138 



Hydration and Growth. 



morning or at some time in the forenoon and continues to do so until 

 some time late in the afternoon, at which point a shrinkage ensues which 

 continues until the next morning. The rate of water-loss was not paral- 

 lel with the changing volume, as transpiration was most rapid between 

 midnight and morning, and was in excess of the amount which might 



Var. 



Tr. 



Ab. 



12p.m. 6a.m. m. 6p.m 12p.m. 6a.m. m. 6p.m. 



FIG. 27. Absorption, transpiration, and variation in length of growing shoot of Opuntia. Ab- 

 sorption by roots denoted by dotted line is least about midnight and is greatest about midday, 

 exceeding transpiration during the greater part of the daylight period. Transpiration is 

 greatest about midnight and least about midday, according to data by Mrs. E. B. Shreve. 

 The wavy line is an auxographic tracing of actual changes in length of a joint of Opuntia 

 in a mature condition, X 50. 



be absorbed by the roots during this period. It is obvious without 

 further discussion that the variation in volume of a member like that 

 of a joint of Opuntia must be the resultant of the transpiration, absorp- 

 tion, and water capacity of the cells modified by the action of the new 

 colloidal material which may 



be added to the cell-masses. TABLE 106. 



It is notable, however, that 

 at some time approaching 

 midday the pen recording 

 the variations in volume 

 traces a line not far from the 

 horizontal for a brief period, 

 perhaps half an hour, at 

 which time absorption and 



transpiration balance, and the absolute water capacity of the cell-masses 

 is greatest (see figs. 28 and 29). An equally marked confluence of fac- 



