STRUCTURE OF STEMS 



707 



constitute a still larger proportion of the stem ; for in the transverse 

 section (fig. 552) they are seen as very broad bands (b, 6), alternating 



FIG. 550. Transverse section of a 

 fossil wood, showing the medullary 

 rays, a, a, a, a, a, a, running nearly 

 parallel to each other, and the 

 openings of large ducts in the midst 

 of the prosenchymatous tissue. 



Fi3. 551. Vertical (tangential) sec- 

 tion of the same wood, showing the 

 prosenchymatous cells separated 

 by the medullary rays, and by the 

 large ducts, b &, 6 b. 



with plates of woody structure (a a), whose thickness is often less 

 than their own ; whilst in the tangential section (fig. 553) the cut 



FIGS. 552 and 553. Transverse and vertical sections of a fossil wood, 

 showing the separation of the woody plates, a a, a a, by the very 

 large medullary rays, b b, b b. 



extremities of the medullary rays occupy a very large part of 

 the area, having apparently determined the sinuous course of the 

 prosenchymatous cells, instead of looking (as in fig 548) as if they 



zz 2 



