LECT. VII.] ANATOMY OF STEMS. 333 



Mirbel not as vessels of the wood ; but of a dis- 

 tinct sheath lining the wood, which he has deno- 

 minated F6tui me*dullaire ; and there is un- 

 doubtedly some reason for this distinction, inas- 

 much as these vessels have not been detected in 

 the wood, but always in immediate contact with 

 the pith. I shall more particularly examine this 

 opinion before describing the structure of the pith. 

 2. The Divergent layers * consist of flattened 

 masses of cellular substance, which cross the con- 

 centric layers at different parts, and, separating 

 the bundles of longitudinal tubes of which they 

 consist from each other, produce the reticulated 

 arrangement seen in the tangental section of any 

 steal ; the oblong tubes and vessels forming the 

 tissue of the network, the meshes of which are 

 filled up by the cells of the divergent layers. The 

 individual cells, which are narrow and horizontal 

 in their length, extend in series from the centre 

 to the circumference of the wood; and conse- 

 quently form nearly right angles with the tubes 

 of the concentric layers^. They communicate 

 with each other by pores ; so that fluids may 

 readily pass through the whole series, and of 



between c. d., the smaller vessels being marked by the figure 

 5, and the larger by 6, showing distinctly their porous and an- 

 nular structure. 



* Dr. GREW was the first author who described these layers 

 under the name of insertions. 



f Vide Plate 6. fig. 7. *. *. 



