84 BOTANICAL GAZETTE [january 



periodic acidity changes and the respiratory ratios in cacti, a group heretofore 

 not sufficiently studied. Extensive work has been done, principally with 

 Opuntia versicolor, with results in general agreement with what is already 

 known regarding respiration in succulents. The paper presents a large mass 

 of data, and considers the influence of light, temperature, oxygen supply, and 

 wounding on the acidity of the tissues, and devotes considerable space to the 

 relation of acidity, light, temperature, oxygen, etc., to the rate and ratio of 

 gas interchanges. The production of the acid, chiefly malic acid in cacti, is 

 thought to be due to lack of oxygen in the tissues, owing to anatomical struc- 

 tures which, to restrict transpiration, restrict the other gas exchanges as well. 

 During the night the acid accumulates, because the chief factors capable of 

 causing deacidification, namely, light, high temperature, prolonged darkness, 

 and unusually high oxygen pressures, are absent. 



The true respiratory quotient for cacti is low, and can be measured accu- 

 rately only when acidity is stationary or rising. For during falling acidity, 

 the approach of the ratio to the typical ratio, unity, is not real, because the in- 

 creased C0 2 is furnished merely by the decomposition of the acid, which is not 

 considered a respiratory process. Some of the minor points brought out are 

 that while C0 2 production closely parallels rise and fall of temperature, it lags 

 behind by about an hour, maximum and minimum C0 2 production being 

 reached about an hour later than maximum and minimum temperature; and 

 that total acidity increases more rapidly than the acid concentration of the 

 juice. This is reasonably traced to greater hydration of the colloids in the 



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presence of the acids, and to an increased osmotic pressure in. the cell sap 

 leading to greater turgidity. 



The main point of interest to physiologists is the interpretation of the 

 phenomena, which differs somewhat from that of Nathansohn, who looked 

 upon the breaking down of the acids by day as a completion of the respiratory 

 process at a time when C0 2 could be used in photosynthesis. This view makes 

 the C0 2 production during deacidification a source of respiratory energy, and 

 at the same time of great biological significance in conserving the raw materials 

 for photosynthesis. Richards considers the acid the end product of respira- 

 tion rather than an intermediate product. The breaking down of the acid by 

 day is due chiefly to light, aided by the accompanying high temperature. The 

 reaction is photolytic and not respiratory, probably takes place in the cell 

 sap, and therefore probably yields its energy not in connection with the living 

 protoplasm. He points out that C0 2 production during deacidification may 

 be so rapid as to exceed photosynthetic use of the gas, and states that 

 "whatever of energy there may be from the final oxidation of the acid 

 outside the sphere of protoplasmic activity is simply the result of anatomical 

 peculiarities of the plant, the advantages of which may well outweigh this 

 loss." 



The whole problem of acidity and gas exchange under life conditions is 

 necessarily a very complex one because so many variable factors are involved, 



