


THE WOOD. : a7) 
The radial section (fig. 7, b.), exhibits the broad tracheids (¢.) 
of early wood (as seen in longitudinal section) and the radially 
flattened tracheids of late wood (fig. 7 6, p. r.). Ordinarily pith 
rays, those without resin ducts, are from 1 to 20 cells high, and 
always 1 cell wide, while rays with resin ducts, the 
more conspicuous form, are from 4 to 6 cells wide 
and from 10 to 25 cells high. Resin ducts within 9 
the pith rays communicate with the resin ducts 
(fig. 7 6, vr. d.) running parallel to the axis or 
lengthwise of the trunk and its branches. 2 
A tangential section* (fig. 7, ¢) of western yel- rh 
low pine wood shows two kinds of pith rays (as a2 
> 
9 


seen in transverse section) which together consti- 
tute about one-tenth of the volume of the wood 
and are composed of continuous rows of short cells , 
extending in a radial direction (from the center or 
pith of the tree toward the bark). One form of 
ray consists of a single row of cells, as noted above, 
while the other is made up of from 4 to 6 rows of 
cells, including also a resin canal. 



bp.- 



550° © 
CITARACTERISTICS OF CELL ELEMENTS. 

The following is a more detailed description of 
the cell elements pointed out as visible in trans- 
verse, tangential, and radial sections of the wood. 
Tracheids (figs. T a, t. and fig. 9), which form 
the principal bulk of the wood, vary from 1.7 to 
3.2 mm. in length, with an average length of about 
2.5 mm. Those of the early wood have an aver- 
age diameter of 0.05 mm., while tracheids of late 
wood have the same tangential diameter, but are 
considerably narrower radially. The length of 
tracheids varies not only in different parts of 
the same tree, but within the same annual ring 
at the same distance above ground. The aver- 
age length of tracheids in both the trunk and _ F's. 9.—Two trache- 
branches increases from the center toward the A eee e 
outside of the trunk until the tree reaches its bordered pits ; s. p., 
maximum height growth, after which the length preGar's 
remains nearly constant. The average length of tracheids increases 
also from the base of the tree upward until the tree attains its 
maximum height growth, after which it again decreases. The great- 










.+ Unless cut accurately with its surface exactly parallel with a plane from the center to 
the circumference of the tree the section will present a misleading view. 
This should be cut in a plane exactly perpendicular to the pith rays. 
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