32 



OF WOOD IN GENERAL. 



conspicuous than those of conifers. In Oak the large primary 

 pith-rays extending from pith to cortex are often twenty or more 

 cells in width, appearing as long, clearly denned, grayish lines in 

 a transverse section of the stem (Fig. 19). The secondary pith- 

 rays are much narrower as well as shorter (Fig. 20). In a tan- 

 gential section (Figs. 21 and 22) the primary rays may be several 

 hundred cell-rows, i.e. upwards of an inch, in height, and, however 

 wide at the middle, taper to one cell at each end. On a radial 



FIG. 24. Two annual rings of wood and the bark of the Oak, the upper 

 surface in transverse section, part of the inner ring (unshaded) in tangential, 

 and the front view of both rings in radial section. The medullary rays are 

 shown black in transverse, shaded in radial section. (After Hough.) 



section they appear as broad, shiny bands, the " mirrors " or 

 "silver grain," so that they are conspicuous on any section, in 

 whatever plane it may be. In Oak they constitute 16-25 per 

 cent, of the wood (Figs. 23 and 24). 



The protoxylem of broad-leaved trees differs from that of 

 conifers mainly in that its spirally-thickened elements are trachea 

 or true vessels, owing to the absorption of the transverse walls of 

 a vertical row of tracheids. But it is in the elements of the 

 secondary xylem that we find the greatest complexity and 

 variety. This may contain from three to five of the following 

 six kinds of elements : tracheids, vessels, woody fibres, fibrous 

 cells with thick or with thin walls, and wood-parenchyma. The 

 tracheae or true vessels vary considerably in transverse diameter, 

 some of them being the widest pores seen in a transverse section 



