e classification of the Pines
with the same reservations that apply to external characters.

Ray-tracheids with dentate walls and ray-cells with large pits are
peculiar to Pinus. Therefore the presence of these characters, alone or
in combination, is sufficient evidence for the recognition of Pine-wood.
But the combination of smooth tracheids with small pits (subsection
Paracembra) Pinus shares with Picea, Larix and Pseudotsuga.

Among Hard Pines the size of the pits has a certain geographical
significance. The large pits are found in all species of the Old World
except P. halepensis and P. pinaster; the small pits in all species of
the New World except P. resinosa and P. tropicalis. The Asiatic P.
Merkusii with both large and small pits is not strictly an exception to
this geographical distinction. The four exceptional species by this and
by other characters unite the Hard Pines of the two hemispheres.

THE BARK.

Bark is the outer part of the cortex that has perished, having been cut
off from nourishment by the thin hard plates of the bark-scales. In the
late and early bark-formation is found a general but by no means an
exact distinction between Soft and Hard Pines. In the Soft Pines the
cortex remains alive for many years, adjusting itself by growth to the
increasing thickness of the wood. The trunks of young trees remain
smooth and without rifts. In the Hard Pines the bark-formation begins
early and the trunks of young trees are covered with a scaly or rifted
bark. The smooth upper trunk of older trees is invariable in Soft Pines,
but in Hard Pines there are several exceptions to early bark-formation.
These exceptions are easily recognized in the field, and the character
is of decisive specific importance (glabra, halepensis, etc.).

Among species with early bark-formation are two forms of bark: 1,
cumulative, sufficiently persistent to acquire thickness and the
familiar dark gray and fuscous-brown shades of bark long exposed to the
weather; 2, deciduous, constantly falling away in thin scales and
exposing fresh red inner surfaces. The latter are commonly known as Red
Pines, as distinguished from Black Pines with dark cumulative bark.
Deciduous bark changes after some years to cumulative bark, and the
upper trunk only of mature trees is red. Red Pines, although usually
recognizable by their bark, are by no means constant in this character.
Oecological or pathological influences may check the fall of the
bark