66 ACIDITY AND GAS INTERCHANGE IN CACTI. 



according to the notes kept, the highly colored plants seem to have been much 

 less frequent, than in late June and early July. A continuous series of obser- 

 vations during the seasonal changes would be of considerable interest in throw- 

 ing light on this question as well as for determining, what very well may be 

 the case, whether there is any individual predisposition on the part of certain 

 plants to form and to retain a reddish color more or less permanently. The red 

 is not exclusively limited to the flaccid plants. It will be remembered in this 

 connection that the flowers of Opuntia versicolor exhibit a considerable range of 

 color, but no satisfactory evidence was forthcoming that any particular color 

 in the flower characterized plants with red color present or absent in the stem. 



QUANTITIES IN GAS INTERCHANGE. 



An inspection of the results set forth in table 42 indicates the same difference 

 in the activity of the plants, according to their age and condition, that was 

 seen in the results of the Pettenkofer experiments. They also show that there 

 is a great variation in the volumes of the gases which are given off and ab- 

 sorbed. This, as will be shown in the subsequent discussion, is to a large 

 extent dependent upon the temperature and increasing or decreasing acidity. 



In the study of the complete gas interchange we have, of course, to con- 

 sider not only the carbon dioxide evolved, but also the oxygen absorbed. 

 The former does not vary alone, but with the latter, although not always in 



the same degree. Consequently, under changing conditions there may be an 



f O 



alteration in the -Q^ quotient. We have here attempted to trace these 



changes and to find how the gas-interchange ratio is related to both the 

 external and internal conditions. 



Before discussing the details it is well to consider the general rate of gas 

 interchange in relation to the fundamental question of the age and state of 

 the tissues. Table 42 gives the general averages of the whole series of 

 gas-interchange experiments in which the external conditions, except 

 temperature, are relatively constant. All of these experiments were in 



normal air and in the dark. No account has been taken of the individual 



c*o 



-Q^ ratios or acidities in these averages, but the results have been arranged 



in three (in one case four) temperature groups. The average gas-interchange 



ratio has been given for each group, but does not have great significance, save 



CO 

 to show in a very general way the gross average -Q^ quotient. The amounts 



of gases exhaled and absorbed are given in terms of both fresh and dry weights 

 of the plant substance. 



Attention is again called to the fact that in this section of this paper all 

 quantities of carbon dioxide and oxygen are expressed in cubic centimeters 

 unless it is specifically stated otherwise. The acidities are given in terms of 

 cubic centimeters of decinormal alkali per cubic centimeter of juice or per 

 gram fresh or dry weight. 



For the same amount of dry substance the young joints respire at a rate 

 about three times that of the flaccid ones and twice that of the turgid plants. 

 The relative quantities of carbon dioxide evolved and oxygen absorbed are 



essentially the same in the. young and the turgid material, at the temperatures 



PO 

 from 31 to 36 C., so that their ratios are similar. In the 40 to 41 C. 



