16 COLOMBIAN MAHOGANY. 



may be from a few to 10 or more in a single wood-parenchyma fiber, 

 either occupying the whole or only a part of the fiber. (The wood- 

 parenchyma fibers of true mahogany are similar to those of Cariniana, 

 but smaller and contain crystals, fig. 9, b.) 



Wood-parenchyma fibers are shorter than wood fibers, but their 

 diameter is slightly greater. The walls of the former are much 

 thinner and their cavities relatively larger than those of the latter. 

 Wood-parenchyma fibers are subdivided by horizontal, oblique, or 

 somewhat rounded cross walls, into from 6 to 10 cells. The upper 

 and lower cells always have blunt ends, while the walls are marked 

 by numerous simple pits, the latter being less numerous on the walls 

 adjoining wood fibers, and largest on the terminal walls. 



Pith rays (figs. 6, d, and 11, d) appear, under a pocket lens, as 

 numerous, fine, uniform, slightly undulating, nearly equidistant 

 lines, always growing around the larger vessels. The cells of which 

 the rays are formed vary in form from oval, the commonest form, to 

 oblong, and usually have oblique cross walls. As seen in tangential 

 sections the pith rays are evenly distributed and constitute about 25 

 per cent of the wood mass. They are from 1 to 2, rarely 3, rows of 

 cells wide, and from a few to 35 cells high. In tangential sections, 

 the cells of pith rays are usually round and somewhat smaller in 

 diameter than those of the wood-parenchyma fibers. The diameter 

 of the upper and lower cells is larger in a vertical direction, but 

 shorter in a horizontal direction. These marginal cells, as they are 

 called, arc only one-half as long as the ordinary pith-ray cells. The 

 cavities of pith-ray cells are filled with a dark-brown mass consisting 

 of starch grains and other materials, as yet unidentified. Crystals of 

 calcium oxalate are seldom present in these cells. The walls of pith- 

 ray cells are marked with simple pits, which are usually very numerous 

 on the sides adjoining other pith-ray cells, and vessels, and wood- 

 parenchyma fibers. 



Approved : 



James Wilson, 



Secretary of Agriculture . 

 Washington, D. C, May 22, 1911. 



O 



