varieties of cabbages, of lettuces, &c. Most of these
variations are mentioned under the head of the particular morphological
change of which they are illustrations.

The effect of a change in the conditions of growth in producing
diversity in the form of the leaf may be here alluded to. _Ficus
stipulata_, a plant used to cover the walls of plant-stoves in this
country, and growing naturally on walls in India, like ivy, produces
leaves of very different form, size, and texture, when grown as a
standard, from what it does when adhering to a wall. _Marcgraavia
umbellata_ furnishes another example of a similar nature, as indeed, to
a less extent, does the common ivy.

Allusion has been already made to the occasional persistence of forms in
adult life, which are commonly confined to a young state, as in the case
of some conifers which present on the same plant, at the same time, two
different forms of leaves. Mention has also been made of the presence of
adventitious buds on leaves and in other situations. The leaves that
spring from these buds are usually of the same form as the other leaves
of the plant, but now and then they differ. Of this a remarkable
illustration is afforded by a fern, _Pteris quadriaurita_, in which the
fronds emerging from an adventitious bud are very different from the
ordinary fronds.

[Illustration: FIG. 178.--Portion of a frond of _Pteris quadriaurita_,
with an adventitious bud, the form of the constituent foliage of which
is very different from that of the parent frond.]

=Dimorphism.=--This term, applied specially to the varied form which the
flowers or some of their constituent elements assume on the same plant,
is an analogous phenomenon to what has been above spoken of as
heterophylly, and, like it, it cannot, except under special
circumstances, be considered as of teratological importance. A few
illustrative cases, however, may here be cited.

Sir George Mackenzie describes a variety of the potato[367] (_Solanum
tuberosum_), which produces first double and sterile flowers, and
subsequently single fertile ones; the other portions of the plant do not
differ much.

_Stackhousia juncea_, according to Clarke, has mixed with its perfect
flowers a number of apetalous blossoms destitute of anthers.[368]

This peculiarity is well exemplified in the tribe _Gaudichaudieae_ of the
order _Malpighiaceae_. A. de Jussieu, in his monograph, speaks of these
flowers as being very small, green, destit