Page:The Oak.djvu/74

 placed cylindrical cells, standing end to end; it is because the adjacent ends become resorbed and disappear that the rows of cells at length form long, continuous tubes—vessels, or tracheæ.

Turning once more to the transverse section, as the eye follows the bundle radially outward the lumina of the vessels in the radial rows are found to become wider and wider, until we meet with vessels with diameters many times greater than that of the pith-cells. The walls of these wider vessels, however, are not strengthened with spiral thickenings, but are thickened and furnished with bordered pits, the shape and characters of which are best seen from the illustrations (Figs. 14-16). These larger vessels are not always associated with the radial rows of spiral vessels, but may be scattered between them.

The vessels intermediate between the spiral and the pitted ones are thickened sometimes with reticulations. All these larger vessels have septa inclined towards the medullary rays, and perforated with several long, oval, parallel, horizontal holes: hence the segments are easily macerated and distinguished, and their lengths are found to be variable (Fig. 16, pv).

The large pitted vessels form groups with parenchyma and wood-cells scattered between, and are confined chiefly to the inner parts, forming radiating series side by side; in the outer parts of the bundle are various groups of smaller vessels—the groups being rounded, or in radial rows, or curved or oblique rows.