20 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



Thus it might be supposed that the transpiration loss might reach 

 an amount in the afternoon which would result in a condition 

 analogous to wilting. Extensi\e measurements, however, ha\e 

 shown that the maximum transpiration of these cacti within the 

 ranges of temperature 60-80° F. occurs during the night period 

 and that it comes down to a minimum during the forenoon at 

 about the time elongation begins to accelerate, and that it does 

 not rise materially until the end of the daylight period. 



That the water-balance is actually decreased at night and 

 increased by day has been found by Mrs. E. B. Shreve, who 

 says of a cylindropuntia,^ "It w'as found, under conditions of 

 average transpiration, such as occur in the greenhouse in summer, 

 that the water intake at night is less than the outgo, while during 

 the day the intake is greater than or at least equal to the outgo. 

 . . . An examination of the water-content of stems from plants 

 in the open and from the greenhouse showed that the highest 

 water-content is at 5 P.M. after the close of a bright day, and the 

 lowest just before daylight the next morning, with an intermediate 

 amount at noon." 



Next there is the s\velling capacity of the material of the joints 

 to be considered, with especial relation to the decreasing acidity. 

 It has been well established by the work of Spoehr, Richards,- and 

 Long that the acidity of the sap of these cactus joints decreases 

 steadily throughout the day and reaches its minimum at a varying 

 time late in the afternoon— about two hours after the rate of 

 growth manifests such a remarkable decrease. According to 

 Long's results,^ the hydratation capacity of separated living 

 sections increases with the decrease of the acidity during the day, 

 being at its greatest at 5:30 P.IVI. It is evident therefore that 

 growth elongations do not follow a course parallel to the capacity 

 for distention, whether this be caused by swelling (hydratation) 

 or osmosis and resultant turgidity. The rate of growth is at a 

 minimum or api)roaching it at a time when the cajjacity for the 

 tissues for taking up water is greatest, and w^hen they actually 

 contain the highest proportion. Growth in these cacti therefore 



' Rep. Dupt. Bot. Research, Carnegie Inst, of Washington, 98, 99. 1915. 



2 Richards, H. M. Acifiity and gas interchange in cacti. Publ. Carnegie Inst, of 

 Washington, no. 209. 1915. 



' Long, E. R. (irowlh and c(;lloid hydratation in cacti. Bot. Gaz. 59: 491-497. 

 1915- 



