112 



FORESTRY INVESTIGATIONS U. S. DEPARTMENT OF AGRICULTURE. 



MINl'TE ANATOMY. 



The minute structure or histology of the wood of the five species under consideration is that 

 of a group whose position in a general classification of the wood of piues is indicated in the follow- 

 ing scheme, suggested by Dr. J. Schroeder, and more completely by Dr. H. Mayr, 1 in which they 

 appear as part of group 2 of Section I. 



Section I. Walls of the tracheids of the pith ray, with dentate projections. 



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



Represented iu this country by P. rezinosa. 



b. Three to six simple pits t<> each tracheid on the walls of the cells of the pith ray .Group 2. P. ta-da, palustris, 



etc., including most of our "hard" and "yellow" 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. P. strobus 



lambertiana, and other true white pines, 

 ft. Three to six small pits on the radial walls of each cell of the pith ray. Group 4. P. parryana, and other nut 



pines, including also P. balfouriaua. 



The general features of structure of coniferous woods are represented in the accompanying 

 cut (fig. 20). 



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) medullary or pith rays, minute 



FIG. 20. Schematic representation of coniferous wood struc. Flo. 21. Cell endings in pine, 



tare : wood of spruce 1, natural size ; 2, small part of oue 

 ring magnitifd 100 times. The vertical tubes are wood fibers, 

 in thia case all "tracheids;" m. medullary or pith ray; n, 

 transverse tracheids of pith ray; a, b, and c, bordered pits 

 of the tracheids more enlarged. 



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 species ; (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. 



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

 pines: they are slender tubes, 4.5 to 6 mm. (about one-fourth inch) long, forty to oue hundred 

 times as long as thick, usually hexagonal in cross section, with sharp or more or less rounded 

 outlines (see PI. XX), flattened in tangential direction at both ends (see PI. XX, A/), the diameter 

 iu radial direction being 45 to 55 /u (about 0.002 inch) in the springwood, and about half that, or 

 21 to 25 /<, in the summerwood, and in tangential direction about 40 yi/, ou the average in their 



'Dr. J. Schroeder, Holz der Coniferen, Dresden, 1872, p. 65; Dr. H. Mayr, Waldungeu von Nordanierika, Miin- 

 chen, 1890, p. 426. 



