Baillon ('Etudes du groupe des Euphorbiacées,' p. 205) mentions the following species of that order as having been seen by him with monœcious inflorescence: Schismatopera distichophylla, Mozinna peltata, Hermesia castaneifolia. Oliver mentions ('Hook. Icon. Plant.,' t. 1044) that in Leitneria floridana the upper scales of the male catkin occasionally subtend an ovary.
It would seem that external conditions have some effect in determining the formation of one sex, as in some species of Carex, while in the case of Salix repens, Hampe[192] says that when grown partially or for a time under water, those twigs which are thrust up above the surface bear female flowers, while those twigs that blossom after the water is dried up, produce male flowers only.
Carrière[193] says that a plant of Stauntonia latifolia which for some years produced stamens only, now produces flowers of both sexes; it was diœcious, but is now monœcious. 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 Müller 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, 'Württemberg Naturwissenshaft Jahreshefte,' 1866, p. 30. Other references to less accessible works are given in 'Linnæa,' 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 Ranunculaceæ constantly exhibit a tendency towards the diœcious 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 "blind" strawberries has been accounted for. Mr. R. Thompson, writing on this subject, speaks of a plantation of Hautbois strawberries which in one season were wholly sterile, and accounts for the circumstance as follows: the plants were taken from the bearing beds the year previous, and were planted in a rich well-manured border, in which they started rapidly into too great luxuriance, the growth being to leaves rather than to fruit. The following season these same plants bore a most abundant crop, hence these plants were accidentally prevented from perfecting their female organs.[195]
Mr. Darwin[196] cites from various sources the following details relating to strawberries which it may be useful to insert in this place, as throwing some light upon the production of unisexual flowers. "Several English varieties, which in this country are free from any such tendency, when cultivated in rich soils under the climate of North America commonly produce plants with separate sexes. Thus, a whole acre of Keen's seedlings in the United States has been observed to be almost sterile in the absence of male flowers; but the more general rule is, that the male plants over-run the females.... The most successful cultivators in Ohio plant, for every seven rows of pistillate flowers, one row of hermaphrodites, which afford pollen for both kinds; but the hermaphrodites, owing to their expenditure in the production of pollen, bear less fruit than the female plants."
Stratiotes aloides has been said to produce its carpels with greater abundance towards the northern limits of its geographical distribution, and its stamens, on the other hand, are stated to be more frequently developed in more southern districts.