DEPARTMENT OF BOTANICAL RESEARCH. 85 



mination of the gaseous interchange. This apparatus consists of a 

 closed flask with mercury seal, to which is attached a graduated 

 manometer and suitable means of drawing off a known amount of 

 gas from the flask. The decrease in pressure resulting from the with- 

 drawal of gas gave the necessary data for calculating, with considera- 

 ble accuracy, the capacity of the flask containing the experimental 

 material. Previous to the determination of the capacity, however, 

 the gas sample to be analyzed had been drawn off at a much less 

 diminished pressure, usually not more than a negative pressure of 5 

 or 6 mm. of mercury. For this a special arrangement was provided. 

 This is a not unimportant precaution, for under greatly diminished 

 pressure gases held in the plant tissues are drawn out which would 

 entirely vitiate the result. 



Six of these gas-flasks were taken to Tucson in the early part of 

 the summer, so that a considerable number of experiments were car- 

 ried through in a relatively short time. The gas samples were pre- 

 served as before. The object of these experiments was twofold: (1) 

 to fill out certain lacunae in the work of the previous year and (2) to 

 obtain data from which the actual amount of COg evolved and of Oj 

 absorbed could be more accurately calculated than by the previous 

 method. 



Especial attention was paid to experimenting with plants the 

 acidity of which might be presumed to be falling from a near maxi- 

 mum or rising from an approximate minimum of acidity. Parallel 

 acidity determinations were made in every case. From the analyses 

 of last year's experiments it also appears that the gas interchange of 

 the cacti under exposure to diffuse daylight and bright sunlight is 

 of great interest. Series of experiments were therefore run under 

 these conditions. Analyses of the gas samples taken from all these 

 experiments are being made by Miss Latham. At the lower temper- 

 ature and equable conditions found at Carmel experiments have been 

 carried on, the results of which will be determined later. 



CO 



The ratio of the -~ interchange appears to be a function of the 



acidity rather than a matter of the age of the cactus joint and is 

 more nearly unity when the acidity is high. Consequentlj^, as stated 

 in last year's report, the younger joints, in which the highest acidity 

 has been found, behave more like the ordinary plant in this regard 

 than do the older portions; or, in other words, the greatest absorption 

 of oxygen comes in the process of disappearance of the acid and is less 

 marked during the accumulation thereof. 



From the results obtained in this work, as well as from the informa- 

 tion afforded by the work of Dr. Spoehr, it seems undoubtedly to 

 be the case that much of the evolution of CO2, at least during the 

 period of diminishing acidity, is not respiration in the restricted 

 sense of the term. This is especially true probably of the COg evo- 



