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the tree's subterranean anatomy, the outbranching roots and rootlets. Outside the cambium and phloem layers grow two layers of bark, the inner one corky and porous for air-breathing, the outer one also porous and fissured but hardened for protection. The bark layers are capable of expanding, sometimes by flaking off (as in sycamores and birches), to accommodate the tree's growth, which is in its girth as well as at its extremities. The tree swells by annual production of new xylem layers. These new cells are large in spring, becoming smaller toward autumn until growth pauses during winter dormancy. Each year's growth can be traced in the sapwood "rings" thus formed, marked off by the darker autumn cells. Darker also, as a rule, are the heartwood cells formed by aged sapwood. Fig. 1 shows in cross-section a tree's structure, which is continuous through all its members. |
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