Text-book of structural and physiological botany . ays reach to the thickening-ring ; but they never get asfar as the pith. The breadth of these rays varies in differentplarts ; in some species they are one cell, in others severalcells wide; broad and narrow bands sometimes alternate withone another ; but they never penetrate through the wholelength of the plant, as can be easily seen on tangential sec-tions (Fig. 474). Both the primary and secondary medullaryrays are generally easily recognised by the naked eye, andform the pattern on the wood ordinarily known as the silver grain. They consis
Text-book of structural and physiological botany . ays reach to the thickening-ring ; but they never get asfar as the pith. The breadth of these rays varies in differentplarts ; in some species they are one cell, in others severalcells wide; broad and narrow bands sometimes alternate withone another ; but they never penetrate through the wholelength of the plant, as can be easily seen on tangential sec-tions (Fig. 474). Both the primary and secondary medullaryrays are generally easily recognised by the naked eye, andform the pattern on the wood ordinarily known as the silver grain. They consist of parenchymatous cells usuallyelongated in the radial direction (see Figs. 474, 475). Both the xylem- and the bast-portion of the vascularbundles of the stem of Dicotyledons consist—with theexception of the fundamental tissue of the medullaryrays—of elements which may be arranged into three 364 Structural and Physiological Botany, groups :—prosenchyma, parenchyma, and vessels. Theprosenchymatous cells of the xylem—which are also called. Fig. 474.—^Tangential section through the wood of the maple ; g vessels ; Hy wood-fibres ; s silver-grain { x 200). wood-cdls^ wood-fibres, simple bast-like wood-fibres, orlibriform fibres—-are always fusiform, comparatively stronglythickened and lignified, unbranched, and, as a rule, furnished special Morphology and Classification, 365 with extremely small bordered pits (Fig. 14, p. 13). Evenwhen the latter are larger, they differ in size and form fromthose of the vessels which occur along with them. The
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