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
Thuja occidentalis its common names, it is not a true cedar in the genus "Cedrus", nor is it related to the Australian white cedar, "Melia azedarach". Unlike the closely related western red-cedar ("Thuja plicata"), northern white-cedar is only a small or medium-sized tree, growing to a height of tall with a trunk diameter, exceptionally to tall and diameter. The tree is often stunted or prostrate in less favorable locations. The bark is red-brown, furrowed and peels in narrow, longitudinal strips. Northern white-cedar has fan-like branches and scaly leaves. The foliage forms in flat sprays with scale-like leaves long. The seed cones are