18 LiIvincsToN—EsTABROOK : STOMATAL MOVEMENT IN PLANTS 
table should be proportional, under otherwise identical conditions, 
to the rate of diffusion of water vapor through the corresponding 
stomatal pores. . The studies of Brown and Escomb have left no 
doubt in regard to the validity of the principle of linear dimensions, 
as regards minute circular openings. The two assumptions, (1) 
that the configuration of the cross section of the stomatal pore 
nearly enough approaches that of an ellipse to permit the use of 
such calculations as ours, and (2) that diffusion through minute 
elliptical openings occurs at the same rate as through circular ones 
of the same cross sectional area, need further substantiation, but 
they are highly probable and at least carry us much further toward 
the inception of a quantitative knowledge of stomatal diffusive 
capacity than can any other method of treatment thus far avail- 
able. While it is undoubtedly true that a stoma may close very 
considerably without at the same time producing alteration in 
the long axis of the ellipse, yet by the time closure is nearly com- 
pleted that axis has shortened markedly in many plants. We are 
thus convinced that to assume the long axis as constant and com-- 
pute diffusive capacities on the basis of the length of the short 
axis alone is a method not generally applicable. 
If the square root of the product of the two elliptical axes may 
be taken as proportional to the diffusive capacities of the stomata 
involved, that is, to their power of! transmitting water vapor, 
then the mean of the square roots derived from all measurements 
should be a relative measure of the average stomatal diffusive 
capacity at the hour of sampling. At the base of each part of 
TABLE I is given this mean. 
It is seen at once from TABLE I that all stomata were open at 
the hour of daylight observation while the majority were closed 
at the night hour. The two open pores, out of twenty observed 
for the night hour, are seen to be less widely open than were any 
at the other time. All conditions being equal excepting that of » 
the size of opening, the diffusive capacity of the stomata appears 
to be approximately 69/840, or 8.2 per cent as great at about 
midnight as at 3 in the afternoon. 
In TABLE II are given the final results of the calculation for 
each case tested. Two series are presented, one carried out on 
August 30 and 31, the other on September 7 and 8, 1910. Besides 
