GAS INTERCHANGE. 77 



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is reached at the higher temperatures, whereas in the highest acidity group, 

 from 5.00 to 7.85 per gram, the reverse is the case, as the acidity is highest 

 at the lowest temperature, 23 to 24 C. This is in accord with what has 

 already been described in the previous chapter on acidity alone. When the 

 acidity is low, high temperatures do not so greatly affect it; and if conditions 

 are otherwise favorable for an increase, under such circumstances it will 

 commonly rise. On the other hand, when the acidity is high it will fall if the 

 temperature is raised, since the breaking down of the acid is considerably 

 hastened thereby. This accounts for the condition just described. Left to 

 itself without external stimulus, it may be supposed that the tissue establishes 

 a certain equilibrium of acid concentration within itself, which is higher than 

 that found in the lowest acidity group and lower than that of the highest 

 group. In the former case, rising temperature may serve as a stimulus to 

 acid formation, while in the latter case it acts in the opposite way. In this 

 connection attention should again be called to the fact that too much stress 

 should not be laid on the results of the series at 40 to 41 C. There were only a 

 few experiments at this temperature and in these it chanced that the acidity of 

 the juice was very high in proportion to the total acidity in the material used. 



The relation between pure-juice acidity and dry-weight acidity must be 

 dependent on the actual amount of juice present in the tissue. A high acidity 

 of the expressed juice, with a low total acidity dry weight, must mean a 

 relatively desiccated tissue; and the opposite must also be true. There is a 

 considerable range of individual variation in this regard aside from the large 

 differences which are to be noticed between what can readily be distinguished 

 as turgid and flaccid joints. A study of loss or gain in acidity in relation to 

 water loss or gain in the tissues would throw light on this question and would 

 no doubt be of no little interest, but no such investigation was attempted. 



In the matter of the relation of total acidity as calculated for dry and fresh 

 weight, the proportion of one to the other must depend directly on the per- 

 centage of dry substance present, and naturally there will be greater disparity 

 between the two with young and turgid joints than with old and flaccid ones. 



PURE-JUICE ACIDITY AND RATE OF GAS INTERCHANGE. 



It should be noted that the acidity of the undiluted juice as expressed from 

 the tissues really means the concentration in terms of acid of that juice. Con- 

 centration should influence at least the speed of the reactions which are taking 

 place and it is not without interest to examine the results considered on the 

 basis of this pure-juice acidity, as shown in table 51. The irregularity of the 

 gas interchange at different concentrations does not suggest that it plays a 

 very important part, and a comparison with the previous table (50) of aver- 

 ages, where the basis of separation was dry-weight acidity, makes it seem 

 that the actual quantity of acid present most greatly influences these phenom- 

 ena. From the averages given here, however, it does appear that the evolution 

 of carbon dioxide and absorption of oxygen are highest when the juice acidity is 

 from 0.71 to 0.80 per cubic centimeter. To establish any certainty in this 

 matter would require a very much larger number of experiments as well as the 

 observation of certain precautions that were not taken inasmuch as this idea 

 was an afterthought. 



