STRUCTURE OF STEMS 



705 



thus added to the exterior of the duramen. More generally, how- 

 ever, this consolidation is gradually effected, and the alburnum and 

 duramen are not separated by any abrupt line of division. 



The medullary rays which cross the successive rings of wood 

 connecting the cellular substance of the pith with that of the bark, 

 and dividing each ring of wood into wedge-shaped segments, are thin 



FIG. 545. Portion of transverse section of the stem of cedar : a, pith ; 

 b, b, b, woody layers ; c, bark. 



plates of cellular tissue (fig. 541, c, c), not usually extending to any 

 great depth in the vertical direction. It is not often, however, that 

 their character can be so clearly seen in a transverse section as in 

 the diagram just referred to ; for they are usually compressed so 

 closely as to appear darker than the wedges of woody tissue between 

 which they intervene (figs. 543, 545), and their real nature is best 

 understood by a comparison of longitudinal sections made in two 

 different directions namely, 

 radial and tangential with the 

 transverse. Three such sec- 

 tions of a fossil coniferous wood 

 in the Author's possession are 

 shown in figs. 546-548. The 

 stem was of such large size that, 

 in so small a part of the area of 

 its transverse section as is re- 

 presented in fig 546, the medul- 

 lary rays seem to run parallel to 

 each other, i listen d of radiating 

 from a common centre. They are FIG. 546. Portion of transverse section of 

 very narrow ; but are SO closelv large stem of coniferous wood (fossil), 

 set together that only two oV tt&*Xftl&ff 

 three rows of trachei'ds (no numerous medullary rays. 



ducts being here present) in- 

 tervene between any pair of them. In the longitudinal section 

 taken in a radial direction (fig. 547), and consequently passing in the 

 same course with the medullary rays, these are seen as thin plates 

 (a, a, a) made up of superposed cells very much elongated, and 

 crossing in a horizontal direction the trachei'ds which lie parallel to 

 one another vertically. And in the tangential section (fig. 548), 



z z 



