
1905.] On the Physiological Processes of Green Leaves. 79 
Should the variations in solar radiation produce directly or indirectly any 
effect on the guard-cells which control the stomatic openings, there will be a 
consequent increase or decrease in transpiration due to this cause, and this. 
will again produce its effect on W and. The same will also be true of any 
variation in the hygrometric state of the outer air, and of the slightest change 
in its temperature. But of even more importance than any of the above 
mentioned disturbing factors is the influence exerted by variations in the 
velocity of the air-currents passing over the leaf. These variations act mainly 
through their effect on the thermal emissivity of the leaf, e, and have been the 
subject of a special investigation* which brings out the important fact that 
the thermal emissivity of a leaf increases over “still air” conditions by about 
00017 calorie per square centimetre of leaf surface per minute for every 
increase in the velocity of 10 metres per minute. Thus a leaf which has an 
emissivity in still air of 0015 calorie per square centimetre of leaf surface per: 
minute for a temperature excess of 1° will have this doubled by the very 
moderate air-velocity of 44-2 metres per minute (2°65 kilometres per hour). 
and trebled at double this velocity. It only requires reference to the formula 
connecting leaf-temperature with emissivity to see that this would mean a, 
corresponding diminution of the leaf-temperature to one-half and one-third 
respectively of that in still air, when all other conditions remain the same. 
But in addition to the influence of moving air-currents on-the “rate of 
cooling” of the leaf induced by alterations in the thermal emissivity of its: 
two surfaces we have also to consider the direct effect of the currents in 
promoting transpiration, and here we must draw attention to one very 
essential point of difference between the loss of water from the surface of an 
actively transpiring leaf and evaporation from a free surface of water. The 
difference is one of fundamental importance to the well-being of the leaf and 
depends on its physical structure. If we imagine a free limited surface of water 
exposed to evaporation in unsaturated air which is in very slight steady 
horizontal movement,f the partial pressure of the water-vapour in the air may 
be regarded as varying in a direction normal to the surface of the liquid from 
a maximum of p, at the immediate surface to a partial pressure of p at some 
distance / above the surface, p expressing the partial pressure (“tension ”) of 
the water-vapour in the surrounding air before it comes under the influence 
* See Brown and Wilson, infra, p. 122. 
+ For the sake of simplicity we have imagined the air to be in very slight movement just 
sufficient to displace the curved lines of equal density of the over-lying water-vapour and to- 
render these practically horizontal. The exact form which the lines of equidensity would take in 
perfectly still air over a circular disk of liquid has been fully discussed elsewhere (see Brown and _ 
Escombe, ‘ Phil. Trans.,’ B, vol. 193, 1900, p. 223), 
