TRANSPIRATION IM CACTI. Ill 



does the latter. This must mean that while the evaporating 

 power of the air is still increasing the plant in some way begins 

 to exert a retarding influence upon its own rate of water loss. 

 The curve of absolute transpiration may or may not follow that 

 of relative transpiration. In a very general way, as has been 

 pointed out, it is apt to follow the curve of evaporation. The 

 relation of the physiological retardation just mentioned to the 

 acceleration of the increasing evaporating power of the air will 

 determine what form the curve of absolute transpiration will 

 assume. The absolute magnitude of the retardation will, on 

 the other hand, determine the form of the cuiwe of relative trans- 

 piration. The latter curve is thus seen to be a curve of the re- 

 tardation in question. The curve of absolute transpiration may 

 ])c entirely misleading with regard to the retardation due to the 

 plant as an organism, and this curve is therefore not available 

 at all for a study of such retardation. 



The author has studied the curves of relative transpiration 

 for a number of leafy plants and for several forms of cacti, with 

 the interesting result that, while the curves for leafy ]dants are 

 in good agreement with one another and uniformly show a high 

 period in the day and a low one in the night, the curves for the 

 cacti exhibit the opposite condition of affairs, having a high 

 period in the night and a low one in the day. Of twenty-six 

 tests so far made at the Desert Laboratory, with eight species 

 and three genera of cacti (growing in tin cylinders standing in 

 the open), 84.6 per cent, were in agreement with the alx)ve 

 statement, the others showing erratic variations probably due 

 to pathological conditions. The forms studied comprise two 

 flat-jointed and one cylindrical Opuntia, three Mammillarias 

 and two Cereuses. 



The method used in obtaining the curves, some samples of 

 which are shown in Figure 1, was as follows: Potted plants, ap- 

 parently in good growing condition, were sealed so as to prevent 

 water loss excepting through transpiration, and then weighed at 

 intervals of from one to four hours for at least one full day. 

 The evaporation record for the same time periods was taken by 



