22 



WOOD AND FOREST. 



pith). 'J'liese are ilic large "silver liakes" 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 oir edge and radiating lengthwise 

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

 woods. In sycamoi'e they are very prominent. Fig. 13. In oak tliey 

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



This may amount to an 

 inch or two. Thev are 

 ofteir twenty cells thick, 

 tapering to one cell at 

 the edge. In oak very 

 many are also small, 

 even microscopic. But 

 iir the conifers and also 

 m some of the broad- 

 leaved trees, altho they 

 can be discerned with 

 the naked eye on a split 

 indial surface, still they 

 are all very small. In 

 ]iiiie there are some 1.5,- 

 OdO of them to a square 

 inch of a tangential sec- 

 tion. They are to be 

 fiumd in all exogens. In 

 a cross-section, say of oak. Fig. II. 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 i)ith 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, ai'c called secondarv ravs. 



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

 formative material fi-oni one jiart of a stem to another, communicat- 

 ing with both wood and bark liy nutans of the simple and bordered 

 pits in tliem, and (2) they bind the trunk together from pitli to 

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

 wood to s])lit I'adiallv. 



F]is. 13. Tang-eiitial Section of Sycamore, Mag-- 



nilifd 37 Diameters. Note the largfe size of the 



pith rays, A, A (end view). 



