54 



ACIDITY AND GAS INTERCHANGE IN CACTI. 



be supposed for this reason that the alterations in acidity are dependent 

 simply upon temperature changes. It has been shown previously that while 

 high temperatures affect acidity to a certain extent, the decrease which is due 

 to this cause is not nearly so great as that caused by the photolytic action of 

 sunlight. The fact that the carbon-dioxide output is so markedly influenced 

 by temperature, falling as it does in the range of temperatures employed in 

 these experiments to only 25 per cent of the maximum, gives another indication 

 that decrease in acidity is probabh r not a respiratory phenomenon in the 

 strictest sense. Were it so, temperature would be expected to exert as great 

 an influence upon acidity as it does on the production of carbon dioxide, 

 which is not the case. 



I3HT3 



2ihrs. 



3hrs 



6hrs. 



24hrs. 



FIG. 6. Graph showing evolution of carbon dioxide by Opuntia versicolor with progressively 



changing temperature. 



In milligrams of CC>2 per gram hour, fresh weight. The upper graph is of the COj; the lower 

 one of the temperature. The graphs are composite ones from the data given in tables 31, 32, 

 and 33. 



It is interesting to observe in these graphs how sharply the curve rises at 

 34 C. and drops correspondingly rapidly as the temperature falls. At temper- 

 atures below 39 C. this cactus is decidedly sluggish; but at 5 higher, which 

 may be considered as an average normal temperature in the sun, its production 

 of carbon dioxide is not inconsiderable. The common statement of the exceed- 

 ingly slow rate of respiratory activity in cacti is largely due, no doubt, to the 

 fact that most investigators used temperatures optimal for the ordinary 

 plant, but not so favorable for the cacti. 



It is to be observed, in looking over the whole series of results, that there 

 is a very considerable variation in the rate of carbon-dioxide evolution, even 

 at similar temperatures. In general, the determinations made in New York 

 with material sent from Tucson are higher than those made at Tucson itself. 

 The material used in New York was all sent during the winter months and the 

 joints used were those of the previous season's growth. They were fully 

 mature, but at the same time were younger than the youngest mature joints 

 used during the summer. This may very well account for their greater 

 activity over those used the following summer. 



