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76 PATTERSON v. M'CAUSLAND.
of each summer during the time of its growth, may be estimated
by the respective thickness of the rings of timber.' (r)
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 stems increase by the regular addition
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, (s) Again it was observed,
(r) Darwin's Phytologia, 476.
(s) 'The wood, which exists more or less abundantly, even in herbaceous stems,
and which forms so large a portion of those of 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
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