MICROSCOPICAL STRUCTURE. 



139 



Sriiioii I. ANallsiil' tlio ti'achcklH of tlic pith ray with ilentatc ])r<>joction8. 



a. One to two lar^e, siiniile pits to each traoheid cm tlie radial wallH ol" tlic colls ot' the ]iith ray. — (iroiip 1. 



Kepreseuted in this country by J', rcsiiiom. 



b. Three to six simple pits to each trachcid on the walls of the cells of the pith ray. — Grouj) 2. /'. Imla, 



paluatris, etc., inchidin;; most of our "hard'' and "yellow" 

 pines. 

 Sci'tiou II. Walls of tracheids of iiith ray smooth, witliout di'Utate 

 projections. 



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

 each cell of the ])ith ray.— Group 3. /'. utrobita, lamheri'mna, 

 and other true white pines. 



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

 pith ray. — Group 4. P. parrxjana, and other init pines, includ- 

 ing also 7'. batfonriana. 



Fig. 16. — Schematic representation of conilerous wood 

 structure; wood of sjirucc— 1. natural size: 2. small 

 part of one ring ma^nilied 100 times. The vertical 

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

 VI, medullary or pith ray ; n, transverse tracheids of 

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

 more enlarged. 



The general features of structure of coniferous woods ""^^g] 



are represented in the accompanying cut (fig Ki). "i&Sfi 



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; (h) medul- 

 lary or jiith 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; ((') 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 occui)ying the middle 



portion of medullary rays (see PL XX^'II). 



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 C 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), flattened in tangen- 

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

 in radial direction being 45 to 55 /( (about 0.002 inch) in tlie 

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

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

 in their middle. They are arranged in regular radial rows 

 (see PI. 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 tlieiv ends they are often bent, 

 slightly distorted, and usutilly separated (see PI. XXI, B c; 

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

 is no constant ditference in the dimensions of these fibers 

 in the ditferent species here considered. In every tree the 



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



l)itli outward, and reacliing their full size in about the tenth to tweiititth ring from the pitli. 



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



in radiiil direction is in /(=0.0()1 mm: 



l'"lG. 17. — Ci-ll endings in pine. 



