EVOLUTION OF CARBON DIOXIDE AS DETERMINED BY THE 



PETTENKOFER METHOD. 



NORMAL RESPIRATION. 



The rate of liberation of carbon dioxide in the cacti shows considerable 

 variation, which is to be expected if it is in any way connected with the acidity 

 and the fluctuations thereof. In the first place, there must be distinguished 

 here, as in the case of acidity, three classes of tissue. These are as follows: 

 that which is mature and turgid; that which is mature, but flaccid by reason of 

 desiccation; and that which is young. Table 28 shows the relative activity 

 of all three at 30, 35, 40, and 45 C. These temperatures were chosen for 

 experimentation as representing the normal and the higher ranges to which 

 the plant may be subjected in its native habitat during the summer season. 



It is evident that the young joints, either those which arise in the natural 

 course of events as a result of the summer rains or those the formation of 

 which is induced by irrigation, have a much higher rate of carbon dioxide evo- 

 lution than the mature ones. If the comparison is made on the basis of equal 

 weights of fresh tissue the amount given off is from 80 to 100 per cent higher in 

 the young material than in the old. The disparity is considerably greater if 

 the dry weight is taken as the standard since the young tissue contains pro- 

 portionally more water, the excess of carbon dioxide produced by the young 

 joints over that of the mature ones being from 118 per cent at the lowest 

 temperature, to almost 200 per cent at 40 C. Reference to table 26 will 

 show this. It will be remembered that the young tissue is also more acid 

 than the older parts, which indicates what the subsequent experiments show, 

 that acidity and evolution of carbon dioxide are closely connected. 



In comparing the turgid and flaccid joints there does not appear to be as 

 great a difference in the rate at which the gas is given off as might be expected. 

 On the basis of fresh weights, the latter appear to be more active than the for- 

 mer, which is of course due to the considerably larger amount of dry substance 

 in the flaccid material over that in the turgid when equal weights of fresh tissue 

 are taken. The true comparison should be made on the basis of dry weights. 

 With this standard it will be seen that the flaccid material at 30 C. gives out 

 about 90 per cent as much carbon dioxide as does the turgid, diminishing 

 proportionally with rising temperature until at 45 C. it is not much over 80 

 per cent. The rate, however, is considerable even in the most desiccated and 

 shriveled plants found under normal conditions. It was, indeed, a matter of 

 some surprise to find such an active evolution of carbon dioxide with the 

 tissues in an apparently quiescent condition. It appears that the water- 

 balance maintained in the absence of external water-supply, while not suffi- 

 cient to keep the cells of the plant turgid or to afford the conditions necessary 

 for growth, was evidently enough to allow for a not very greatly diminished 

 metabolic activity. Thus it can hardly be said that the cacti enter into a 

 period of rest when they become desiccated during the dry season. This 

 probably applies to the constructive as well as to the destructive processes, 

 and, as will be seen for reasons given later, it seems not unlikely that the 

 actual accumulation of substance by this group of plants takes place not so 

 much during the hot and relatively moist period of the summer rains as 



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