspection of the table that high -^ 



ratios, high acidities, and high rate of gas interchange are all associated. 

 It is indeed what would be anticipated and is in keeping with what 

 has been announced in previous reports. When the acidity is high 

 there is, other things being equal, a marked tendency for it to fall if 

 the slightest stimulus which favors the breaking up of the acid is 

 present, such as increase in illumination, prolonged darkness, or rising 

 temperature. The splitting of the acid results in the great absorption 

 of oxygen and the still greater formation of carbon dioxide. Thus the 

 ratio rises and approaches or even exceeds unitj^ The actual rate of 

 gas interchange which accompanies these disruption processes must 

 naturally be greater than at periods when the acidity is low, for since the 

 acid is the origin of at least a large part of the carbon dioxide formed, 

 it is obvious that when there is less actual acid present there could not, 

 under any circumstances, be so much carbon dioxide produced. 



