128 DISTINCTIVE CHARACTERS OF STEMS AND ROOTS. 
as already noticed, in a similar manner, and, like it, have 
commonly neither buds nor leaves. To this latter character, 
however, there are many exceptions, for although the root has 
no power of forming regular buds, yet adventitious buds may 
be developed, in the same manner as we have seen that under 
certain circumstances they may be produced from any paren- 
chymatous tissue (page 111). The power which the root thus 
possesses of producing adventitious buds may be observed in 
the Plum-tree, the Moutan Peony, the Japan Anemone, and 
many other plants. The latter plant especially exhibits this 
tendency in a remarkable degree. 
Distinctive Characters of Stems and Roots.—From the above 
general description which has been given of the growth, struc- 
ture, and characteristics of the true or primary root, we find 
that the chief distinctive characters between it and the stem in 
Dicotyledons may be summed up as follows:—1st. The ten- 
dency of the root at its first formation to develop in an opposite 
direction to the stem, and thus withdraw from the light and air. 
2nd. By not growing throughout the entire length of its newly 
formed parts like a stem, but only by additions just within its 
apex, which is covered by a root-cap or pileorluza. 3rd. The root 
under ordinary circumstances, when fully developed, has no pith 
or medullary sheath. 4th. It has no true epidermis with stomata, 
but in place of this an integument composed of cells without 
stomata, to which the name of epiblema has been given. 5th. 
It has no foliage leaves, or scales (cataphyllary leaves). 6th. It 
has no regular buds, and has consequently no provision for a 
regular ramification. 
2. ADVENTITIOUS OR SEcoNDARY Roots.—This name is ap- 
plied to all roots which are not produced by the direct elonga- 
tion of the radicle of the embryo; because such roots, instead 
of proceeding from a definite point as is the case with the true 
or primary root, are, to a certain extent at least, accidental in 
their origin, and dependent upon favourable external circum- 
stances for their development. All branches of a true root, 
except those originally produced from its apex, are of this nature, 
as are also those of the different modifications of the stem, such 
as the rhizome, runner, sucker, stolon, corm, bulb, &c. ; those of 
slips and cuttings of plants, &c. ; and those of nearly all Mono- 
cotyledons and of Acrogens or Cormophytes. In some plants 
roots are also developed from the stem or branches of plants 
in the air, and are hence called Aerial Roots. Such roots are 
likewise necessarily of an adventitious nature. 
The adventitious roots of Monocotyledons make their first 
appearance as little more or less conical bodies formed by divi- 
sion and subsequent growth of the cells constituting the 
pericambium or outer layer of the plerome or procambium ; 
these soon break through the tissue which envelops them, and 
appear externally, at first as parenchymatous elongations, but 
