MEDULLARY RAYS. 381 



peculiar appearance presented by a section of Hazel- stem (in the 

 Author's possession), of which a portion is represented in Fig. 

 169 ; for between two layers of the ordinary thickness, there in- 

 tervenes a band whose breadth is altogether less than that of 

 either of them, and which is yet composed of no fewer than six 

 layers, four of them (e) being very narrow, and each of the other 

 two being about as wide as these four together. The inner layers 

 of wood are the oldest, and the most solidified by matters de- 

 posited within their component cells and vessels ; hence they are 

 spoken of collectively under the designation duramen or " heart- 

 wood." On the other hand, it is through the cells and ducts of 

 the outer and newer layers, that the sap rises from the roots 

 towards the leaves ; and these are consequently designated as 

 alburnum or " sap-wood." The line of demarcation between the 

 two is sometimes very distinct, as in Lignum-vitse and Cocos 

 wood ; and as a new layer is added every year to the exterior of 

 the 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 al- 

 burnum and duramen are not separated by any abrupt line of 

 division. 



239. 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 

 Fig. 170. of wood into wedge-shaped seg- 



ments, are thin plates of cellular 

 tissue (Fig. 165, c, c), not usually 

 extending to any great depth in 

 the vertical direction. It is not 

 often, however, that their cha- 

 racter can be so clearly seen in 

 a transverse section, as it is in 

 that just referred to; for they 

 are usually compressed so 

 closely, as to appear darker than 



Portion oi traiisvei-se seclioii of large stem of thc WedgCS of WOOdy tisSUe bc- 



omiferous Wood (fossil), showing part of two ^^ceen wMch thcv intervene 



annual layers, divided at a, a, and traversed by ..^. ^ nn ^ r»c\\ i j_i^ • l 



very thin but numerous Medullary Rays. {-t IgS. 166-169) ; and their real 



nature is best understood by a 

 comparison of longitudinal sections made in two different direc- 

 tions, — namely, radial and tangential, — with the transverse. Three 

 such sections of a fossil Coniferous wood in the Author's pos- 

 session are shown in Figs. 170-172. 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. 170, 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 



