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PATTERSON v. M'CAUSLAND. 77
on the first appearance above ground of the nascent plant, that it in
many cases exhibited a pair of thick fleshy lobes of the seed, hav-
layer being formed annually, as long as the plant lives. The wood of an exogen, of
one year's growth, may be viewed as an elongated hollow cone, extending from the
base to the summit of the stem, and enclosing the pith. This cone docs not extend
further, nor does it enlarge in any way; but is surrounded the next year by another
cone, which, like the first, after being formed, undergoes 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 ascer-
tained by counting the number of rings presented on a transverse 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 consequence of the periodical suspension
of vegetation, the annual layers are distinctly marked; but in the case of trees of the
torrid zone, where vegetation 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 deter-
mined 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 be-
ing determined by previous observation. In these several ways, the ages of nume-
rous 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 transverse section of the trunk, cannot be regarded as any thing
more than approximations 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-diame-
ter 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 colour and density, between old
and recent wood. The outer and more recent portions of the stem, have been called,
in allusion to their colour, alburnum; and in allusion to their office, sap wood; the
inner and older portions are termed the heart wood. After a few years, the colour
of a layer of wood is changed, its density is increased, and it takes thereafter little
part in the transmission of the sap. During the winter, it is true, it generally con-
tains sap, but then this sap is rather deposited in it, than circulating through it.
The change in colour 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 colour is discharged,
and the piece again assumes toe appearance of sap wood. Where the matter depo-
sited 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, become to all intents and purposes dead matter, it would naturally
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