280 BOTANICAL GAZETTE [OCTOBER 
A curious kind of quick adaptation to dry conditions may some- 
times be noted. Young leaves of Ipomoea, grown in nearly saturated 
air, were found to wilt in air of less than 30 per cent. relative humidity 
in about two minutes, though the stem was cut off under water and 
kept immersed in water. After being left for some hours in a satu- 
rated atmosphere until the wilting had entirely disappeared, the 
shoots were left, with the cut ends in water, in an atmosphere of less 
than 30 per cent. relative humidity for 48 hours without showing any 
signs of wilting. More than go per cent. of the stomata were at this 
time found to be perfectly closed. 
The relative transpiration rate in diffuse light of moist-air and 
dry-air leaves of nearly all the kinds of seedlings grown was carefully 
determined. Attempts to make use of very slender (and therefore 
quick-reading) burettes as potometers were not successful. It was 
found too difficult to attach the soft and readily crushed stems of @ 
young seedlings to the burettes in such a way as to be sure to obviate 
leakage. All losses by transpiration were therefore estimated by 
weighing the shoots and the tubes of water which contained them on a 
balance sensitive to 5™6. The relative humidity of the air in which 
the transpiration took place was measured by the sling psychrometer 
(sometimes twice) during each experiment. As might have been 
expected, the inequality of transpiration was found to be greatest in 
the case of fully developed leaves, half-grown ones showing less, 
though notable, differences. The values given below are for the 
ratio M/D, in which M is the transpiration of the moist-air leaf and 
D the transpiration of the dry-air leaf. ; 
In discussing the results above given, it should be noted that a 
considerable range of values in the ratios obtained is almost unavoid- 
able. In the first place we have to reckon with the great variability 
of transpiration in individuals of the same species grown under 
identical conditions. F. Hapertanpr® found in the case of rye 
plants that the transpiration per day varied (in round numbers) from 
2 to 7™ per square decimeter for different individuals. Also, if the 
transpiration were allowed to take place in a nearly saturated atmos- 
phere (to prevent sudden wilting), the leaves would be under condi- 
6 HaBERLANnr, F., Wissensch.-prakt. Untersuchungen auf dem Gebiete des 
Pflanzenbaues 2:146. Wien. 1877. 
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