112 VEGETABLE PHYSIOLOGY 
made to emerge from its cut surface in a continuous stream 
by reducing the pressure above the water by means of an 
air-pump. 
The facility of the interchanges will largely depend 
upon the number, size, and position of these orifices. A 
lenticel will allow more gas to pass between its loosely 
arranged cells than will a stoma, but their relative numbers 
make the stomata much more important than the lenticels. 
In most cases there is a free passage through the stomatal 
pore, but in others considerable difficulty is afforded by 
the aperture being sunk in the epidermis or situated ina 
Fic, 81.—TRANSVERSE SECTION oF Rou~LEeD Lear or Heat. 
depression of the leaf. In the rolled leaves of heaths and 
certain grasses this difficulty is frequently partially com- 
pensated by the lacunar character of the parenchyma 
which is in the immediate neighbourhood of the stomata 
(fig. 81). : 
It must be noted in this connection that the stomata 
and the lenticels are passive with regard to the process of 
aeration, and do not exert an active influence upon it. 
The variations in the width of the stomatal apertures 
which are of so much importance in the regulation of 
transpiration must be regarded as bearing upon that 
function alone, being caused by fluctuations in the amount 
