120 THE OAK 
phellogen layers may be formed so far down in the cor- 
tex that they cut out tissues of the secondary cortex— 
2.é. phloém 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. 
