THE AEKATION 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 he 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 



effort Of the lungS of the FIG. 79. SECTION OF A LENTICEL. 



observer, and can be seen i, lenticei ; JM, 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 

 lenticei 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. 



