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
Chrysolepis three sweet, edible nuts, eaten by the indigenous peoples. The fruit also provides food for birds and wildlife. "Chrysolepis" is related to the subtropical southeast Asian genus "Castanopsis" (in which it was formerly included), but differs in the nuts being triangular and fully enclosed in a sectioned cupule, and in having bisexual catkins. "Chrysolepis" also differs from another allied genus "Castanea" (chestnuts), in nuts that take 14–16 months to mature (3–5 months in "Castanea"), evergreen leaves and the shoots having a terminal bud. There are two species of "Chrysolepis" — "Chrysolepis chrysophylla" and "Chrysolepis sempervirens" — which like many species