32 ACIDITY AND GAS INTERCHANGE IN CACTI. 



placed in the dark, shows an increase of acid up to a certain point, which is 

 followed by a slow decrease if the darkness is prolonged. Material of high 

 acidity under similar conditions commonly shows a slow decrease. Such 

 results are not surprising when it is understood that the most important part 

 of the process of deacidification is a photolytic one. Experimentally considered, 

 this close connection of acidity with weather conditions gives an exceedingly 

 small expectation of precisely similar results from day to day, even with 

 material from the same plant. 



Owing to such causes difficulties in the comparison of different series tend 

 to increase, and the necessity of making large numbers of observations in order 

 to obtain reliable averages is obvious. Even under apparently like conditions 

 the plants are not constant in acidity. No attempt was made to discover 

 to what causes fluctuations of this kind were due, and while some might be 

 more or less easily traced others are probably very -obscure. It is possible 

 that there may be a correlation between degree of acidity and transpiration- 

 rate, and certainly the amount of water held at any one time in the tissues 

 would affect the acidity of the pure juice, even if it did not greatly influence the 

 total amount of acid. To establish points of this nature would require further 

 investigations with a large number of experiments designed to bring out such 

 relationships. In this work no especial effort was made to go further into the 

 matter than has been indicated. 



EFFECT OF INCREASE AND DECREASE OF OXYGEN SUPPLY. 



While deacidification has, for a long while, been considered an oxidation 

 process, it was not until the appearance of Spoehr's work that we were in- 

 formed in what manner this process proceeded. His experiments, however, 

 were not with the living plant, so that the behavior of the acid in Opuntia 

 versicolor in atmosphere containing more or less oxygen than air is of especial 

 interest. 



Various trials were made with joints in atmospheres practically devoid of 

 oxygen, in those in which the oxygen had been cut to a few per cent, and in 

 others containing 75 to 80 per cent of the gas. The first were with material 

 sent from Tucson to New York, and the reactions, while not as marked as those 

 carried on in the natural habitat of the plant, are worthy of record. This 

 series of determinations is shown in table 17, series I. The material was ex- 

 posed in an atmosphere of hydrogen, 97 per cent pure. The balance of 3 

 per cent was presumably air, which gives a concentration of oxygen of 0.6 

 per cent, which is almost negligible. Analyses of the atmosphere of the 

 receiver at the end of the experiment showed no trace of oxygen, which must 

 have disappeared almost at the outset. Under these conditions there is a 

 small but well-marked increase in the acidity of the juice of approximately 15 

 per cent. The increase in total acidity is not quite so great, amounting in the 

 average of the six experiments to 12 per cent. A similar series is shown in 

 table 17, series II, but with the addition of experiments in a 75 to 80 per cent 

 atmosphere of oxygen. In this set the gain of acidity in an atmosphere of 

 hydrogen is somewhat more marked than in series I. In the atmosphere 

 enriched with oxygen there is a distinct fall in acidity, amounting to 27 per 

 cent. But in none of the experiments made in New York were the reactions 

 as well marked as in the experiments carried on at Tucson. It must be remem- 



