Page:The Oak.djvu/73

Rh where in the pith, but otherwise their shape, etc., are similar; all the pith-cells are vertically twice or three times as long as broad. Thus the shape of the cells is that of short, polygonal prisms, standing on end and closely packed.

Imbedded, as it were, in the smaller pith-cells at the rounded angles of the vascular wedges are the oldest—i. e., first-formed—vessels, looking like small holes with very firm outlines (Fig. 12, r). These are the tracheæ, or vessels with unrollable spiral thickenings on their walls. From their shape and peculiarities they are called spiral vessels, and from their position and development they constitute the first-formed elements of the xylem or wood. They are of very narrow caliber, and stand in radial, short rows, single or branched; those first developed—i.e., nearest the pith—are the narrowest, their diameter being often even less than that of the smallest pith-cells among which they lie. As we pass radially out towards the cortex these vessels get wider and wider, but the true spiral vessels are always very narrow (Fig. 16, sp). Occasionally some of these vessels have annular instead of spiral thickenings.

Of course, their true characters are not elucidated until we compare longitudinal sections of the stem. It is then seen that the spiral thickenings are very closely wound, sometimes to the right, sometimes to the left, and occasionally double. Comparative studies of longitudinal sections also show that these vessels at first simply consist of longitudinal rows of very narrow,