84 ACIDITY AND GAS INTERCHANGE IN CACTI. 



exposed to the sunlight, so that it is probable that some of the latter gas is 

 afforded the plant from that which is liberated by the feeble action of the 

 chlorophyll-bearing tissue. 



Appended to table 63 are the results of four experiments (RR, WW, SS, ZZ) 

 carried on in sunlight, which are exceedingly perplexing. Since there was no 

 known reason for excluding them, they have perforce been incorporated with 

 the rest. Two of them were with young material and two with mature-turgid 

 joints. In all, there was a very small evolution of carbon dioxide and a very 

 large absorption of oxygen. In fact, the oxygen absorption exceeds that found 

 in any other series of experiments. What the cause may have been for this 

 extraordinary behavior there is no clue. It is possible that there was some 

 hidden experimental error, but the records of the experiments give no informa- 

 tion which will afford an explanation. 



The question of the reaction of these cacti in the sunlight brings up many 

 interesting problems. It would seem that at the time of year when there is 

 large storage of acid and when the temperatures are relatively high there must 

 be an actual loss of weight as a result of the action of the light upon the stored 

 acids. It is not to be denied that during such periods in the summer rainy 

 season Opuntia versicolor makes its growth, and makes it seemingly at the 

 expense of its stored food supply. The photosynthetic activities do not keep 

 pace with the growth metabolism. It is suggested by these results that the 

 actual accumulation of energy for growth must take place during the cooler 

 periods of the year, but at a time when the tissues are turgid and consequently 

 active. This would be during the winter rains, and the relatively cooler 

 spring season which follows them. It is at that time, indeed, that many of the 

 other Opuntias make their growth and when the majority of them produce 

 their flowers. The subject is one of no little interest and demands investiga- 

 tion in an extensive manner. It is to be regretted that no determinations of 

 gas interchange in sunlight were made at lower temperatures, which might 

 have afforded the answer to this question. It is, of course, quite impossible 

 that this evolution of carbon dioxide is a constant phenomenon under all con- 

 ditions of illumination unless we were to imagine the preposterous possibility 

 that these cacti are nourished by the mycorrhizas which exist on their root 

 systems. There can not be any reason for doubting the experiments, and the 

 fact that another experimental method, that of the Pettenkofer apparatus, 

 yielded similar results, gives additional consideration for accepting them. 

 Care was taken to exclude the possibility of external non-chlorophyll-bearing 

 organisms influencing the results, and the material used was inspected with 

 especial care to make sure that no insect grubs were harbored in the tissues, 

 so that possible contamination of the results by carbon dioxide from these 

 sources may be neglected. 



EVOLUTION OF CARBON DIOXIDE IN THE ABSENCE OF OXYGEN. 



In the experiments tried with an atmosphere of practically pure hydrogen, 



CO 

 there is naturally no question of a ^gp ratio (table 65). The small percentage 



of oxygen which was present at the beginning of the experiment had disap- 

 peared at the time the gas sample was collected. The main point of interest 

 here is the relatively high rate of the carbon-dioxide production, despite the 

 absence of oxygen. 



