120 THE OAK 



phellogen layers may be formed so far down in the cor- 

 tex that they cut out tissues of the secondary cortex 

 i.e. phloem and bast fibres. It is, of course, this gradual 

 exfoliation of the cut-out areas of bark that explains 

 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. 



