MICROSCOPICAL STRUCTURE. 



149 



Section I. Walls of the trachcids of the pith ray with dentate projections. 



a. One to two large, simple pits to each tracheid on the radial walls of the cells of the pith ray. Group 1. 



Represented in this country by P. resinosa. 



i. Three to six simple pits to each tracheid on the walls of the cells of the pith ray. Group 2. 1'. tcvda, 

 palustris, etc., including most of our "hard r and "yellow" 

 pines. 



Section II. War of tracheids of pith ray smooth, without dentate 

 projections. 



a. One or two large pits to each tracheid on the radial walls of 



each cell of the pith ray. Group 3. P. strobits, lambertiaiia, 

 and other true white pines. 



b. Three to six small pits on the radial walls of each cell of the 



pith ray. Group 4. P. parryaiia, anW other nut pines, includ- 

 ing also P. balfouriana. 



The general features of structure of coniferous woods 

 are represented in the accompanying cut (fig, 16). 



The structural elements, as in all pine, are few and 

 simple and consist of () tracheids, the common wood 

 fibers, forming over 90 per cent of the volume; (b) medul- 

 lary or pith rays, minute cell aggregates composed of two 

 kinds of cells, scarcely visible without magnifier and then 

 only on the radial section, yet forming about 7 to 8 per 

 cent of the volume and weight of the wood in these spe- 



FlG "-** representation of coniferous 



structure: wood of spruce 1, natural size; 2, small 

 part of one ring magnified 100 times. The vertical 

 tubes are wood fibers, in this case all "tracheids," 

 in, medullary or pith my; n, transverse tracheids of 

 pith ray ; a, &, and c, bordered pits of the tracheidx 

 more enlarged. 



the middle 



cies; (c) resin ducts, small passages of irregular length 



surrounded by resin- secreting cells, scattered through the 



wood, but forming two more or less connected systems, 



one running in the direction of the fibers, the other at 



right angles to the first, the individual ducts of the latter system always occupyin 



portion of medullary rays (see PL XXVII). 



The tracheids, or common wood fibers, are alike in all five species, and resemble those of 



other pines; they are slender tubes, 4.5 to 6 mm. (about one- 

 fourth inch) long, forty to one hundred times as long as 

 thick, usually hexagonal in cross section, with sharp or more 

 or less rounded outlines (see PL XXI), flattened in tangen- 

 tial direction at both ends (see PL XXI, A/), the diameter 

 in radial direction being 45 to 55 yu (about 0.002 inch) in the 

 springwood, and about half that, or 21 to 25 /<, in the sum- 

 nierwood, and in tangential direction about 40 /< on the average 

 in their middle. They are arranged in regular radial rows 

 (see PL XXI), which are continuous through an indefinite 

 number of rings, but the number of rows increasing every 

 year to accommodate the increasing circumference of the 

 growing stem. (See PL XXI, C c.) The fibers of the same 

 row are practically conterminous, i. e., they all have about 

 the same length, though at their ends they are often bent, 

 slightly distorted, and usually separated (see PL XXI, B c; 

 also fig. 17), their neighbors filling out the interspaces. There 

 js no constant difference in the dimensions of these fibers 

 in the different species here considered. In every tree the 



libers are shortest and smallest near the pith of any section, rapidly increasing in size from the 



pith outward, and reaching their full size in about the tenth to twentieth ring from the pith. 



To illustrate: In a section of Lougleaf Pine, 10 feet from the ground, the diameter of tracheids 



in radial direction is in /<=0.001 mm: 



17. (Jell endings in pine. 



