plant. The affection cannot be cured by any
change of treatment, and is propagated by layers, cuttings, &c., and
perhaps even by seed. In contabescent plants the female organs are
seldom affected, or merely become precocious in their development. The
cause of this affection is doubtful, and is different in different
cases. Until I read Gaertner's discussion I attributed it, as
apparently did Herbert, to the unnatural treatment of the plants; but
its permanence under changed conditions, and the female organs not
being affected, seem incompatible with this view. The fact of several
endemic plants becoming contabescent in our gardens seems, at first
sight, equally incompatible with this view; but Koelreuter believes
that this is the result of their transplantation. The contabescent
plants of Dianthus and Verbascum, found wild by Wiegmann, grew on a dry
and sterile bank. The fact that exotic {166} plants are eminently
liable to this affection also seems to show that it is in some manner
caused by their unnatural treatment. In some instances, as with Silene,
Gaertner's view seems the most probable, namely, that it is caused by
an inherent tendency in the species to become dioecious. I can add
another cause, namely, the illegitimate unions of reciprocally
dimorphic or trimorphic plants, for I have observed seedlings of three
species of Primula and of _Lythrum salicaria_, which had been raised
from plants illegitimately fertilised by their own-form pollen, with
some or all their anthers in a contabescent state. There is perhaps an
additional cause, namely, self-fertilisation; for many plants of
Dianthus and Lobelia, which had been raised from self-fertilised seeds,
had their anthers in this state; but these instances are not
conclusive, as both genera are liable from other causes to this
affection.

Cases of an opposite nature likewise occur, namely, plants with the
female organs struck with sterility, whilst the male organs remain
perfect. _Dianthus Japonicus_, a Passiflora, and Nicotiana, have been
described by Gaertner[407] as being in this unusual condition.

_Monstrosities as a cause of Sterility._--Great deviations of
structure, even when the reproductive organs themselves are not
seriously affected, sometimes cause plants to become sterile. But in
other cases plants may become mons