THE SEEDLING AND YOUNG PLANT 55 
tannin, and starch in winter. At the rounded angles 
of the vascular wedges the cells are smaller than else- 
where in the pith, but otherwise their shape, &Xc., 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. 
Embedded, 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, 7). These are the trachee, 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 calibre, and stand in 
radial, short rows, single or branched; those first de- 
veloped—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 
