62 PATTERSON v. M'CAUSLAND.—3 BLAND.

having some resemblance to leaves; these were called cotyledons.
The common garden bean, the peach, the oak, and a great variety

no change in dimensions Hence, as the necessary result of this mode of
growth, the stem of an exogen is more or less conical "

" Each layer, or to speak more accurately, each hollow cone of wood, is
the result of a single year's growth, it is evident, that the age of an exogen
may be ascertained by counting the number of rings presented on a trans-
verse section of the stem, made near its base This may be done with great
accuracy, in most trees of the temperate and cold climates, in which, in con-
sequence of the periodical suspension of vegetation, the annual layers are
distinctly marked but in the case of trees of the torrid zone, where vege-
tation goes on throughout the year, this cannot be so readily done In old
trees, the rate of increase being very uniform, their age may be determined
with considerable accuracy, by the inspection of a mere fragment of the
stem the diameter of the whole stem from which it was taken being known
A rough estimate of the age of a tree is sometimes made by dividing the
semi-diameter of its base by the average increase of the species to which it
belongs, that average being determined by previous observation. In these
several ways the ages of numerous very old trees have been determined.
It should be remarked, however that these determinations except where
they are based upon an actual counting of the rings presented by a trans-
verse section of the trunk, cannot be regarded as any thing more than ap-
proximations to true age A tree growing in peculiarly fertile ground, will
enlarge much more rapidly than most other trees of the same species and
of course, with a given diameter will have a less number of zones than the
average In the case of a tree growing in peculiarly barren ground, just
the opposite effect would ensue An estimate of the age of the first, made
by dividing its semi diameter by the average thickness of the zones of that
particular species, would give too great an age An estimate of the age of
the last, made by this same method, would give an age less than the true
one "

" There is almost always a marked difference in color and density, between
old and recent wood The outer and more recent portions of the stem, have
been called m allusion to their color, alburnum, and in allusion to their
office, sap wood the inner and older portions are termed the heart wood
After a few years the color of a layer of wood is changed, its density is in-
creased, and it takes thereafter little part in. the transmission of the sap
During the winter, it is true, it generally contains sap, but then this sap is
rather deposited in it than circulating through it The change in color and
density by which sap •wood is converted into heart wood, is caused by the
deposition of a solid matter, peculiar to each species, in the tissues of that
part This matter is, in most cases, soluble in nitric acid, and hence it is,
that if a piece of heart wood be immersed in that acid, the color is dis-
charged, and the piece again assumes the appearance of sap \vood. Where
the matter deposited is of a resinous character as in the pines, it adds very
much to the durability, and consequently, to the value of the heart wood
On this account, as well as on account of its greater solidity and strength,
the heart wood is universally preferred to the sap wood, for use in the arts
As the layers of wood, in the course of a few years after their formation,
cease to take any active part in the circulation of the sap, and, in time, be-
come to all intents and purposes dead matter, it would naturally follow, that
the central part of the stem would be first to decay Where the matter de-
posited in those parts is not of such a character as to protect them from