410 



STRUCTURE OF DICOTYLEDONOUS STEM. 



to occupy the place of a single one. Thus in a section of Hazel- 

 stem (in the Author's possession), of which a portion is represented 

 in Fig. 214, between two Layers of the ordinary thickness there 

 intervenes 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 (c) being very narrow, and each of the other 



Fig. 214. 



a h c 

 Portion of Transverse Section of Stem of Hazel, showing, in the 

 portion a, b, c, six narrow layers of Wood. 



two (a, b) being about as wide as these four together. — The inner 

 Layers of Wood are the oldest, and the most solidified by matters 

 deposited 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-vitas 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, how- 

 ever, this consolidation is gradually effected, and the Alburnum 

 and Duramen are not separated by any abrupt line of division. 



302. 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 (Plate xn., fig. 1), 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 it is in that just referred to ; for they are 

 usually compressed so closely as to appear darker than the wedges 

 of Woody tissue between which they intervene (Plate xn. , figs. 3, 4) ; 

 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 sections 

 of a fossil Coniferous wood in the Author's possession are shown in 

 Figs. 215-217. The Stem was of such large size, that, in so small 

 a part of the area of its Transverse section as is represented in 



