GENERAL CONCLUSIONS. 77 



in the forms examined. This acidity results from a modification of the 

 photosynthetic processes and hence is not directly connected with the state 

 of the water-balance, although it is probable that some slight concentra- 

 tion or heightening of the acidity might ensue with desiccation. Thus an 

 Echinocactus after the summer rains showed an acidity equivalent to 0.090 

 grams H^SOj per 100 cc, while another that had lost nearly seven- 

 tenths of its water showed 0.106 gram. A Carnegiea in a condition of 

 maximum turgidity showed an acidity equivalent to 0.161 gram H^SO^, 

 and one in a state of nearly maximum desiccation near the close of the 

 arid foresummer gave 0.187 gram, but a specimen desiccated for six 

 months in the open and then a similar period in a shaded room yielded 

 but 0.161 gram. Two plants growing within a few meters of each other 

 taken two days after the beginning of the summer rains gave 0.150 gram 

 and 0.181 gram, respectively. 



A careful control of the water-supply and the illumination might reveal 

 some heightening of the acidity with the depletion of the water-balance, 

 although the data given above do not demonstrate such a relation. The 

 two recognized characteristics of succulents — the high osmotic activity and 

 the modified photosynthesis resulting in great acidity — are not sufficient 

 to explain their occurrence and development in deserts and saline situations , 

 and it is evident that a more extensive investigation of their chemistry 

 must be made before an adequate interpretation can be offered for the 

 origination of the capacity for accumulation and retention of great water- 

 balances such as are carried by the succulents of the desert and the seashore. 



