aced in one group as in another.

=Formation of adventitious roots.=--This is of exceedingly common
occurrence in a vast number of plants, so much so that in most cases it
cannot be considered as in any way abnormal; there are, however, a few
instances where the formation of these organs may be considered to come
within the scope of teratology, or, at least, where their production is
the result of injury or of some unfavorable condition to which the plant
is exposed.

Thus the production of adventitious roots on the stem of the vine is
considered to be due to untoward circumstances impairing the proper
action of the ordinary subterranean roots. So, too, the formation of
roots on the upper portions of stems that are more or less decayed
below, as in old willows, is to be considered as an attempt to obtain
fresh supplies through a more vigorous and healthy channel.

A similar occurrence often arises as a consequence of some injury.
Virgil had this circumstance in view when he wrote

"_Quin et, caudicibus sectis, mirabile dictu,
Truditur e sicco radix oleagina ligno._"--'Georg.' Bk. ii.

I have seen many specimens of adventitious roots produced on the olive
in the way just mentioned.

In the 'Gardeners' Chronicle,' January 8th, 1853, p. 21, is described a
curious formation of roots in the fissure between two divisions of a
laburnum stem. In the same journal, January 1st, 1853, p. 4, Mr. Booth
mentions the case of a Cornish elm, the trunk of which was divided at
the top into two main divisions, and from the force of the wind or from
some other cause the stem was split down for several feet below the
fork. Around the edges of the fracture, layers of new bark were formed,
from which numerous roots issued, some measuring an inch in diameter and
descending into the cleft portion of the tree: similar instances must be
familiar to all observers.

It may happen that these roots sent down into the cavity of a decaying
trunk may, after a time, become completely concealed within it, by the
gradual formation and extension of new wood over the orifice of the
cavity formed by the death and decay of the old wood. Such is presumed
to be the explanation of a specimen of this kind in the possession of
the writer, and taken from a cavity in an apparently solid block of
rosewood; externally there were no marks to indicate the existence of a
central space, but when the block was sawn up for the use of the
cabinet-maker, t