122 



PRACTICAL COURSE IN BOTANY 



planks can be obtained in this manner. A better way of 

 treating small logs is shown in Fig. 138, where the three 

 central planks, r;r,r, on and near the diameter, will give the 

 " quartered " effect, while the rest can oe used for the cheaper 

 tangential cuttings. Examine a piece of quartered board, or 

 a log of wood that has been split down the center, and notice 



Fig. 140. — Sections of sycamore wood : a, tangential; h, radial; 

 c, cross. {From Pinchot, U. S. Dept. of Agr.) 



Fig. 141. — Section of 

 U. S. Dept. of Agr.) 



{From Pinchot, 



that the medullary rays appear as silvery bands or plates 

 (Figs. 140, 141). This is because the cut runs parallel to 

 them. It is the medullary rays chiefly that give to commer- 

 cial woods their characteristic graining. Knots, buds, and 

 other adventitious causes also influence it in various degrees. 

 136. The swelling and shrinking of timber. — The ca- 

 pacity possessed by certain substances of bringing about an 



