1917] CURRENT LITERATURE 173 
by shorter periods of darkness and light. Humidity of the air was figured to 
a standard. 
Potometer determination of stomatal aperture involves mass movement 
of air through the stomates under differential pressure, while transpiration 
involves static diffusion of water- vapor through the stomates. To clear up 
the physics of the problem, DARWIN gives two quotations from Sir J. Larmor: 
“The speed of diffusion through a narrow aperture between two open spaces . 
is proportional to its diameter. The speed of a stream of air through such an 
aperture, between open spaces having different pressures on them, is propor- 
tional to its area if the effect of viscosity can be neglected, but proportional to 
the 4 power of its area if viscosity is — Which of these conditions 
prevails, or whether the circumstances intermediate, in a given case, 
depends upon the diameter of the mem ca de “Diffusio n through a long pipe 
or channel varies as the area, and flow through it depends upon a reduced 
area owing to the flowing air adhering to the walls of the tube; in fact it varies 
as the square of the area if viscosity is predominant. Thus if this be the case, 
-provided the channels are of fairly uniform width, transpiration would be 
_ portional to the square root of flow, the same law as that obtained for the case 
of holes in a thin plate.” Darwin b th t nearly 
represents the situation, for the first applies only to tubes whose lengths are 
less than one-fifth of their diameter. As might be deduced from either of the 
physica eigrag y zl. at L skh 
l ) 
root of the rate of potometer flow, and another by plotting the rate of trans- 
Piration for various stomatal apertures, and for 18 separate experiments finds _ 
RS tcl aah acai although there are many minor dis- . 
temperature and 1 hum conten, which might go far to eliminate r minor 
were not carried out in closely controlled — . 
"abo a in explaining these discrepancies. ‘Much has been done since 1900 to ee 
the air nnon 
Put the material and 
a -antien peigeopi sityecleigarighn step in that direction. mes a 
