120 THE OAK. 



trees the pliellogen layers may be formed so far down in 

 the cortex that they cut out tissues of the secondary cor- 

 tex i. e., phloem and bast fibers. It is, of course, this 

 gradual exfoliation of the cut-out areas of bark that ex- 

 plains the relative thinness of the bark in very old stems 

 and branches ; the whole of the primary cortex, and most 

 of that formed from the cambium, have been thrown 

 off as bark long before. 



