MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS. 



371 



alburnum, an additional layer of the innermost part of the alburnum is 

 every year consolidated by internal deposit, and is thus added to the 

 exterior of the duramen. More generally, however, this consolidation 

 is gradually effected, and the alburnum and duramen are not separated 

 by any abrupt line of division. 



370. 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 plates 

 of cellular tissue (Fig. 253, c, c), not usually extending to any great depth 

 in the vertical direction. It is not often, however, that their character 



F.ic. 257 



vm "258. 



Portion of Transverse Section of the Stem of Cedar: a, pith; 6, 6, 6, woody layers; c, bark. 



can be so clearly seen in a transverse section as in the diagram just re- 

 ferred to; for they are usually compressed so closely as to appear darker 

 than the wedges of woody tissue between which they intervene (Figs. 255, 

 257;; and their real nature is best understood by a comparison of longi- 

 tudinal sections made in two different directions, namely radial and 

 tangential, with the t r a n s v e rse. 

 Three such sections of a fossil Coni- 

 ferous wood in the Author's possession 

 are shown in Figs. 258-260. The 

 stem was of such large size, that, in 

 so small a part of the area of its 

 transverse section as is represented in 

 Fig. 258, the medullary rays seem 

 to run parallel to each other, instead 

 of radiating from a common centre. 

 They are very narrow ; but are so 

 closely set together, that only two or 

 three rows of woody fibres (no ducts 

 being here present) intervene between 



, i T 4.1 i Portion of Transverse Section of large 



any pair of them. In the longitu- stem of coniferous wood (fossil), showing 



rlinnl cppfirm tnkpii in a Vflflial flirPoHon part of two annual layers, divided at a, a, 



. " and' traversed by very thin but numerous 



(Fig. 259), and consequently passing Medullary Rays, 

 in the same course with the medul- 

 lary 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 

 woody fibres which lie parallel to one another vertically. And in the 

 tangential section (Fig. 260), which passes a direction at right angles to 



