The general body plan of a tree must be briefly outlined so that all subsequent information can be understood in its proper context within the living organism. A living, growing tree has two main domains, the shoot and the roots. The roots are the subterranean structures responsible for water uptake, mechanical support of the shoot, and storage of biochemical. The shoot comprises the trunk or bole of the tree, the branches, and the leaves (Raven et al. 1999). It is with the trunk of the tree that the remainder of the chapter will be concerned.
If one cuts down a tree and looks at the stump, there are several gross observations that can be easily made. The trunk is composed of various materials present in concentric bands. From the outside of the tree to the inside there are six layers: outer bark, inner bark, vascular cambium, sapwood, heartwood, and the pith (Figure 2.1). Outer bark provides mechanical protection to the softer inner bark, and also helps to limit evaporative water loss. Inner bark (phloem) is the tissue through which sugars produced by photosynthesis (photosynthate or “food”) are translocated from the leaves to the roots or growing portions of the tree. The vascular cambium is the layer between the bark and the wood that is responsible for producing both these tissues. The sapwood is the active, “living” wood that is responsible for conducting the water (or sap) from the roots to the leaves. It has not yet accumulated the often-colored chemicals that set apart the nonconductive heartwood found as a core of darker-colored wood in the middle of most trees. The pith at the very center of the trunk is the remnants of the early growth of the trunk, before wood was formed.