106 



LEAF-FALL. 



[OH. VII 



still considerably. It will be more easily realised how 

 porous a thing the epidermis must be, when it is remem- 

 bered that there are thousands of stomata on a square 

 inch. 



Leaf -fall. 



That the leaves of many trees fall from the tree 

 in autumn is a familiar fact, but the physiology of the 

 process is not so well known. It is easy to prove, by a 

 simple experiment, that leaf-fall is a process requiring a 

 certain mechanism for its accomplishment, that it is not a 



al: 



FIG. 48. 



LONGITUDINAL SECTION THROUGH A BRANCH AND PART OF A 

 LEAF-STALK OF THE POPLAR 



The absciss layer and a layer of cork are shown at a.l. 



ck, cork; c, cortex; /, bast fibres; 



xy, xylem; p, pith. 



