Page:The Oak.djvu/138

 120 trees the phellogen layers may be formed so far down in the cortex that they cut out tissues of the secondary cortex—i.e., phloëm and bast fibers. 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.