ABSTRACT
Wood in the living tree serves two functions. First, it supports the foliage or canopy above, allowing it to reach above competing plants or trees to capture sunlight for photosynthesis. At the same time, the wood is a conduit for water and nutrients to move up and down the tree. If we look at a cross-sectional cut from a tree, we can see the bark, which protects the living tree from injury (Figure 23.1). Beneath the bark, we can see sapwood and heartwood (USDA 1996). Sapwood is the living part of the tree that conducts uids. Inside that zone is the heartwood, which is the collection of nonliving cells that provides some support for the canopy above but no longer conducts uids. If we look closer on our cross section, we see numerous cells with large openings called lumens. ese hollow tubes or cells are both conducting elements and structural support for the tree.
