THE VZGETATIVE ORGANS OF PLANTS. 269 
the firmer vascular bundles unaltered in form. A portion 
of the base of such a stalk, cut lengthwise, is represented 
in figure 47, where are seen the duct-fibers arranged par- 
allel to each other in the internodes, and curiously inter- 
woven and branched at the nodes, either those, a and 8, 
from which roots issue, or that, c, which was clasped by 
the base of a leaf. 
The endogenous stem, as represented in the maize-stalk, 
has no well-defined bark that admits of being stripped off 
externally, and no separate central pith of soft cell-tissue 
free from vascular bundles. It, like the aérial portions of 
all flowering plants, is covered with a skin, or epidermis, 
composed usually of one or several layers of flattened 
cells, whose walls are thick, and far less penetrable to 
fluid than the delicate texture of the interior cell-tissue. 
The stem is denser and harder at the circumference than 
towards the center. This is due to the fact that the fibers 
are more numerous and older towards the outside of the 
stem. The newer fibers, as they continually form, grow 
in the inside of the stem, and hence the designation endog- 
enous, which in plain English means inside-grower. 
In consequence of this inner growth, the stems of most 
woody endogens, as the palms, after a time become so in- 
durated externally, that all lateral expansion ceases, and 
the stem increases only in height. It grows, nevertheless, 
internally, new fibers developing in the softer portions, 
until, in some cases, the tree dies because its interior is so 
closely packed with fibers that the formation of new ones, 
and the accompanying vital processes, become impossible. 
In herbaceous endogens the soft stem admits the indefi- 
nite growth of new vascular tissue. 
The stems of the grasses are hollow, except at the 
nodes. ‘Those of the rushes have a central pith free from 
vascular tissue. 
The Minute Structure of the Endogenous Stem is ex- 
hibited in the accompanying cuts, which represent highly 
