SUMMARY AND CONCLUSIONS. 101 



in the flaccid ones. The gas-interchange ratios vary according to the acidity 

 conditions rather than according to the age of the tissue. 



7. In general, high acidities, high rate of evolution of carbon dioxide, and 

 high CO 2 /0 2 ratios are associated and the reverse of this statement is true. 



8. No difference in acidity was found between the base and the tip of a 

 joint, but the major portion of the acid is held in the layers of soft, highly 

 turgid cells outside of the bundle ring. 



The rise and fall of acidity, as far as external conditions are concerned, 

 seems to be influenced mainly by two factors: light, which is the most im- 

 portant, and temperature which, while it plays a secondary role, is by no 

 means to be neglected. In the daytime, particularly during the brilliant and 

 hot days that are characteristic of the native climate of the cacti, both of these 

 factors are in an optimal condition for the processes of de-acidification. Con- 

 sequently, the diurnal acidity curve follows closely the diurnal temperature 

 curve, but lags behind it by about an hour and must also be fairly close to 

 that of diurnal light intensity. At night not only is light absent, but also the 

 temperature is lower, and in the arid regions inhabited by these plants the 

 difference between night and day temperatures is very marked with a con- 

 sequent accumulation of acid. This relation to light and temperature is so 

 intimate that changes in the intensity of either are quickly reflected in the 

 acidity of the tissues. After a cool night the acidity is measurably higher 

 than after a warm one, and a cloudy day greatly reduces the rate of de-acidi- 

 fication. Such being the case, the degree of acidity in any one plant varies 

 greatly from day to day. Photolytic action is unquestionably the most 

 powerful factor in the fall of acidity. 



That the water-content of the tissues appears to increase somewhat more 

 rapidly than does the total acid present, in consequence of which the increase 

 in the concentration of the juice lags behind, is a point of some interest. It 

 has already been noted that there might be applied here the idea brought out 

 by Borovikow concerning the increased hydratative power of acidified colloids. 

 Under such conditions as he postulates, water which might otherwise be trans- 

 pired is held in the tissues. Also it may be remarked that the increase of 

 acids in the vacuoles might well increase the turgor pressure which in a measure 

 would act in the same way. 



That acidity declines in an excess of oxygen does not seem surprising, not- 

 withstanding the statement of Astruc that an increased partial pressure of the 

 gas is favorable for acid formation. Both the accumulation and loss of acid 

 are undoubtedly oxidation phenomena, but under ordinary circumstances 

 oxygen is more essential to the latter than to the former process. That there 

 should be even a slight rise in acidity with a complete or partial absence of 

 oxygen is a matter of some surprise. Under such conditions the rise is, indeed, 

 slow and is not very great compared with that found in normal air, but it is 

 a definitely marked one. Since the acids are themselves the product of a 

 partial oxidation of substances in the cells, the question arises how can they 

 be formed when oxygen is absent. The explanation must be that acids are 

 also formed in the course of " intramolecular respiration." This would not, 

 indeed, be wholly in accord with some of the prevalent ideas regarding the 

 phenomenon, but at the same time it is hard to see why the auto-oxidations 

 of the protoplasm might not yield, in some measure at least, end-products 



