EXOGENOUS STEM: — MEDULLARY BAYS. 



383 



only deeper than the preceding from above downwards, but also 

 to nave a much greater thickness. This section also gives an 

 excellent view of the ducts b b, b b, which are here plainly seen 



Fig. 174. 



Fm. 175. 



Mum 



H.WJ i^ljife 



Ml ! 



»>'« 



m 



ct u u, b b 



Fig. 174. Portion of transverse section of Fossil Wood, showing the Medullary Rays, a a, a a, a a, 



running nearly parallel to each other, and the openings of the large Ducts in the midst of the woody 



fibres. 

 Fig. 175. Porlion of vertical (tangential) section of the same Wood, showing the woody fibres 



separated by the Medullary Rays, and by the large Ducts, 6 6, 6 6. 



to be formed by the coalescence of large cylindrical cells, lying 

 end to end. In another fossil wood in the Author's possession, 

 the medullary rays constitute a still larger proportion of the 



Pm. 176. 



Fig. 177. 



Pig. 176. Portion of transverse section of a Fossil Wood, remarkable for the very large size of 

 the Medullary Rays, &, t, which separate the woody plates, a, a. 



Fig. 177. Vertical (tangential) section of the same wood, showing, a, a, the woody bundles, and 

 6, 6, the Medullary Rays. 



stem ; for in the transverse section (Fig. 176), they are seen as 

 very broad bands (6/&), alternating with plates of woody struc- 



