Page:The Oak.djvu/78

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60
THE OAK.

Successive sections prove that the vessels in the bundle change in number—i.e., there are fewer when passing from stem to leaf. A vessel may end in an interpectinating, pointed, terminal cell; or it may branch, as it were, dichotomously, owing to fusions with other similar elements; or such a fusion may occur lower down, the original vessel ending blindly.

In the vicinity of the reticulated and first pitted vessels, following on the spiral vessels, we find libriform fibers, tracheids, wood-parenchyma, and secondary rays of parenchyma; the tracheids are especially in the neighborhood of the vessels (see Fig. 14).

The tracheids are long cells with gradually tapering ends, and the walls rather thick but by no means obscuring the lumen; on the walls are numerous, usually elongated, oblique or horizontal bordered pits. These pits occur whether the next element is a tracheid, a vessel, or fibers or cells of any kind (Fig. 16, tr).

The length of the tracheids varies, and the diameter is also variable.

The libriform fibers are also long cells, but often more pointed at the ends, and their very thick walls almost obliterate the lumen (Fig. 16, f); their length is about that of the tracheids, but slit-like, small, simple pits are rare on their walls. In the wood of later years, however, the lengths may be different.

There are also elements which stand midway between the true fibers and tracheids; they occur in those parts where masses of true fibers abut on the groups consist-