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, &c., 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, r). These are the tracheae, or 

 vessels with unreliable 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 j 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 



