66 REPORTS ON INVESTIGATIONS AND PROJECTS. 



As the period at Tucson comprised the end of the dry fore-summer and 

 the subsequent rainy season, a very favorable opportunity presented itself 

 to study the activity of the young joints which in Opuntia versicolor are 

 found at this time. Consequently especial attention was paid to a compari- 

 son of the behavior of the young tissue and the mature joints of the previous 

 year. It was also possible during late June and early July to compare the 

 condition of mature joints which were turgid and others in an inactive state. 



In general, the respiration of the young joints is far higher than that of 

 the older but also turgid tissue, while that of the latter greatly exceeds the 

 respiration of the mature but inactive plant. 



Corresponding to the respiration activity there is a marked difference in 

 acidity. That of the young joints is high, in some cases i c.c. of pure juice 

 being the equivalent of 1.75 decinormal alkali, though usually between 1.4 

 and 1.5. The acidity of the old dry joints ranged from a third to a fifth of 

 the above, while that of the mature turgid joints, which varies considerably, 

 occupied a middle place. The range of acidity at different times in the day 

 was enormous in the young tissue, in some cases falling in the late afternoon 

 to nearly a tenth of that in the morning. With the fall in acidity there is 

 a corresponding fall in respiration, most marked, of course, in the young 

 joints where the acidity range is greatest. 



In the gas interchange there is also a difference between the young and 

 old shoots, the younger ones approximating more nearly the usual absorption 

 of oxygen and evolution of carbon dioxide, while the older joints show 

 the larger use of oxygen, characteristic of succulent plants. The acidity 

 increases in the absence of oxygen and decreases very greatly with an 

 increased supply in the atmosphere. 



The experiments during the summer seem to show very conclusively that 

 the diminishing acidity from early morning to late afternoon is a combined 

 temperature and light reaction. In joints kept in the dark a fall in high 

 acidity at high temperatures and a rise from low acidity at low temperatures 

 was repeatedly obtained. On the other hand, a decrease in acidity in plants 

 kept at moderate temperatures (about 15 C.) but exposed to the brightest 

 illumination possible was noted in comparison with a practically stationary 

 acidity of similar material, also kept cool, but in diffuse light. This latter 

 observation is in keeping with results obtained by Dr. Spoehr as to the 

 effect of sunlight on the hydroxy-acid. 



With every precaution taken to obviate contamination of the results by 

 wound respiration or possible foreign organisms, a decided evolution of 

 carbon dioxide at temperatures above 30 C. was obtained, even when the 

 joints were exposed to the direct rays of the sun as filtered through a layer 

 of water and the walls of the glass container. 



Opuntia versicolor was used; in the major part of the work other cacti 

 with 0. discaia, O. blakeana, 0. leptocaulis, and Mamillaria grahami were 

 employed to some extent, both in the respiration experiments and in those 

 on gas interchange. 



