Tree tissue. It protects the trunk against the elements, disease, animal attack and fire. It is perforated by a large number of fine breathing pores called lenticels, through which oxygen diffuses. Bark is continually replaced by a living layer of cells called the cork cambium or phellogen. The London plane ("Platanus × acerifolia") periodically sheds its bark in large flakes. Similarly, the bark of the silver birch ("Betula pendula") peels off in strips. As the tree's girth expands, newer layers of bark are larger in circumference, and the older layers develop fissures in many species. In some trees such as the
Crataegus Crataegus Crataegus (), commonly called hawthorn, quickthorn, thornapple, May-tree, whitethorn, or hawberry, is a large genus of shrubs and trees in the family Rosaceae, native to temperate regions of the Northern Hemisphere in Europe, Asia and North America. The name "hawthorn" was originally applied to the species native to northern Europe, especially the common hawthorn "C. monogyna", and the unmodified name is often so used in Britain and Ireland. The name is now also applied to the entire genus and to the related Asian genus "Rhaphiolepis". The Latin name "Crataegus" is from the Greek "kratos" "strength" and "akis" "sharp", referring