PERMANENT WILTING IN PLANTS 93 



is usually present in non-submerged plants. As the saturation 

 deficit increases, the condition of incipient drying ensues, 

 where the transpiration rate is decreased as a concomitant of 

 lowered water content of the transpiring organs. As incipient 

 drying increases, temporary wilting eventually results (if the 

 plant be of a kind which wilts), and with still further increase 

 of incipient drying (in a manner made relatively clear by Cald- 

 well's discussion) permanent wilting is at length brought about. 

 It appears probable that the balancing method of Briggs and 

 Shant? should detect incipient drying directly, at some fairly 

 well defined stage, and this before any sort of wilting might have 

 become evident. The soil moisture residue at this supposed 

 stage of incipient drying may be expected to possess a higher 

 magnitude than that concomitant with permanent wilting. This 

 matter is worthy of an experimental study. 



Another source of possible error in the present experimenta- 

 tion may lie in the use of the white porous cup atmometer for 

 indicating evaporation conditions. In some of the exposures 

 here employed direct sunlight was surely an important factor 

 in the determination of the velocity of transpirational water loss 

 from the plants. As has been shown by Livinsgton,^'^ where 

 radiant energy is involved it is expedient to employ the dark cup 

 of the radio-atmometer, which integrates not only the evapor- 

 ating power of the air but also the influence of direct radiation 

 upon evaporation. The tests of the last named author show that 

 the black porous clay cup is considerably more sensitive to the 

 accelerating influence of sunlight upon evaporation than are 

 ordinary plant leaves, so that the employment of this cup may 

 have introduced an error in the opposite direction. The evapo- 

 ration rate which should have been employed in the pres- 

 ent work, — if these considerations are sound, — should have had 

 a magnitude somewhat above that given by the ordinary at- 

 mometer and somewhat below that given by the radio-atmometer. 

 In further work of this kind this matter should receive attention. 



The result of five series of wiltings will now be presented and 



1" Livingston, B. E., Light intensity and transpiration. Bot. Gaz. 52: 418-38. 



i9n. 



