ome
years produced stamens only, now produces flowers of both sexes; it was
dioecious, but is now monoecious. The same author alludes to a
similar occurrence in _Juniperus Virginiana_. The hops is also said to
vary in sexual characteristics from time to time.[194] In addition to
the genera, already named, in which this production of flowers of both
sexes has been observed may be mentioned _Taxus! Gunnera! Urtica!
Mercurialis! Restio! Cannabis! Salix! Humulus!_ as well as others in
which the change is less frequent.

Among cryptogams a similar change occurs. As an illustration may be
cited _Leucobryum giganteum_, as quoted from Mueller in Henfrey's
'Botanical Gazette,' i, p. 100.

As to androgynous willows, in addition to the references given
under the head of Substitution of stamens for pistils, see
Schlechtendal, 'Flora Berol.,' ii. p. 259. Tausch, 'Bot.
Zeit.,' 1833, i. p. 229. Koch, 'Synops. Flor. Germ.,' 740.
Host, 'Flor. Aust,.' ii, p. 641 (_S. mirabilis_). See also
Hegelmaier, 'Wuerttemberg Naturwissenshaft Jahreshefte,' 1866,
p. 30. Other references to less accessible works are given in
'Linnaea,' xiv, p. 372.

=Change from hermaphroditism to unisexuality.=--Many flowers ordinarily
hermaphrodite as to structure, become unisexual by the abortion or
suppression of their stamens, or of their carpels, as the case may be.
This phenomenon is lessened in interest since the demonstration of the
fact by Darwin and others, that many plants, structurally hermaphrodite,
require for the full and perfect performance of their functions the
cooperation of the stamens and pistils, belonging to different
individuals of the same species.

Some of the _Ranunculaceae_ constantly exhibit a tendency towards the
dioecious condition, and the rarity with which perfect seeds of
_Ranunculus Ficaria_ are formed is to be attributed, in great measure,
to the deficiency of pollen in the anthers of these flowers. _Ranunculus
auricomus_ also is frequently sterile. Specimens of _Ranunculus
bulbosus_ may be met with in which every flower is furnished with
carpels, most of which have evidently been fertilised, although there
are no perfect stamens in the flowers.

Knight and other vegetable physiologists have been of opinion that a
high temperature favours the production of stamens, while a lower degree
of heat is considered more favorable to the production of pistils, and
in this way the occurrence of