MICROSCOPICAL STRUCTURE. 



139 



■tioii I. Walls of the tracheuls of tlu-pitli ray with dentate in-ojections. 



a. One to two large, simple pits to each traeheiil on the ratlial walls of the cells of tlie pith ray.— Grou]> 1. 



Represented in this country by 1'. rcaiiiosa. 



b. Three to six simple pits to each tracheid on the walls of the cells of the pith ray.— Group 2. /'. Inda, 



/^ It m^ r^^TtTT^ 



Fig. 16 Schematic representation of couilVrona wood 



.structure: wood of wpruco — 1, natural size; 2, Hmall 

 jiart of one rinj^ majiuilied 100 time.s. The vi-rlical 

 tubi's are wood fibers, in this case all " trachcids," 

 7*1, medullary or ]iith r:iy ; /i, transverse traclieids of 

 pith ray; a, &, and r, horth^red pits of the traclieid.s 

 more enlarged. 



the iniildlc 



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

 pines. 

 Section TI. Walls of tracheids of pith ray smooth, without dentate 

 projections, 

 ii. ( )ne or two large pits to each tracheid on the radial walls of 

 each cell of the pith ray.— Group 3. /'. strohns, laiiilierliiiiiti, 

 and other true white pines. 

 /). Three to six small j>its i>n the radial walls of each cell of the 

 pith ray. — Group 4. I', parrijana, and other nut pines, includ- 

 ing also /'. ha1ffniriti»a. 



The general featuies of structure of coniferons woods 



are represented in the aecompaiiying cut (fig Ki). 



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 foniiiiig 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 occupyinj. 



portion of medullary rays (see PI. XXVII). 



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



other pines; they are slender tubes, i.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 Tl. XXI), llatteued in tangen- 

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

 in radial direction being 45 to 55 /< (about O.OOli incli) in tlie 

 springwood, and about half that, or 21 to 25 //, in tlie siim- 

 merwood, and in tangential direction about40 /< 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 

 glowing stem. (See PL XXI, 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, 1! c; 

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

 is no constant difference in the dimensions of these fibers 

 in till' different species here consitlered. In every tree the 



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



pith outward, and reacliing their full size in about the tenth to twentiith ring from the pith. 



To illustrate: In a sectiim of Longleaf I'ine, 10 feet from the ground, the diameter of tracheids 



in radial direction is in /(=0.001 mm: 



D_OOC1 



(\h fU(lin;;a in ])in*^ 



