34 ACIDITY AND GAS INTERCHANGE IN CACTI. 



by oxidation from outside. It will be shown later that the so-called intra- 

 molecular respiration of this cactus form is remarkably high, and it must be 

 supposed that the acidity is maintained or increased by such activity, just as it 

 is by normal aerial respiration. That there is so considerable a fall of activity 

 in the presence of an excess of oxygen is no more than would be expected in 

 view of the probable manner of acid formation in these plants. There is 

 evidently no support in these experiments for the claim that Astruc makes 

 to the effect that with an enriched oxygen supply the formation of acid is 

 encouraged. 



As the experiments described above were all carried on in the absence of 

 light, a supplementary series was made where, under the same conditions, the 

 plants were exposed to diffuse light. Here in the presence of excess oxygen 

 the acidity falls to a lower point than under normal conditions, while in the 

 absence of that gas it is maintained at somewhere near its initial amount, 

 although there is a slight decrease. This material was exposed to daylight 

 for the major portion of 2 days (table 20). With an initial acidity of 1.25 

 per cubic centimeter pure juice the mature joints, after exposure as indicated 

 above, gave in air an acidity of 0.60. In the absence of oxygen it dimin- 

 ished only to 0.96, while with greatly increased oxygen pressure it fell to 

 0.39. The young joints behaved in a similar fashion. With them the initial 

 acidity was higher, namely, 1.76; in ordinary air it fell to 0.46, while in 

 oxygen it was as low as 0.38. As in the dark, there was a much smaller dimi- 

 nution in atmospheres of hydrogen, the acidity amounting to 1.23 per cubic 

 centimeter. With the supply of oxygen less than in air, but still amounting to 

 as much as 10 per cent, there is no marked difference from the normal in the 

 acidity, but at a concentration of 5 per cent the effect of the lesser oxygen 

 supply begins to manifest itself. 



Behavior of acid formation in light seems to accord with that found in con- 

 tinued darkness. The greater decline of acidity in absence of illumination is 

 entirely what might be expected when the effect of the light in deacidification 

 is recalled. Nor is it difficult to account for the greater decline in the speci- 

 mens exposed to hydrogen. In the light, of course, the photosynthetic 

 activity of the plant supplied oxygen for the splitting of the organic acids, 

 but that this supply was limited is shown by the comparatively partial deacidi- 

 fication, a fact that is somewhat significant, suggesting as it does how relatively 

 feeble the photosynthetic activity of plants of this type must be. In all but 

 one of the cases given, the degree of acidity after exposure to an increased 

 supply of oxygen was about that which experience teaches is the natural 

 minimum. In other words, the action of the oxygen was to carry the breaking- 

 down of the acids as far as it could be carried. In normal air no such limit 

 was reached, nor, indeed, could it be expected that the illumination afforded 

 by diffuse light would be sufficiently effective. The influence of the excess 

 oxygen is in the direction of hastening the deacidification, but not beyond a 

 certain minimum. 



EFFECT OF INJURY ON ACIDITY. 



Wounding, which has the effect of increasing the area directly exposed to the 

 air, has somewhat the same effect as does excess of oxygen. A number of 

 experiments were tried to show this point (table 21). A sharp decline in 

 acidity of more than 25 per cent was noticed 24 hours after injury and this 



