30 ACIDITY AND GAS INTERCHANGE IN CACTI. 



as shown in the control experiments which were not inclosed in glass, where, 

 at the end of a similar exposure, the acidity amounted to only 0.36 per cubic 

 centimeter juice, or not much more than 25 per cent of the original quantity. 

 It must be remembered that in the latter case the material was under the 

 action of light free from any screen whatsoever, and also under the influence of 

 the outside temperatures, which were higher in all cases than those within the 

 cooled glass chambers. It would be better, indeed, to expose the material in 

 quartz containers or under some form of glass which allowed all of the sunlight 

 to pass through, in which manner the full effect of the sunlight could be 

 obtained. 



A more detailed analysis of the figures from these experiments shows that 

 the range of acidity was greater in the young shoots used than with the older 

 ones. In the young tissues an average shows the initial acidity to have been 

 1.70 per cubic centimeter pure juice, which fell to 0.71 ; while the mature joints, 

 which in the beginning stood at 1.27, dropped only to 0.76 per cubic centimeter 

 juice. There does not seem to have been any marked effect of the tempera- 

 ture, though there was a slightly less proportional drop at 5 to 6 C. than at 

 20 C. As has already been said, the temperatures were all below the critical 

 point where this factor markedly affects the disintegration of the acids. 



EFFECT OF TEMPERATURE ON ACIDITY. 



The series to test the effect of temperature alone were, of course, kept in the 

 dark. Here not only was the action of heat on the decrease of acidity studied, 

 but also upon its increase. 



The results of experiments with mature joints that have a relatively high 

 acidity showed that at 20 to 30 C., after several hours' exposure, there is only 

 a slight decline from the original amount of acid; but at 40 C., on the other 

 hand, the decrement is nearly half (table 15). 



The actual averages of the various experiments are as follows : initial acidity, 

 1.32 per cubic centimeter juice; final acidity at 20 C., 119; at 30 C., 1.24; 

 at 40 C., 0.79. With young joints under similar conditions the same phenome- 

 non is noticed, with the exception that at 20 C. there is an increase. The 

 figures are as follows: initial acidity, 1.58 per cubic centimeter juice; final 

 acidity at 20 C., 1.67; at 30 C, 1.45; at 40 C., 0.81. As in the case with 

 mature joints, the decrease at 40 C. is nearly 50 per cent. The difference of 

 the response of the old and young tissue at 20 C. is interesting, the latter 

 showing a rise in acidity at that temperature, and even at 30 C. an increase 

 was noted in the young joints in one instance. The reason may be that the 

 young joints were probably not so near their highest possible acidity, while the 

 old joints were; consequently the equilibrium of the latter was more easily 

 disturbed, with the resultant natural tendency for the acidity to fall when it 

 has accumulated to its maximum. It is known from the results of de Vries and 

 others that continued darkness tends, other things being equal, to induce a 

 diminution of acidity; but from the experiments cited above it is evident that, 

 aside from any influence the protracted absence of light may have on tissue 

 of high acid-content, the decrease is hastened by rising temperature. When it 

 comes not to the decrement of acid in material of high acidity, but to the incre- 

 ment of it in plants where it is already low, that of the three temperature 

 ranges employed, the accumulation of acid in both young and old tissue is 



