STOMATA AND GASEOUS EXCHANGE 85 



one hand, and with the tension of gas in solution in the water 

 saturated cell walls on the other. It is saturated, or 

 nearly so, with water vapour ; in light it is relatively rich 

 in oxygen, in the dark in carbon dioxide. By day there is 

 diffusion of water vapour out through the stomata, and 

 constant evaporation from the cell walls results ; the oxygen 

 which, liberated by the reduction of carbon dioxide, saturates 

 the cell water, passes into the air spaces and so outwards ; 

 the carbon dioxide in solution is continually used up, and 

 fresh supphes pass into solution from the intercellular air, 

 and diffuse from the atmosphere in through the stomata. 

 At night as assimilation ceases the conditions affecting oxygen 

 and carbon dioxide are reversed ; frequently the evapora- 

 tion of water practically ceases as the atmosphere becomes 

 saturated with falling temperature, and as the stomata 

 close. This continual drift of gases takes place primarily 

 by diffusion and not by any sort of pumping action on the 

 part of the leaf, though possibly mechanical bending by 

 air currents has a certain accelerating effect by alternately 

 increasing and decreasing the volume of the internal air 

 space. Neger (1918) has shown that in many cases diffusion 

 is free throughout the internal air spaces of a leaf, e.g. in 

 the holly, ivy, and spindle tree ; in most leaves, however, 

 intercellular spaces are not continuous and the leaf is 

 divided into airtight compartments, usually bounded by the 

 major veins, each of which is independent as regards gas 

 exchange : such are the leaves of sweet chestnut, oak, 

 beech, and elm. 



Diffusion is governed by physical laws. The molecules 

 of a gas are in continual motion, and in a gas mixture 

 where the different components are unequally distributed, 

 the molecules of each gas drift from the higher towards the 

 lower concentration, the rate of drift depending on the 

 difference of concentration — the steepness of the diffusion 

 gradiefit — on the nature of the gas, and on the temperature. 

 Between the intercellular spaces and the atmosphere, gases 

 can pass by two ways — through the cuticle, and through the 

 stomata. Since the cuticle is more or less impervious and 



