PERMANENT WILTING IN PLANTS 119 



ranging from 0.1 or 0.2 cc. to 3 or 4 cc. per hour, loss from the 

 standard porous cup atmometer, with other conditions such as 

 were provided in the present study, and with plants such as 

 were here used. 



The studies of Caldwell and of the present writers have thus 

 established a range of atmospheric evaporation intensities 

 within which this universal conclusion of Briggs and Shantz 

 does not, at least generally, obtain; it remains to be determined 

 under just what sort of conditions the moisture residue at per- 

 manent wilting, for any given soil, may be practically unaffected 

 by variation of evaporation intensity within a considerable 

 range. The descriptions of the exposures employed in their 

 studies, as given by Briggs and Shantz, lead to the conviction 

 that their evaporation intensities were almost certainly not 

 below, 0.1 or 0.2 cc. per hour, loss from the standard atmometer, 

 and it is highly improbable that their intensities were above 3 

 or 4 cc. per hour. The range of atmospheric evaporating power 

 covered in the present studies thus appears probably to have 

 included the corresponding range used by Briggs and Shantz. 



From these considerations it is suggested that the special con- 

 ditions above mentioned, in regard to which the Washington 

 experiments must have differed from those reported by Caldwell 

 and in the present paper, may have been within the plant body. 

 Brown {loc. aL, page 132) emphasizes the observation from his 

 experiments, that the atmospheric condition under which a plant 

 has grown may frequently determine its nature as a water ab- 

 sorbing, conducting and transpiring system, and this idea is 

 generally held by students of field ecology. It is true that Cald- 

 well found the conditions under which his plants were grown to 

 have no appreciable effect upon their behavior in regard to 

 permanent wilting, but this may mean merely that he did not 

 happen to have the requisite differences in these conditions. 

 It remains at least possible that the explanation of the diametric 

 opposition involved between the conclusions derived from the 

 Briggs and Shantz experiments and those indicated by the work 

 of Caldwell and of ourselves, may lie in some internal difference 

 between plants grown in summer at Tucson, and those grown in 



