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On the way up, the moisture takes an inner course through deep layers of xylem or sapwood cells, just outside the heartwood (which is old xylem cells grown inert). Surrounding this thick cylinder of sapwood is a thin outer one composed of tubular phloem cells through which the enriched sap is conducted earthward. Where the two parts of this pipe-within-a-pipe touch is called the cambium (exchange) layer. From it extend lateral fissures called medullary rays, through which both water and sap are transferred inward. This dual circulatory system (See Fig. i.) is present all the way from the slimmest leaf stem down through twig, branch, limb, and sturdy trunk into the tree's subterranean anatomy, the out branching 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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