250 



INDUSTRIAL PLANTS 



its separation into more or less delicate strands when crushed. 

 Those who have had experience in chopping Avood know that 

 the ax cleaves as a rule most easily when cutting toward the 

 center of the log; less easily in an}' other lengthwise direc- 

 tion, and least easil}' when directed slantingly or directly 

 across the grain. This shows that the structural parts have 

 a peculiarly definite arrangement. Something of this appears 

 when we examine, for example, with a strong magnifier, the 

 surface of a piece of pine wood, cut radially, i. e., toward 

 the center of the log. We see, as shoA\ai in Fig. 229, that the 

 wood is made up mainly of very slender, thin-walled tubes 



Fig. 229. — Radial section of white pine wood, 

 ters. (Original.) 



Magnified about 50 diame- 



each closed and tapering at the ends; and besides these are 

 numerous flat bundles of much smaller tubes running at 

 right angles to the others and radially. These bundles of 

 finer structure are called pith-rays because they are some- 

 what similar in texture to a cylinder of 29?7/z in the center 

 of the log, and some of them at least, are extensions of it. 

 Their relative softness makes the wood most easily separated 

 along the planes in which they lie. Even to the naked eye 

 their peculiar sheen makes the pith-rays apparent on a radial 

 surface, and gives an especially attractive prominence to 

 them in Avhat the dealers call " quarter-sawed " timber. It is 

 plain also that the fibrils, by which name we shall understand 



