92 College of Forestry 
hight yellowish-brown color, the heartwood somewhat darker, 
there being no sharp line of demarcation between them. In 
a transverse section of the wood the pores are numerous, 
indistinct to the unaided eye, and uniformly distributed 
throughout the growth ring. The pith-rays Lkowise are 
mostly indistinct without a lens. 
Microscopically the wood of yellow birch presents the 
usual features of the diffuse-porous type. The annual rings 
are defined principally by a tangential row of wood paren- 
chyma which terminates the growth ring. The definition of 
the annual rings is somewhat further enhanced in that the 
vessels are slightly smaller and more sparsely distributed 
in the late wood. (Plate XXI, Fig. 1.) Microscopically 
the wood of the yellow birch is seen to consist of (a) uni- 
seriate and multiseriate pith-rays, (b) pitted vessels, (¢) 
metatracheal parenchyma with simple. pits, and (d) wood 
prosenchyma with either simple or bordered pits. The 
medullary rays are from 1—3, more rarely 4, seriate, and 
homogeneous. The ray cells are mostly elongated im a radial 
direction and aie connected with one another by numerous 
simple pits. As units the rays are peculiar in that the 
lateral walls of the uniseriate forms and similarly the lateral 
walls of the peripheral cells, in the case of the multiseriate 
forms, are characterized by having very thin walls, while 
the interior cells in the latter case have fairly thick walls. 
The vessels occur either solitary or in radial groups of from 
two to three; they are uniform in size and distribution 
throughout the growth ring. Their walls are comparatively 
thin and densely covered with small bordered pits with nar- 
row, slit-like openings. These occur on the contiguous walls 
of the vessels and where they are in contact with wood or 
ray parenchyma. The vessel segments are characterized by 
very oblique end walls which are provided with numerous 
sealariform perforations. A small amount of wood paren- 
chyma is scattered irregularly throughout the growth ring 
(metatracheal), sometimes appearing in broken tangential 
lines in the late wood. 
