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, 7,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 
Re ae ES a 
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IY = 
= 
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Fie. 140.— Sections of sycamore wood: a, tangential; b, radial; 
c, cross. (From Pincuort, U.S. Dept. of Agr.) 
Bide VAS VE 
Fic. 141.— Section of white pine wood. (From Pincuot, 
U.S. Dept. of Agr.) 
aa kk 
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 
