Page:The Oak.djvu/131

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113
THE TREE—ITS SHOOT-SYSTEM.

through which the protoplasmic and other contents of the continuous segments pass uninterruptedly. Similar sieve-plates occur on the lateral walls of the segments also. The walls are not thickened and not lignified, and thus the morphological similarities between the sieve-tubes of the bast and the vessels of the wood (which only contain air and water, have their septa absorbed, and their walls lignified and covered with bordered and simple pits) depend almost entirely on the similar development. The sieve-pores are very fine, and easily overlooked.

(3) The bast fibers (Figs. 17 and 18 b), which are homologous with the libriform fibers of the wood, and are developed in the same way from single cells of the cambium. They are short, blunt, very thick-walled fibers, grouped in strands which appear on the transverse section of the bast as tangential bands 2-4 deep, alternating (in the radial direction) with broader bands of sieve-tubes and parenchyma. These bands of fibers (hard bast) are accompanied at their outer and inner boundaries by parenchyma-like cells arranged in vertical rows, each of which contains a large simple crystal of calcium oxalate imbedded in yellowish substance, and the walls of which are slightly sclerotic. Similar vertical series of cells are found in the soft bast, but they contain compound (clustered) crystals of the same salt (Figs. 17 and 18, e).

The soft bast also contains scattered roundish groups of short sclerenchyma cells, the thickened walls of which