SUMMARY AND CONCLUSIONS. 103 



is the result of the deacidification, which does not seem to be connected with 

 the actual vital processes. The breaking down of the acid by sunlight is an 

 unavoidable consequence of its accumulation in the tissues and does not seem 

 to be connected with the energy-releasing reactions which are of a true respi- 

 ratory nature. 



Carbon-dioxide evolution as a whole, while not so great as in more typical 

 plants, is by no means as small in the cacti as is commonly supposed. Carbon- 

 dioxide evolution as representing true katabolic activity is probably much less. 

 In estimating its output it is of course impossible to say, except by inference, 

 how much of the gas is from the latter source and how much simply from the 

 splitting of the stored acids. 



By both the methods which were used it was found that the newly formed 

 joints have a much higher rate of carbon-dioxide formation, which is probably 

 due as much to the increased amount of acid stored as to any other cause. 

 If activity is to be measured in terms of this sort, the mature-turgid joints are 

 much less active than the young tissue and the mature joints which are desic- 

 cated are still more quiescent, but not by any means in what could properly 

 be called a resting-state. The proportional relation of carbon-dioxide evolu- 

 tion in the three conditions of the tissues is represented by the ratio 6:3:2. 

 The relatively slight decrease in the rate in mature-flaccid joints, as compared 

 with the mature-turgid ones, when considered in connection with the fact 

 that their acidity is much lower than the more active tissues, again suggests 

 that the relation between respiration in the proper sense and de-acidification 

 is not as close as has been assumed. 



Increased heat hastens the production of carbon dioxide 100 per cent or 

 more for every 10 increase, thus conforming measurably to the principle of 

 van't Hoff regarding the acceleration of chemical reactions by rise in tempera- 

 ture. However, since the absorption of oxygen does not increase as rapidly, 

 the gas-exchange ratio with oxygen naturally rises under these conditions, which 

 only goes to support the idea that a considerable part of the increased evolu- 

 tion of carbon dioxide is not truly a result of the respiratory function. 



The maximum carbon-dioxide formation is at 45 C. and from this point 

 there is a gradual decrease as the temperature rises. It is, however, question- 

 able if at the high temperatures of 60 to 65 C. the continuance of an evolution 

 of carbon dioxide may not be wholly due to destructive processes not properly 

 respiratory. High temperatures in the presence of oxygen result in the decrease 

 of acid in the living plant, and it is possible that there may be some katalytic 

 agent present which initiates this action even in the absence of light. Were 

 it not for the evidence which Spoehr has brought forward the presence of some 

 enzyme might be suspected. Finally it may be added that some of the carbon 

 dioxide exhaled might be due to gas dissolved in the cell fluids which the high 

 temperature drives off. 



"Intramolecular respiration" is relatively high in these plants. If the 

 amount of carbon dioxide evolved under normal conditions be taken as 100, 

 then in the absence of oxygen the quantity at 23 C. is 66 and at 35 is 85. 

 That there is proportionally so large an amount of the gas produced does not 

 seem to be due to deacidification, for as we know acidity does not fall rapidly 

 in the absence of oxygen and it may rise. As has been pointed out, however, 

 it is not impossible that the preliminary stages of deacidification which involve 

 the formation of simpler acids than malic may be going on, so that when the 



