=Change from the monoecious to the dioecious condition.=--This is of less frequent occurrence than might have been anticipated. In the 'Gardeners' Chronicle,' 1847, pp. 541 and 558, several instances are noted of walnut trees bearing female flowers to the exclusion of males. The mulberry tree has also been noticed to produce female blossoms only, while in other plants male flowers only are developed. It seems probable that the age of the plant may have something to do with this production of flowers of one sex to the exclusion of the other. =Change from the dioecious to the monoecious condition.--Androgynism.=-- This is of far more common occurrence than the preceding. [Illustration: FIG. 101.--Monoecious inflorescence of Hop.] In the hop (_Humulus Lupulus_), when monoecious, the female catkins are usually borne on the ends of the branches as shown in the cut (fig. 101), and a similar thing has been noticed in _Urtica dioica_ by Clos, 'Bull. Soc. Bot. France,' vol. 9, p. 7. Baillon ('Etudes du groupe des Euphorbiacees,' p. 205) mentions the following species of that order as having been seen by him with monoecious 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. Carriere[193] says that a plant of _Stauntonia latifolia_ which for some 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.