106 ACIDITY AND GAS INTERCHANGE IN CACTI. 



summer only at the cooler part of the day, or it may take place mainly at the 

 cooler time of the year, when, by reason of the lower temperature and some- 

 what diminished intensity of the sunlight, the breaking-down of the acids is 

 less. At such times the photosynthetic activity of the plant is able to more 

 than utilize all the carbon dioxide which is evolved in the processes of deacidi- 

 fication. Such a relatively restricted period during which the cactus is actually 

 gaining in substance would mean a slow accumulation of stored energy and 

 might account for the relatively slow growth of the plant. The first expansion 

 of the young joints in Opuntia versicolor is, it is true, rapid, so rapid indeed 

 that it must be accomplished on the basis of material already accumulated, 

 but after the first growth the increment is very gradual. In this species the 

 formation of new joints takes place during the summer rains, a period of high 

 temperature and ample water-supply, and, if the above is true, on the basis 

 of the potentital energy stored during the previous spring. This whole ques- 

 tion is one of such interest that it would be well worth while to investigate 

 the matter closely, as it appears to represent a periodicity which is partially 

 inverted as compared with the ordinary type of plant, when the hottest time 

 of the year is time of greatest food accumulation. In this research, however, 

 no attempt was made to investigate the photosynthesis of these plants, so no 

 further explanation is at present possible. 



It has been pointed out by various authors that the anatomical structure of 

 the cactus stem must greatly interfere with the absorption of both carbon 

 dioxide and of oxygen. A conservation, therefore, of the carbon would, it 

 is maintained, become a matter of great moment to the plant. Hence it is 

 considered that the retention of the combined carbon in the form of acid is an 

 effective means of this conservation, for in the daytime, even if the acid may 

 not be directly built up again by photosynthesis into plastic food material, it 

 may at least serve as an additional source for carbon dioxide when it is broken 

 down by the sunlight. This may well be the case in a large measure, but it is 

 evident from the results just discussed that photolj-sis of the acids and the 

 consequent production of carbon dioxide can take place more rapidly than the 

 photosynthesis can, so that under these conditions there must be an actual 

 loss of dry weight. 



It has been said above, that in the estimation of the writer the carbon 

 dioxide arising from deacidification which is the result of the further oxidation 

 of the malic acid present is not to be considered as a consequence of a true 

 katabolic process and is not respiratory carbon dioxide. As this point of view 

 is not in agreement with the usual supposition further analysis of the matter 

 is necessary. 



It has been stated that the partial oxidation processes which result in the 

 formation of acids, of which malic is undoubtedly the chief, constitute the 

 respiratory phenomena and may be conceived as taking place at all times, 

 although during the day the accumulation of the acid does not ensue because 

 of the effect of light and higher temperature. In the writer's opinion, the 

 substance oxidized to form these acids is the complex of the living protoplasm 

 itself, which in turn no doubt draws upon the reserve food supply to make up 

 the loss. This reserve food supply may very well be in its last analysis the 

 accumulated carbohydrates. That the sugar is directly consumed as such, 

 and that the energy liberated directly from such consumption can be that 



