1912] LIVINGSTON & BROWN—TRANSPIRATION 313 
stomata being open, a more rapid diffusion, or possibly a molar 
movement,* of water vapor through these openings into the outer 
air. As long as the sun shines, then, a high rate of transpiration 
may be maintained, and this without excessively high temperatures 
in the foliar tissues. The radio-atmometer (LivincsTon, see foot- 
note 3) furnishes an excellent example of this kind of effect; with 
continuous influx of solar energy evaporation from the black porous 
cup is maintained at a high rate, but the black cup does not exhibit 
any markedly higher temperature than does the white one, from 
which water loss is much less rapid. 
The Piche atmometer’ illustrates this process, and also shows 
another phenomenon which we need to consider here. It consists 
of a graduated glass tube, closed above and covered below with a 
circle of filter paper, the latter having a pin hole to admit air to 
the tube. The tube is filled with water, the paper disk applied 
and fixed in place, and the whole inverted. The entire disk soon 
becomes wet, and evaporation therefrom draws water from the 
tube, air rising through the pin hole to replace the water withdrawn. 
Now, if such an instrument be arranged with a relatively large disk 
of paper and placed in conditions of pronounced evaporation, it 
may often be noted (we are unable to find any mention of this in 
the literature) that the outer edge of the disk becomes dry, thus 
vitiating the readings as a measure of the evaporating power of 
the air. The instrument is unsuited to its purpose unless care be 
exercised to have the paper so small that the evaporation rate 
never surpasses the possible rate of outward diffusion to the periph- 
ery of the disk. As long as the peripheral portion of the paper 
remains unsaturated, the actual rate of water loss is maintained 
nearly constant, the dry area automatically increasing or decreasing 
in extent according to the fluctuations in the amount of energy 
4 According to the researches and calculations of Brown and Escomse (Static 
diffusion of gases, etc., in plants. Phil. Trans. Roy. Soc. London 193:223~291. 
molar streaming may not occur about midday in the Arizona desert remains an open 
question. 
$ oe iterature bearing 0 on this instrument, see LrvincsTon, GrRAcE J., An anno- 
tated b Monthly Weather Review, 1908-1909. 
The original deacitption is abstracted at page 48 of the reprint under “1872, Piche.’’ 
