Page:The Oak.djvu/124

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l06
THE OAK.

thirteen rays to the millimetre may be counted on the transverse section of the wood.

(2) The cambium cells situated between the rays—except when they suddenly commence to form a new ray, as just described—pass over into one or more of the following elements of the wood proper—viz., wood-parenchyma, libriform fibers, tracheids, segments of the vessels (see Fig. 28).

When a cambium cell passes over into wood-parenchyma it first undergoes a few horizontal divisions transverse to its long axis, and then we have a vertical row of five or six parenchymatous cells, the walls of which do not thicken much, but obtain small simple pits, and retain part of their living contents—protoplasm, nucleus, starch-forming corpuscles, etc.—and indeed present much resemblance to the cells of the medullary rays themselves.

When the cambial cell becomes transformed into a libriform fiber it does this simply by thickening its walls at the expense of the living contents, etc., which soon disappear. The cell undergoes no horizontal divisions, and probably elongates very slightly. The thickened walls become pitted with minute simple pits, and are stratified and eventually lignified.

In the case of the transformation of a cambial cell into a tracheid everything is essentially as described in the last paragraph, except that the diameter increases and the thickening walls become marked with bordered pits, quite similar to those of the pine, except that they