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 in a 



Fia. 81. TRANSVERSE SECTION OF KOLLED LEAF OF HEATH. 



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 



