Fig. 232. Wedge of a four year old pine stem cut in winter, showing, somewhat diagrammatically, a transverse surface (q), a radical surface (1) and a tangential surface (t); f, f, f, spring wood; s, summer wood; m, pith; p, first formed wood; 1, 2, 3, 4, the four successive annual rings of the wood; i, i, i, junction of spring and summer wood in successive years; ms, ms', ms', pith rays extending through the wood; ms, pith rays extending into the inner bark (b); h, resin ducts; br, outer bark, 6/1.

Fig. 232.-Wedge of a four-year-old pine stem cut in winter, showing, somewhat diagrammatically, a transverse surface (q), a radical surface (1) and a tangential surface (t); f, f, f, spring wood; s, summer wood; m, pith; p, first-formed wood; 1, 2, 3, 4, the four successive annual rings of the wood; i, i, i, junction of spring and summer wood in successive years; ms, ms', ms"', pith-rays extending through the wood; ms", pith-rays extending into the inner bark (b); h, resin-ducts; br, outer bark, 6/1. (Strasburger.)

In the woody plants which form annual rings there is also formed each year in the new shoots a ring of strands each consisting of an inner wood-part and an outer bark-part with a layer of cambium between. These strands connect with similar ones in the leaves, and are continuous below with the ring of strands forming between the wood and the bark which was fully formed when the season began (Fig. 233).

Fig. 233. Diagram of maple stem showing the development of wood and bark through first and second years. At the tip is a mass of living formative material (shown unshaded) from the sides of which arise protrusions that finally become leaves. Also arising from the formative region, just above the base of the very young leaves, are protrusions which develop into formative regions like that of the main tip, and, as growing points, produce leaf bearing branches of the main stem. In the center, around the axis, the formative material as it grows older becomes pith (shown as dotted), and this pith is continuous with that of the branches. The surface becomes changed into a skin or epidermis (shown by coarse shading), covering both stem and leaves. Parts of the formative material between the epidermis and the pith become variously hardened into bundles of fibrous material: around the central pith arise strands of wood (shown by fine shading); near the epidermis arise corresponding strands of bark (shown by black), surrounded by more or less pith like material which may become green, corky, or otherwise peculiar (shown dotted like the pith); and between the rings of wood and bark is a layer of formative material which is continuous with that of the tip and is called the cambium. From this cambium in successive years new wood is added to that within and new bark to that on its outer side, and thus both wood and bark increase in thickness by annual layers. But on the outside the epidermis, and then the older bark, is pushed off or worn away so that the total thickness of the bark is limited. Both wood and bark are continued into the leaves, but not the cambium. The strands of wood and those of bark are so connected as to form a sort of network through the meshes of which extend radially the plates of pith called pith rays.

Fig. 233. - Diagram of maple stem showing the development of wood and bark through first and second years. At the tip is a mass of living formative material (shown unshaded) from the sides of which arise protrusions that finally become leaves. Also arising from the formative region, just above the base of the very young leaves, are protrusions which develop into formative regions like that of the main tip, and, as growing-points, produce leaf-bearing branches of the main stem. In the center, around the axis, the formative material as it grows older becomes pith (shown as dotted), and this pith is continuous with that of the branches. The surface becomes changed into a skin or epidermis (shown by coarse shading), covering both stem and leaves. Parts of the formative material between the epidermis and the pith become variously hardened into bundles of fibrous material: around the central pith arise strands of wood (shown by fine shading); near the epidermis arise corresponding strands of bark (shown by black), surrounded by more or less pith-like material which may become green, corky, or otherwise peculiar (shown dotted like the pith); and between the rings of wood and bark is a layer of formative material which is continuous with that of the tip and is called the cambium. From this cambium in successive years new wood is added to that within and new bark to that on its outer side, and thus both wood and bark increase in thickness by annual layers. But on the outside the epidermis, and then the older bark, is pushed off or worn away so that the total thickness of the bark is limited. Both wood and bark are continued into the leaves, but not the cambium. The strands of wood and those of bark are so connected as to form a sort of network through the meshes of which extend radially the plates of pith called pith-rays. (Original.)

The cambium is a continuation of the formative living material out of which the whole young shoot is developed. The living part of such a tree as a maple or pine is thus seen to be like a mantle completely covering the older wood and lining the older bark, renewing each, somewhat as our skin and nails are renewed.

Some trees, such as palms, have no cambium. The trunk in such cases may be regarded as a sheaf of numerous mixed fibers embedded in pith, and growing upward by additions formed in the terminal mass of living material from which the continually expanding terminal bud is derived. As shown in Fig. 234, a cylindrical shaft is the result, instead of the tapering shaft formed by concentric conical layers where a cambium is present. A stem made up of tough strands thus embedded in pith, forms a flexible column often of considerable strength as a whole, and therefore well adapted to resist tropical gales and carry aloft a heavy crown of leaves, flowers, or fruit. At the same time such a stem is economically of some use as a log in rough building, but it does not make serviceable planks, and is quite un-suited for most of the uses of ordinary wood. Like bamboo, the hard shell of the coconut, and other materials sharing some of the essential properties of ordinary wood but differing from it decidedly in structure, the palm-trunk can scarcely be regarded as a true wood at all. Such materials are best called pseudo-woods, to distinguish them from true woods which are always formed by a cambium layer.

Fig. 234. Diagram of palm stem showing development, for comparison with that of maple. Corresponding material is shaded as in the previous figure. Note the absence of a cambium, and the uniform diameter of the stem, which is here surrounded entirely by the bases of the leaves.

Fig. 234.-Diagram of palm stem showing development, for comparison with that of maple. Corresponding material is shaded as in the previous figure. Note the absence of a cambium, and the uniform diameter of the stem, which is here surrounded entirely by the bases of the leaves. (Original.)