EVOLUTION OF CARBON DIOXIDE. 49 



acidity was relatively high, being 1.03 per cubic centimeter juice, and the 

 amount of carbon dioxide that was liberated was 0.72 mg. per gram-hour, dry 

 weight ; while in the afternoon, when the acidity had sunk to 0.38 per cubic 

 centimeter, it was only 0.42 mg. per gram-hour. It is to be remembered that 

 in the first case the acidity was falling, though not very greatly, while in the 

 second case it was already low and was rising slightly. A similar state of 

 affairs is to be observed in the three series with young joints. The evolution 

 of carbon dioxide in the morning, which was 4.08 mg. per gram-hour, dry- 

 weight, was accompanied by an initial acidity equivalent of 1.49 per cubic 

 centimeter, while in the afternoon, when the acidity had fallen to 0.28 per 

 cubic centimeter, the amount of the gas given off was only 1.81 mg. for the 

 same unit. This was in the face of the fact that the temperature in the after- 

 noon was, if anything, slightly higher than in the morning. 



The results of the temperature series already discussed might seem at first 

 sight to be antagonistic to that which has just been said, but a closer con- 

 sideration shows that there is no real contradiction. In all of the experiments 

 given in table 28 the acidity was fairly high and was falling, and so it is only 

 with the morning determinations that they should be compared. It is to be 

 regretted that we have no determinations of the effect of temperature upon 

 tissue with low acidity. 



The general conclusions which seem justified from these data are that the 

 higher the initial acidity the higher also is the rate of respiration, and the lower 

 it is the lower also will the rate be. Plants of high acidity kept in the dark 

 at normal temperatures, while they do not show a very great decrease in acid- 

 content, do not ordinarily show an increase thereof. On the other hand, 

 plants of low acidity in the dark at temperatures not above 35 C. universally 

 show a gain in acid-content up to a certain point. Thus it would seem that the 

 formation of acid is accompanied with a decreased carbon-dioxide formation, 

 which is to be expected. Again, fuller discussion must be deferred until the 

 consideration of the gas-interchange determinations which also bear upon this 

 question. 



During the normal day in the warm seasons in their native habitat around 

 Tucson, the cacti must be subjected at times to temperatures as high as 50 C., 

 when the fact is taken into consideration that within the tissue itself the tem- 

 perature rises several degrees above the air maximum. This phenomenon 

 has already been spoken of in connection with the course of acidity from 

 observations made in April 1911. A more extended series of observations 

 on this point were made during the summer of 1912. With an external 

 maximum of 45 C., the temperature within the joint was found to be 

 47.5 C. In order to determine how such high temperatures affect carbon 

 dioxide evolution, three series were made at temperatures from 35 to 65 C., 

 at 5-degree intervals (table 31). In all cases it will be seen that the maximum 

 carbon-dioxide production is at 45 C., and it is almost twice that at 35 C. 

 At 50 C. in two cases there was a somewhat greater evolution of carbon 

 dioxide than at 40 C., but in one case slightly less. Even at 55 C. there is 

 still as much as at 40 C., while at 60 C. the amount of carbon dioxide formed 

 is about that found at 35 C., showing a diminution from the 55 C. rate. 

 At 65 C. there is still some carbon dioxide given off, though much below the 

 normal. It is hardly to be supposed that the tissues are still alive at tempera- 



