24 ACIDITY AND GAS INTERCHANGE IN CACTI. 



In order to learn what effect upon the evolution of carbon dioxide the 

 detaching of the shoots might exert, joints which had been removed from the 

 plant for a week, and hence must have fully recovered from any possible wound 

 stimulus, were placed in the Pettenkofer apparatus and their respiration-rate 

 determined. The same joints were then cut at the base as they would be on 

 removing them from the plant and their respiratory-rate was again obtained. 

 No difference in carbon dioxide was observed over a period of hours, so that 

 here, as with the acidity, it is evident that the removal of the joints from the 

 plant causes no measurable wound reaction. The question of wound respira- 

 tion which follows considerable injury will be considered later. The point 

 under discussion is merely whether the necessary injury done in detaching the 

 joints from the plant need be taken into consideration. It is evident from our 

 experiments that any such effect is negligible. However, to avoid any possi- 

 bility of traumatic reaction by increased oxygenation of the tissues, and to 

 prevent any undue evaporation of water, the wounds caused by breaking the 

 joints apart were always painted with vaseline. Finally, it will be recalled 

 that the area of the articulation of one joint with another is not at all large in 

 Opuntia versicolor, nor, indeed, in any of the opuntias. 



While it was sometimes convenient to have a considerable supply of material 

 already gathered, particularly when a series was to be run through the night, 

 as a rule the specimens were freshly collected, as after standing there is a 

 noticeable decrease in weight due to loss of water. Under the conditions in the 

 laboratory at Tucson in the spring of 1911, it was found that 1,014 grams of 

 fresh material of Opuntia versicolor lost, after 10 days' exposure to the air 

 in diffuse light, 138 grams weight. Of course all the material sent from 

 Tucson to New York was subject to this drying process. After gathering 

 there must have been some loss of water, though after the material arrived 

 in New York, care was taken to prevent any further evaporation; indeed in 

 some cases there was a slight gain. In this connection it is interesting to note 

 that the material left unprotected in the laboratory in New York lost water and 

 shriveled far more rapidly than that lying on the tables in Tucson. Whether 

 this was wholly because of the aridity of a steam-heated building is perhaps a 

 question, but there can be no doubt that it was an important factor. The 

 material sent from Tucson to New York arrived, without exception, in excel- 

 lent condition, and while some was sent as late as November and some as 

 early as March, the weather conditions at the time of transit were such that 

 there could have been no danger of freezing. There was one thing noticeable, 

 however, namely, that under the relatively feeble illumination of a northern win- 

 ter the acidity showed much less distinct periodicity than it would have shown 

 in its natural habitat For this reason no extended series of acidity determina- 

 tions were attempted in New York, where the work was confined during the 

 winter chiefly to respiration determinations with the Pettenkofer apparatus 

 and later in the spring to the beginning of the gas-interchange experiments. 

 Many of the latter determinations have not been utilized in the final assem- 

 bling of data, as these experiments were more for the purpose of getting control 

 of methods than for final results. 



