26 



OF WOOD IN GENERAL 



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 

 section they appear as broad, shiny bands, the " mirrors," " felt,'* 

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







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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 tracheae 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 



