278 BOTANICAL GAZETTE [OCTOBER 
The relative translucency of moist-air leaves and dry-air leaves was 
approximately measured in the case of Nicotiana and of Ipomoea 
y blue-printing them with various terms of exposure to sunlight, 
until an equal degree of blueness was obtained-for the portions of 
paper covered by each kind of leaves.¢ The moist-air leaves of 
Ipomoea were found to be 3.5 times as translucent as the others, and 
those of Nicotiana 3 times. In the latter plant many of the first 
leaves grown in moist air stand nearly vertical, while those of the dry- 
air plants form approximate rosettes. This vertical growth of the 
moist-air leaves is exactly the reverse of the epinasty of the leaves of 
Sempervivum tectorum and Ovxalis floribunda noted by WIESNER.’ 
The differences in form and size between Nicotiana leaves and 
Ipomoea leaves grown under moist and under dry conditions are 
shown in jigs. 4, 5. 
The histology of the leaves studied did not differ nearly as much 
as it often does in sun plants and shade plants of the same species. 
In house-grown individuals of Phaseolus, leaves developed in dry 
air exceeded in thickness those in the glass cases by 25 to 33 per cent., 
the upper epidermis of the former was about 25 per cent. thicker, and 
the palisade layer was a little thicker. On the other hand, moist-air 
leaves of Sinapis were found to be a little thicker, and of Ipomoea 
sometimes 25 percent. thicker, than leaves of these genera grown in 
dry air. As might have been expected, less notable differences were 
found between leaves grown in air nearly saturated with moisture and 
those grown in the moderately moist air of the greenhouse. 
The behavior of moist-air leaves and dry-air leaves, on being 
deprived of a water supply and exposed to air at a temperature of 
about 21° C. and 25 per cent. relative humidity, differs greatly. Ifa 
shoot of each kind is cut and exposed to such air, in many cases — 
(Brassica, Cucumis, Ipomoea, Oxalis, Phaseolus) wilting begins in 
from 0.5 to 2 minutes. Even if the shoots are cut under water and — 
kept with the cut end always submerged, wilting is prompt and con- 
tinuous. Shoots of Phaseolus were cut and laid in sunshine, in ait | 
of humidity probably below 25 per cent., at a temperature of 23°3 C. 
: 
One shoot was from the saturated air of the glass case, the other from 4 
4 This of course only measures translucency with reference to those rays which 
affect the blue-print paper 
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