es he 
38 WESTERN YELLOW PINE IN ARIZONA AND NEW MEXICO. 
est length of tracheids is usually less in the branches than in the 
trunk; the length in the branches, however, depends upon the part 
of the trunk from which they arise. Moreover, the length of 
tracheids in branches increases upward for some distance from the 
-point where the latter are given off, and then gradually diminishes 
again. It is difficult Dyan m to determine the mean average length 
of tracheids in western yellow pine wood, particularly because the 
tree has such a wide natural range and grows under a great variety 
of soil and climatic conditions. As already pointed out, the length 
of cell elements of this wood is determined by the character of the 
soil, soil moisture, and the length of the growing season. 
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Fie. 10.—An ordinary pith ray cut longitudinally (radial section), magnified 250 diam- 
eters; p. r., pith rays; r. t., ray tracheids; r. c., ray cells; ¢c., thin-walled ray cells; 
6b. p., bordered pits; s. p., Simple pits; ¢., tracheids. 
Tracheids (fig. 9) differ from the wood fibers of broadleaf trees 
in possessing bordered pits, while their ends are blunt and often 
slightly curved, due to the manner in which these elements overlap. 
The bordered pits of tracheids in early wood (fig. 9, 6. p.) are 
arranged in one (rarely two) rows on the radial walls. In late 
wood the tracheids are compressed radially to such an extent that 
the pits are slit-like and usually smaller and less numerous. Where 
pith ray cells are in contact with tracheids, the communicating pits 
(fig. 9, s. p.) within the walls of the latter are small and simple, 1. e., 
without border, and arranged chiefly in two vertical rows. 
Pith rays (fig. 7 a, p. 7.) appear in transverse section as radially 
disposed rows of elongated cells with vertical or oblique cross walls 
(fig. 8, c. w.). The majority of the pith rays are only a single cell 

