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PATTERSON v. M'CAUSLAND. 79
of other plants are alike in this respect; and have thence been
denominated dicotyledonous plants. Another class, on their first
appearance exhibit only one such leaf or cotyledon, such as Indian
corn, the cabbage tree, (chamaerops palmeto,) &c.; and hence they
have been called monocotyledonous plants. Those of the first
kind having been found to have stems of the exogenous structure,
and those of the latter to be always of an endogenous formation,
the two classes have been and may as well be designated by the
one name as the other, (t)
Then assuming that this was the only mode by which exoge-
nous trees were enlarged, and because the sap flowed more freely
and obviously in summer than in winter, it was affirmed, that the
number of those concentric layers, counting from the surface to
the centre, demonstrated the number of years the tree had been
growing. But as has been seen, it is admitted, that in the wood
of forest trees of the temperate zone, in which those concentric
layers have been noticed, it has been observed, that each layer is
composed of a great number of thinner and scarcely distinguish-
able ones, which in some cases assume a more or less conspicuous
appearance than usual, in consequence of the fluctuations of the
seasons, or accidental checks on the growth of the tree; as hard
winters render the outside, or poms part of each circle, more de-
cided; while favourable summers make the circle itself altogether
broader.
Hence it is evident, from what is thus stated by the advocates
of this notion, of each layer's being an evidence of a year's
growth, that it is founded upon the apparent effects of the revo-
lution of the seasons in the temperate zone. But the roots of car-
rots, beets, &c., which are the growth of a single season; and
indeed the roots of all perennial trees, as well those of the endo-
genous as of the exogenous class, are also formed of concentrical
layers; (u) and the wood of the trunks of most of the forest trees
of the torrid zone are evidently formed in the same way; although
some of them may exhibit slighter traces of such concentric rings
than others. But the wood of none of the endogenous class of
plants, among which is the cabbage tree, (chamaerops palmeto,) of
on Vegetable Physiology, by George D. Armstrong, Prof, of Nat. Philosophy and Che-
mistry, in Washington College, Virginia, chap, 5 and 6; The Farmers' Register, by
Edmund Ruffin, vol. 7, No. 3.
(t) Eaton's Botanical Grammar, 18.—(u) Roget Anim. and Veget. Physi. pt. 1,
c. 1, s. 3.
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