20 ACIDITY AND GAS INTERCHANGE IN CACTI. 



GENERAL METHODS OF PROCEDURE AND PRECAUTIONS. 



Most of the gas-interchange and respiration experiments were, of course, 

 carried on with the plants in darkness. Whatever method was used to exclude 

 light, care was taken that it was efficient. Owing to the peculiar behavior of 

 the cacti in the matter of evolution of carbon dioxide, a number of experiments 

 were, however, performed in diffuse light or direct sunlight. Those in diffuse 

 light were exposed, uncovered, on the laboratory table or were placed out of 

 doors in the shade. Where direct sunlight was used the receivers containing 

 the plants were immersed in water cooled with ice. This method of prevent- 

 ing excessively high temperature consequent on the inclosure of the speci- 

 mens in glass was efficient, though somewhat crude. Water, of course, may 

 absorb light to a limited extent, but not as much as does glass. Care was 

 taken to see that the water was clear and that the receivers were not immersed 

 any deeper than necessary. 



In the laboratory at Tucson, where a medium constant temperature was 

 required, the subterranean dark-room proved very convenient. Over any 

 24-hour period the temperature change would rarely be more than a fraction 

 of a degree and not to be considered from the standpoint of a physiological 

 stimulus. For higher temperatures a fairly even rate was obtained by the 

 use of shrouded incandescent bulbs, which, even without regulation, gave a 

 temperature which varied only a degree or two if care was taken to prevent 

 radiation. For short periods of exposure to high temperatures the small 

 greenhouse attached to the laboratory gave, in clear summer weather, a 

 temperature that was often above 40 C., and would remain fairly constant for 

 several hours during the middle of the day. For low temperatures ice cooling 

 was employed, and if the body of ice were large enough surprisingly regular 

 temperatures were obtained by it. While arrangements of this kind may not 

 make claim to be constant in the proper sense of the term, they served for the 

 study of the larger effects of temperature change in a physiological sense. 

 In New York the laboratory temperature was fairly constant during the winter, 

 varying from 29 to 23 C. ; for lower temperatures an ice-box was used and for 

 higher ones the usual type of bacteriological incubator served. 



When the results of any series of determinations were to be compared among 

 themselves, every care was taken to have samples of material as similar as 

 possible. The acidity of different plants is not necessarily the same at the 

 same time of day; indeed, it varies over a considerable range among plants 

 which appear similar and are similarly placed. It was found, however, that 

 tissue of the same age from one plant was fairly constant. Therefore joints 

 from only one plant were used in any one series of observations. To deter- 

 mine the initial acidities it was, of course, impossible to use the actual material 

 that was to be employed in the gas-interchange experiment, but in such cases 

 joints as nearly similar as possible were chosen and have been termed "par- 

 allel" material. The writer is aware that even with these precautions errors 

 may have crept in. It is, for instance, possible that similar joints from the 

 same plant might occasionally be of widely different acidity, and confusion 

 might arise by correlating their respiratory activity with the wrong acidity 

 conditions. It may be said, however, that this would probably not happen 

 often. Experience taught that the acidity distribution in the tissues of a 

 healthy plant is fairly constant. A certain check on such errors was also 

 afforded by the final acidity, which was always determined from the identical 



