PATTERSON v. M'CAUSLAND.—3 BLAND. 61

be estimated by the respective thickness of the rings of timber."
Darwin's Phytologia, 476.

The Linnaean hypothesis was, that the pith added a layer every
year to the wood internally. But on its being observed, that many
trees grew vigorously, the pith or a part of which had rotted so as
to leave them almost entirely hollow, that hypothesis was aban-
doned as totally erroneous. And on its being discovered that the
food of a tree, after having been taken in by the root, and, some-
how, carried up and digested into sap by the leaves, was assimi-
lated and added to the bulk of its trunk and limbs in layers imme-
diately under its bark, the opposite hypothesis was adopted, that
trees were increased in size by those external additions alone.

Hence it was, perhaps, that upon a more careful examination of
the organs of vegetables, they were classed, in reference to the
visible arrangement of those organs, into two great groups, the
first called exogenous, because of their having the vascular tissue
arranged in concentric cylinders around a common axis, the pith;
and the second, endogenous, having this tissue disposed in bundles,
and not in cylinders. In the first class, the tubes and woody fibre
are arranged in concentric bands, having the cellular tissue, in
part, packed in between them; and in part forming lines, called
the medullary rays, cutting them at right angles, and radiating
from the axis of the stem. Such steins increase by the regular ad-
dition of new layers on the outside of the old wood; and are thence
termed exogenous stems, or growers outwardly, as the name im-
ports. This is the structure of almost all the forest trees of our
Union. In the second class, the tubes and woody fibre are dis-
posed in bundles throughout the stem; the interstices being filled
up with cellular tissue. The stems having this structure do not
increase in diameter after they are once fairly formed, but only in
solidity. This they do by the addition of new bundles of tubes
and woody fibre internally. Hence, they have received the name
of endogenous or growers inwardly, (b) Again it was observed,
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,

(b) " The wood, which exists more or less abundantly, even in herbaceous
stems, and which forms so large a portion of those trees and shrubs, in the
stem which we have selected for examination, consists of a single zone or
layer, composed of tubes and woody fibre, disposed without any regular
order, except that the latter is the most abundant on the outside, next the
bark. The second year of a plant's growth, a new layer is formed outside
of the first, and similar to it in every respect. The third year this process
is repeated; and thus the stem increases in size, a new 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 does 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