14 BOTANY. 
The medullary rays, or plates, or the silver grain of carpenters, keep up 
a communication between the bark and the pith, these being generally 
separated by vascular layers. They consist of cellular tissue, which has 
been gradually compressed, so as to give a muriform appearance to the cells. 
The space they occupy, at first large, is diminished more and more with 
increasing age. A transverse section of a woody stem presents the 
appearance of narrow lines running from the centre to the circumference. 
A longitudinal section shows that these rays are not laminz continuous 
from one end to the other, but are broken up by the intervention of woody 
fibres. 
We have thus described the normal character of the exogenous stem. 
There are, however, certain anomalous appearances in certain plants, which 
are not readily reducible to rule. In place of the concentric arrangement 
of the vascular layers, there are sometimes only a few rows of wedge-shaped 
bundles, and additions made by the interposition of new bundles, just as in 
the young herbaceous normal stem; sometimes these vascular bundles are 
arranged in zones. Again, in some cases the separating layers are cellular, 
not fibrous; sometimes the woody layers are arranged in a very irregular 
manner. In some Bignonias the layers are divided into four wedge-shaped 
portions, probably by an introversion of the liber. In Paullinia a central 
woody mass is sometimes surrounded by others likewise cylindrical. - In some 
Malpighiacez, the outer surface, instead of being cylindrical, exhibits very 
irregular lobes and indentations. 
The stems of endogenous plants present many features different from 
those which we have found to exist in exogens, and especially in that there is 
no absolute or visible distinction into pith, medullary rays, wood, and bark. 
There is an intermixture of bundles of fibro-vascular tissue among a mass of 
cellular tissue, the whole overlaid by a zone of denser cellular and woody 
tissue, inseparable from the stem. In the young plant the centre of the stem 
is occupied entirely by cells, around which the vessels are grouped, increasing 
in number towards the circumference. The central cells are sometimes 
ruptured and absorbed, leaving a cavity; more generally, however, they are 
persistent, becoming gradually encroached upon by the increasing vascular 
system. The external layer of the endogenous plant occupying the place 
of bark, and known as false bark, is a dense layer of cellular tissue, into 
which the lower ends of the vascular fibres dip, losing their vascularity as 
soon as they reach it. 
The opinion originally entertained that the new layers of vascular fibres 
were developed inside the old ones, and pushed these out towards the cortical 
envelope, appears not to be strictly correct. as, although at first they are 
thus internal, yet, subsequently, they curve outwards to run into the 
exterior, aS already mentioned. After all, the true distinction between 
exogenous and endogenous stems consists in this: in the former, the woody 
or vascular layers increase indefinitely at their periphery; in the latter, 
they are arrested in lateral growth at a definite epoch. When it is one 
terminal bud alone of an endogen that dev elopes, the stem may be truly 
cylindrical ; when several develope, however, the stem will be conical. A 
14 
a 
