THE AERATION OF PLANTS 117 
the green parts. In woody and corky parts these are supple- 
mented by the lenticels. The evidence for this statement 
does not consist only of microscopic examination of the 
tissues. A direct proof can be afforded by a simple experi- 
ment. If the lamina of a 
leaf is immersed in water, 
air can be driven through 
it by subjecting the cut end _— 
of the petiole to gaseous 
pressure by means of an 
air-pump, or even by the eeu 
effort of the lungs of the Tia. 79.—Snotion or a LENTICET. 
observer, and can be seen J, lenticel ; per, cork layer. 
to emerge from the surface 
of the leaf on which the stomatal apertures are situated. 
If a petiole is passed into a glass bottle through a tightly 
fitting cork, and covered with water, while the lamina 
remains in the air outside (fig. 80), bubbles of gas can be 
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 caused by 
the aperture being sunk in the epidermis or situated in a 
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. 
