PATTERSON «. M'CAUSLAND.—3 BLAND. 63

*of other plants are alike in this respect; and have thence
been denominated dicotyledonous plants. Another class,

decay, this is frequently found to be the case The resinous matter, de-
posited in the heart wood of the pine, is of such a nature as effectually to
resist the disorganizing agencies which operate upon it and hence it is that
pines, even those of the greatest age, are never found hollow But such is
not the fact with respect to the matter deposited in the heart wood of the
sycamore, ( platanus occidentalis,) and hence, all the oldest trees of that kind,
are little more than shells "

" Endogenous stems differ very much from exogenous ones in their
structure The characteristic differences are the arrangement of the tissues,
and the manner of their growth Besides this endogens differ from exogens,
in having neither pith, medullary lays bark, or wood properly so called,
but consisting of a confused mass of woody bundles, imbedded in cellular
tissue In the stalk of the corn (Zea mays } which affords a good specimen
of a stem constructed on the endogenous plan we find an external conical
integument, without liber, and bundles of woody matter, bo arranged
throughout the cellular tissue, as to be much more numerous and compact at
the circumference, than towards the centre In the stem of the garden
asparagus (asparagus officinalis,) the woody bundles are distributed uni-
formly throughout the stem, and so soft as scarcely to be recognized as

woody matter The same arrangement of the woody bundles, exists in the
green brier (simlax rotundifola,) the only endogenous shrub common in
Virginia In the stems of grasses, which have been said to be the least en-
dogenous of all endogenous stems, the structure is so modified as not to be
at once evident The peculiarity of these stems is, that they are hollow, ex-
cept at the nodes, or joints, which are very compact discs, closing the stem
entirely They are, however, in every instance at first solid, and become
hollow in the course of their growth In other respects, the stems of grass
present no variation from the typical structure of endogens "

" The life of endogens, as well as their diameter, is limited by the nature
of their und When the lateral growth of the stem has proceeded to a cei-
tam extent, the rind hardens, and the stem being, in this way, pre\ented
from increasing in diameter, can only grow in length, and as the conse-
quence, stems of this character are generally slender The continual de-
position of new matter within the unyielding rind, finally produces a total
solidification of the stem, and death follows as a necessary consequence.
Thus the life of an endogenous stem is limited, for unless destroyed by
some external agency, it must die of old age The individual, however, it,
seldom destroyed for, whilst the trunk is thus slowly perishing, the great
accumulation of sap in the loots, causes the development of new shoots from
the base of the stem, and these continue the life of the individual when the
original trunk dies down to the ground. In this view, the life of endo-
genous trees is unlimited

In the structure of exogenous stems, on the other hand, there is nothing
to limit either their increase or duration, they never die purely of old age,
but when destroyed are destroyed by some external agency The central
wood of exogens, it is true, dies in the course of time but the death of the
stem does not follow as a consequence of this, for nothing is more common
than to see a tree hollow, destroyed at its centre, whilst it is growing vigor-
ously at its circumference The sycamore, (platanus occidentalis,) furnishes
a remarkable and well known illustration of this The oldest trunks are
generally all destroyed, excepting a few of the outer and recently formed