Trees

What is a tree? The popular concept of a tree, which encompasses everything from treeferns and Joshua trees to coconut palms and oaks, confounds any simple botanical definition. Nevertheless, almost all ‘trees’ have one (rarely a few) above ground, perennial vegetative shoots, of a size requiring significant nonhydrostatic structural support. In a typical seed plant tree, this structural tissue is generally produced by an increase in girth — ‘secondary growth’ by radial cell division — after growth in length has ceased. The resulting structural tissue — the secondary xylem or wood — is composed mainly of cells that have their walls impregnated with the polymer lignin, which is made rigid by three-dimensional covalent bonds. Other kinds of trees have little or no secondary growth, or secondary growth of a different kind, and support is provided instead by increased mechanical provision (often by lignin) in the cell walls of individual cells.