MICROSCOPICAL STRUCTURE. 



149 



Section I. Walls of the tracheitis of the jtith ray with ileutate projections. 



«. One to two large, simple pits to each tracheid on the radial walls of the cells of th 



Represented in this country by P. resinosa. 

 h. Three to six simple pits to each tracheid on the walls of the cells of the pith ray. 



palustris, etc., inclnding most of our " hard " and " yellow " 2 



pines. 

 Section II. Walls 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. /'. atrobus, lamleriimia, 



and other true white pines. 

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



pith ray. — Group 4. P. parryava, and other nut pines, inclnd 



ing also /'. halfouriatui. 



•ith ray.— G 

 iroup 2. /', 



oup 1. 

 tiKiIa, 



^ 



Fig Ifi. — Schematic representation of coniferous ivood 

 structure: wuoil of spruce — 1, uatuntt size: 2, small 

 part of one ring mjignifled 100 times. The vertical 

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

 »(, nu'dullary or pith ray; ?i, transverse tracheids of 

 pith ray: a, 6, and c, bordered pits of the tracheids 

 more enlarged. 



The general features of structure of coniferous woocL-^ 

 are represented iu the accompanying cut (fig. 10). 



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

 simple and consist of (a) tracheids, the common wood 

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

 lary or i)ith 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- 

 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 occupying the middle 

 portion of medullary rays (see PI. 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 PI. XXI), ttattened in tangen- 

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

 in radial direction being 45 to 35 /.t (about 0.002 inch) in the 

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

 merwood, and in tangential direction about 40 /; on the average 

 iu their middle. They are arranged iu regular radial rows 

 (see PI. XXI), which are continuous through an indefinite 

 number of rings, but the number of rows iiicreasiug every 

 year to accommodate the increasing circumference of the 

 growing stem. (See PI. 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 PI. XXI, B c; 

 also fig. 17), their iieiglibors filling out Ihe interspaces. There 

 is no constant difference iu the dimensions of these fibers 

 Fig. 17.-CV11 endings ,n pine. .^^ ^^^ diflerent spccies here considered. In every tree the 



fibers 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 tentli to twentieth ring from the pith. 

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

 in radial direction is iu /(=0.001 mm: 



