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, b), alternating 



Km. 550. Transverse section of a 

 fossil wood, showing the medullary 

 rays, </, a, a, n,n, it, running nearly 

 parallel to each other, and the 

 openings of large ducts in the midst 

 of the prosenchymatous tissue. 



Fij. 551. Vertical (tangential) sec- 

 tion of the same wood, showing the 

 prosenchymatous cells separated 

 by the medullary rays, and by the 

 large ducts, I b, b b. 



with ]>l:itcs 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. Tran-verse and vertical -en ions 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 >iuu<>us c<inr>e of the 

 prosenchymatous cells, instead of looking (as in iig .~>4S) as if they 



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