134. ROOTS OF DICOTYLEDONS AND MONOCOTYLEDONS. 
of the plant which are thus perennial, their stems dying down 
to the ground yearly. Perennial roots are either of woody 
consistence, or more or less fleshy as in those of biennial plants. 
In the case of fleshy roots such as the Dahlia and Orchis, the 
individual roots are not in themselves perennial, but usually 
perish annually ; but before doing so, they produce other roots 
from some point or points of their substance ; hence, while the 
root as a whole is perennial, any particular portion may perish. 
Woody roots are commonly perennial in themselves, and are 
not renewed. 
Roots oF DicoryLEpONS, MoNnocoryLEpons, AND ACcRO- 
GENS.—We have already seen that the stem of Dicotyledons, 
Monocotyledons, and Cormophytes, possesses certain character- 
istic differences in its internal structure. The roots of such 
plants in like manner possess similar distinctive struc- 
tural characters, and also some others, which, although gene- 
rally referred to previously, had better be briefly summed up 
here. 
1. The Root of Dicotyledons.—The root of these plants is 
formed, as we have seen (page 125), by the direct elongation 
of the radicle of the embryo from the formation of new tissue 
just within its apex. Such a mode of root-development has 
been called exorhizal, and a root thus formed is called a true 
root. 
It follows from this mode of development that Dicotyledons 
have generally a tap-root (page 130) or descending axis (fig. 253, 
v), from which branches are given off in various directions, 
in the same manner as such plants have also an ascending axis 
or stem, t, from which its branches arise. These tap-roots do 
not, however, commonly descend far into the ground, but their 
branches become much developed laterally ; in some cases even 
more so than those of the stem ; while in others, as in plants of 
the Gourd tribe, and commonly in succulent plants, to a less 
extent. 
In its internal structure the fully developed root essentially 
resembles the stem, except that, as already noticed (page 127), 
it has no pith or medullary sheath: hence the fibro-vascular 
tissue forms acentral axis. This absence of pith and medullary | 
sheath is general in herbaceous Dicotyledons ; but there are 
some trees, as, for instance, the Walnut and Horsechestnut, 
where the pith is prolonged downwards for some distance into 
the root. 
2. The Root of Monocotyledons.—In these plants the 
radicle does not itself, except in rare cases, become prolonged 
to form the root, but it generally gives off above its base one or 
more branches of equal size, which separately pierce the radi- 
cular extremity of the embryo, and become the roots (fig. 251, 
v); and each of these roots is surrounded at its base, where it 
pierces the integuments, with a kind of cellular collar, termed 
