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 



