4 TIMBER AND TIMBER TREES.  [cuae. 
Poplar and Cypress they are nearly erect ; the Oak, in 
open and exposed situations, taking a wide-spreading 
form, its branches assuming every imaginable curve ; 
while in the Cedar they are nearly at a righé angle. 
The stem is constructed upon the principle of a 
cone, and consists of a series of perfected layers desig- 
nated heart-wood, or duramen, with, commonly outside 
these, some layers that are imperfect, which are known 
as the sap-wood, or alburnum; while the exterior is 
composed of the bark, or liber. The main portion 
of the stem is broadest at the base, and somewhat 
bell-shaped near the root, but gradually diminishes 
upwards to the part where the first branches are thrown 
out, and from this point there is again a still further 
diminution, until it is finally lost in the extremity of 
the branchlets. 
The central and first-formed part of an exogenous 
stem, namely, the pith, is composed of cellular tissues, 
the cells being very numerous and varying considerably 
in size, but generally diminishing towards the outer 
edge. The pith is somewhat large and full of fluid 
in the young plant, but does not increase in bulk as 
the tree grows older ; on the contrary, it appears rather 
to diminish than otherwise, by the fluid drying out. 
It retains, however, its place in the oldest trees, in the 
form of a dry consistent powder, although it is scarcely 
noticeable in some species on their arrival at maturity. 
In the employment of timber in carpentry, due 
regard must always be had to the position of the pith, 
since there is an outside and an inside to every board 
and piece of scantling ; and the careful workman is so 
well aware of this, that he will study to leave, if pos- 
sible, in any work of construction, the outer side only 
exposed. It is, therefore, necessary in every case to 
