WOOD AND FOREST. 



pith). These are the large "silver flakes' 7 to be seen in quartered 

 oak, which give it its beautiful and distinctive grain, Fig. 32, p. 38. 

 They appear as long, grayish lines on a cross-section, as broad, shin- 

 ing bands on the radial section, and as short, thick lines tapering at 

 each end on the tangential section. In other words, they are like 

 flat, rectangular plates standing on edge and radiating lengthwise 

 from the center of the tree. They vary greatly in size in different 

 woods. In sycamore they are very prominent, Fig. 13. In oak they 

 are often several hundred cells wide (i. e., up and down in the tree). 



This may amount to an 

 inch or two. They are 

 often twenty cells thick, 

 tapering to one cell at 

 the edge. In oak very 

 many are also small, 

 even microscopic. But 

 in the conifers and also 

 in some of the broad- 

 leaved trees, altho they 

 can be discerned with 

 the naked eye on a split 

 radial surface, still they 

 are all very small. In 

 pine there are some 15,- 

 000 of them to a square 

 inch of a tangential sec- 

 .tion. They are to be 

 found in all exogens. In 



a cross-section, say of oak, Fig. 14, it can readily be seen that some 

 pith rays begin at the center of the tree and some farther out. Those 

 that start from the pith are formed the first year and are called pri- 

 mary pith rays, while those that begin in a subsequent year, starting 

 at the cambium of that year, are called secondary rays. 



The function of the pith rays is twofold. (1) They transfer 

 formative material from one part of a stem to another, communicat- 

 ing with both wood and bark by means of the simple and bordered 

 pits in them, and (2) they bind the trunk together from pith to 

 bark. On the other hand their presence makes it easier for the 

 wood to split radially. ( 



Fig. 13. Tangential Section of Sycamore, Mag- 

 nified 37 Diameters. Note the large size of the 

 pith rays, A, A (end view). 



