
1905.| On the Physiological Processes of Green Leaves, 81 
and this maximum will be attained at comparatively low speeds of air 
current.* 
But although this statement applies to stomatal transpiration, which is 
responsible for the greater part of the loss of water by the leaf, it is not 
necessarily true for cuticular transpiration which may be more responsive to 
varying wind velocities. The amount of water brought to the free exterior 
surface of the cuticle must depend not only on the permeability of the cuticle 
itself, but also on that of the epidermis and the underlying tissue. The 
epidermal cells with their large lumina must form a bad water-conducting 
tissue, especially where they overlie the spongy parenchyma. The recent 
experiments of Buscalioni and Pollaccif on the absorption of water by 
collodion-films applied to the surface of the leaf indicate that the portions of 
the cuticle in apposition to the anticlinal cell-walls of the epidermis absor) 
more water from below than the other portions, and it seems probable that the 
supply for cuticular transpiration reaches the surface of the leaf mainly 
in this way. Notwithstanding the extreine impermeability of the cuticle, the 
large area which it exposes relatively to the stomatic openings certainly con- 
tributes a sensible proportion of the transpired water, and it seems probable that 
careful observations on the influence of the speed of an air current on the 
rate of loss of water may enable us to more sharply differentiate the two 
forms of transpiration which have hitherto been determined only by 
comparisons of stomatiferous and non-stomatiferous surfaces. 
Sutficient has been said abovegto indicate the complexity of the problem 
with which we have to deal, and with all the constantly varying factors 
acting and reacting on each other, it inay well be understood that under 
natural open-air conditions the thermal relations of a leaf to its surroundings 
must be constantly undergoing re-adjustment, and that the position of the 
point of “static thermal equilibrium ” must change from moment to moment 
with every passing cloud, with every gust of wind, and with each change 
in inclination of the plane of the leaf-lamina to the incident radiation.t 
In the absence of means for instantaneously determining all these factors 
it is manifestly impossible to ascertain the conditions at any particular 
moment of time, and there would, perhaps, be no great advantage in doing so 
even if it were possible. It is the average values of the varying factors, 
* It is quite possible that, quite apart from the question of diffusion-phenomena, the irregular 
action of a strong wind on leaves may result in a certain amount of intermittent compression of 
the leaf-lamina which may produce tidal ebbs and flows of air through the stomata. 
t+ ‘Instit. Bot. Univer. Pavia,’ vol. 7, 1902. 
_ { The intensity of the radiation received on unit-area varies with the sine of the angle of 
incidence. 
VOL. LXXVI.—B. G 
