The elements of botany for The elements of botany for beginners and for schools elementsbotany00gray Year: 1887 432. The green layer of bark seldom grows much after the first season. Sometimes the corky layer grows and forms new layers, inside of the old, for years, as in the Cork-Oak, the Sweet Gum-tree, and the White and the Paper Birch. But it all dies after a while ; and the continual enlargement of the wood within finally stretches it more than it can bear, and sooner or later cracks and rends it, while the weather acts powerfully upon its sur- face ; so the older bark perishes and falls
The elements of botany for The elements of botany for beginners and for schools elementsbotany00gray Year: 1887 432. The green layer of bark seldom grows much after the first season. Sometimes the corky layer grows and forms new layers, inside of the old, for years, as in the Cork-Oak, the Sweet Gum-tree, and the White and the Paper Birch. But it all dies after a while ; and the continual enlargement of the wood within finally stretches it more than it can bear, and sooner or later cracks and rends it, while the weather acts powerfully upon its sur- face ; so the older bark perishes and falls away piecemeal year by year. 433. So on old trunks only the inner bark remains. This is renewed every year from within and so kept alive, while the older and outer layers die, are fissured and rent by the distending trunk, weathered and worn, and thrown off in fragments, — in some trees slowly, so that the bark of old trunks may acquire great thickness; in others, more rapidly. In Honey- suckles and Grape-Vines, the layers of liber loosen and die when only a year or two old. The annual layers of liber are sometimes as distinct as those of the wood, but often not so. Fig. 481. Magnified view of surface of a bit of young Maple wood from which the barlv has been torn away, showing the wood-cells and the bark-ends of medul- lary rays. Fig. 482. Section in the opposite direction, from bark (on the left) to beginning of pith (on the right), and a medullary ray extending from one to the
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