IX FALLING LEAVES. 231 



is formed, as a plane of separation from the parent branch, under- 

 neath the base of petiole. The well-known expansive power of 

 ice acts here as elsewhere. Consequently the leaf and its stalk are 

 gently all but severed from the tree. And when the sun arises, 

 there is in its rays sufficient heat to melt the ice film^ whence, in 

 the morning, leaves drop off in great numbers. 



(4) But the anticipative power of nature is still more manifest 

 vv'hen we consider the means adopted for prevention of injury at 

 the points left bare by the removed petiole. Ordinarily, of course, 

 the fibro-vascular threads which ramify through the leaf-blade are 

 collected together into a string or bundle, passing down through 

 the leaf-stalk and joining others in the parent branch or tree. 

 But, whilst the Absciss layer immediately precedes the fall of the 

 leaf-stalk, there is much earlier a remarkable formation (on the 

 inner side of where it afterwards appears) of what is known as 

 Periderm. 



To make this intelligible to non-technical readers, it must be 

 pointed out, as tersely as possible, that plants require a "skin" 

 as much as animals. They can no more afford to have bleeding 

 spots without a protecting cuticle, than we. Hence, they are 

 provided with an epidermis, which in all trees and large plants 

 involves more or less of cork. Generally, the epidermis serves 

 three functions — (i) prevention of excessive evaporation; (2) pro- 

 tection from injury ; (3) strengthening. Here we are concerned 

 with (i) and (2). It must be remembered, that what is properly 

 termed cork, really involves several distinct layers of cells, histolo- 

 gically considered. The most important is a layer of actively 

 dividing cells, known as Phellogen or Cork Cambium, from which 

 are formed externally Cork proper or Phellem, and internally 

 Phelloderm. The whole of these is included in the name Periderm 

 (see PI. XV., Fig. 2). 



Now, it is of great importance that a corky shield should pro- 

 tectingly cover up for the winter the spots which are left naked by 

 leaf-fall. But this can only be done from within. Nothing can 

 grow out from the external cork to form a covering. As a matter 

 of fact, the Periderm is formed long before leaf-fall, even whilst the 

 Fibro-vascular bundles are in full discharge of their functions. 

 Hence, they pass right through this growing plane of Periderm, 



