ABSORPTION OF GASES. Jl 



not only by microscopical observation, but also by direct 

 experiment. A simple means of doing this is to place the 

 blade of a leaf in the mouth and to immerse the cut surface 

 of the leaf-stalk in water : if now air be forced into the blade 

 by the mouth, a stream of bubbles will escape from the end 

 of the petiole which is in the water. 



Stomata are by no means confined to leaves, although 

 they are most abundant on them. They are present, at some 

 time at any rate, in the epidermis of all subaerial organs 

 excepting, in certain cases, some of the floral leaves : they 

 are never to be found on roots, nor on submerged plants, 

 and they are confined to the Cormophyta. In the case of 

 the stems and branches of perennial plants, in which the 

 epidermis is thrown off and is replaced by cork, the stomata 

 are of course lost, but a means of communication between 

 the internal tissues and the external air is provided by 

 certain structures which are termed lenticels. These consist 



FIG. 14 (after Stahl). Section of a Lenticel of Sambucus nigra: f, lenticel-tissue; 

 c, cork-cambium ; /, cambium layer of the lenticel ; pk, phelloderni ; b, bast- 

 bundles. 



of cells belonging to the cork-layer ; but whereas the cells of 

 the cork are closely packed, the cells of the lenticels have 



