THE SEEDLING AND YOUNG PLANT. 55 



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 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 caliber, and stand in 

 radial, short rows, single or branched ; those first devel- 

 oped 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 longi- 

 tudinal sections also show that these vessels at first sim- 

 ply consist of longitudinal rows of very narrow, verti- 



