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Wood (Botanical sense the hard solid)

in a botanical sense. the hard, solid, non-succulent stem of any plant, and in a chemical sense, lignin, a hardened form of cellulose, the principal constituent of all vegetable life, with a small proportion of resinous and incrusting matter. Wood thus only differs from vegetable fibers-cotton, flax, hemp, etc. -in its physical condition. Woody stems are divided into two great classes, according to the manner in which they grow. The first and most important class is the exogens or exogenous stems, so called on account of the annual increments being added on the outside of the stem just within the bark, and the layers of growth are seen in such stems in a series of concentric circles, each ring of which represents the growth of one year.

This class embraces all the trees of temperate climates, and, indeed, the greater part of all tree life. The second class endogenous stems-shows no rings of annual growth, but a cross section exhibits a series of dark spots, more or less closely studded, and always more densely packed toward the outer circumference.