HISTORICAL. 9 



degeneration of the acids might be utilized at once by the plant for constructive 

 purposes and would not necessarily be further broken down to its final constit- 

 uents of carbon dioxide and water. This, however, would not mean that the 

 deacidification of the tissues could be effected without the evolution of carbon 

 dioxide, for this gas is also formed in the splitting-off of some of the higher 

 acids. 



How far these present studies may help to throw much more light upon these 

 vexed questions is perhaps doubtful. An attempt has been made, however, to 

 accumulate a larger body of reliable data than previously existed, with the hope 

 of being able more closely to analyze the various happenings connected with the 

 appearance and disappearance of the acids among cacti. This has been done 

 especially in relation to the interchange of gases which takes place during the 

 processes that have been commonly included under the general head of respira- 

 tion. 



The work was largely carried on at the Desert Botanical Laboratory at 

 Tucson, Arizona, under the auspices and with the support of the Carnegie 

 Institution of Washington. To be able to work upon these desert plants in 

 their native habitat and with the facilities afforded by the laboratory was, as 

 the writer acknowledges with great pleasure, an unusual opportunity. It is 

 a further pleasure to acknowledge the interest of the director, Dr. D. T. Mac- 

 Dougal, whose suggestions have been helpful, and also of the other members of 

 the staff, particularly Dr. H. A. Spoehr. The gathering of so much experi- 

 mental data in connection with the gas-interchange problem would have been 

 almost impossible without the assistance of Miss M. E. Latham, who col- 

 laborated in the work on gas-interchange. 



