36 
action. Surrounding these ducts are small cells, which are termed 
tracheids, having minute thin places in their sides ; the middle lamella 
of the cell, however, is not visibly perforated. Some cells containing 
starch are also intermingled with these tracheids. If the tree is not 
vigorous or the season favorable for a large growth there are only 
a few other cells formed, and the annual layer will be narrow and im- 
perfect; and if the entire wood is made up of such rings it has a low 
specific gravity, is " brash" and not strong. Such wood is not suitable 
for wheel-pieces of car- trucks, and when used for ties the rails wear 
down into it as they do on chesnut ties. In an annual layer of vigor- 
ous growth large bundles or masses of hard, dense fibers are seen just 
outside of the concentric circles of ducts, and when fully formed extend 
through the outer part of the layer. These fibers vary in diameter from 
.006 to .0075 of an inch. Such Hundles or masses of hard fibers give to 
this wood its peculiar hardness and toughness, making it so valuable for 
railroad ties. The medullary rays run through the bundles and at fre- 
quent intervals are intersected by cells running parallel to the axis of 
the tree, thus connecting radiating bundles of medullary rays, and di- 
viding a mass of hard fibers into small rectangles. On either side of a 
mass of hard fibers will be seen a series of small flattened ducts about 
.001 of an inch in diameter, surrounded by small tracheids and some of 
the small wood-fibers. In this transverse section only single lines of 
the medullary rays are seen, but large bundles do occur, and a portion 
of an end view of one will be seeu in the tangential section. Near the 
top and bottom of the illustration can be seen the lines where the an- 
nual layer joins others. The illustration of the radial section shows 
it to have been made through a mass of dense fibers, exposing also the 
exterior and interior of one of the large ducts. The single rows of cells 
running down the page are the medullary rays. This view shows also 
the firmness and solidity of the wood. The hard fibers running across 
the page give but little indication of where the5 T join or interlace. One 
of the ducts shows the surrounding tracheids and also the cells of the 
medullary rays crossing the duct. 
In the tangential section, as already stated, are seen the ends of the 
cells composing one of the great bundles of medullary rays so common 
to this wood j but few of the cells are as large as .001 of an inch in di- 
ameter, most of them being much smaller. Ou either side of this are the 
hard, dense, woody fibers, interspersed with small bundles of medullary 
rays of a single row of cells superimposed one above the other. From the 
views of the structure of this wood it is seen to be one of considerable 
hardness and density. 
Specific Gravity, — For the heart-wood, this ranges from 0.G5 to 0.90. 
In Vol. IX., of the Tenth Census, Professor Sargent reports the average 
of thirty-one pieces, dried at 100° centigrade, as 0.7470. Ties will never 
be found as dry as this, and in general the average will be much higher. 
