(40). A Bean-stalk and the stem of any common
shrub or tree represent the second; and to it belong all plants with
dicotyledonous or polycotyledonous embryo. The first has been called,
not very properly, _Endogenous_, which means inside-growing; the second,
properly enough, _Exogenous_, or outside-growing.

427. =Endogenous Stems=, those of Monocotyls (40), attain their greatest
size and most characteristic development in Palms and Dragon-trees,
therefore chiefly in warm climates, although the Palmetto and some
Yuccas become trees along the southern borders of the United States. In
such stems the woody bundles are more numerous and crowded toward the
circumference, and so the harder wood is outside; while in an exogenous
stem the oldest and hardest wood is toward the centre. An endogenous
stem has no clear distinction of pith, bark, and wood, concentrically
arranged, no silver grain, no annual layers, no bark that peels off
clean from the wood. Yet old stems of Yuccas and the like, that continue
to increase in diameter, do form a sort of layers and a kind of scaly
bark when old. Yuccas show well the curving of the woody bundles (Fig.
471) which below taper out and are lost at the rind.

[Illustration: Fig. 474. Short piece of stem of Flax, magnified, showing
the bark, wood, and pith in a cross section.]

428. =Exogenous Stems=, those of Dicotyls (37), or of plants coming from
dicotyledonous and also polycotyledonous embryos, have a structure which
is familiar in the wood of our ordinary trees and shrubs. It is the same
in an herbaceous shoot (such as a Flax-stem, Fig. 474) as in a
Maple-stem of the first year's growth, except that the woody layer is
commonly thinner or perhaps reduced to a circle of bundles. It was so in
the tree-stem at the beginning. The wood all forms in a cylinder,--in
cross section a ring--around a central cellular part, dividing the
cellular core within, the pith, from a cellular bark without. As the
wood-bundles increase in number and in size, they press upon each other
and become wedge-shaped in the cross section; and they continue to grow
from the outside, next the bark, so that they become very thin wedges or
plates. Between the plates or wedges are very thin plates (in cross
section lines) of much compressed cellular tissue, which connect the
pith with the bark. The plan of a one-year-old woody stem of this kind
is exhibited in the figures, which are essentially diagrams.

[Illustration: