THE GREEN LEAF 65 
when the surplus vapour is constantly passing 
away through the stomata. 
But the stomata are important as the means 
of gaseous intake, as well as for the output of 
water vapour and other gases. Now, although 
the apertures are very numerous, the total sum 
of their areas reckoned as a fraction of the 
surface of the leaf is still very small. The 
amount of carbon dioxide in the air is likewise 
very minute, and yet the intake of carbon 
dioxide is very large. For many years the 
explanation of this apparent anomaly re- 
mained obscure, but investigations revealed 
the fact that the leaf actually absorbs as much 
carbon dioxide as if its chlorophyll-containing 
cells were exposed freely to air, and were not 
covered by a membrane or epidermis at all. 
The explanation is to be found in a remark- 
able modification of the ordinary conditions 
of diffusion through their perforated mem- 
branes. It is to the effect that when the 
orifices become small enough the rate of 
diffusion through them increases, area for 
area, up to certain limits. Or to express it 
more precisely; while the rate for relatively 
large holes varies very nearly as the areas of 
the holes, it varies as the diameters of small 
holes if these are sufficiently spaced apart. 
In these respects, then, the leaf is an organ 
admirably adapted for the discharge, in the 
most efficient manner possible, of the im- 
portant function of photosynthesis. The 
necessary passage of gases and water vapour, 
E 
