With the development of heterogamy, which is the prevailing method in the plant kingdom, the development of sex in plants is practically complete. Certain resulting conceptions, how ever, should he considered. Among the bryo phytes alternation of generations (q.v.) is estab lished. The sexual plant (gametophyte), which is the ordinary leafy plant of popular conception, usually develops both sex organs upon the same individual, and is said to he moncecions (bi sexual or hermaphrodite). In some cases, how ever, antheridia and archegonia are borne upon different individuals (dicecious or unisexual ). Among the pteridophytes, which is the lowest group to exhibit heterospory (q.v.), the sexual plant (prothallium), which may be either monm cious or diwcions, is very inconspicuous, hut the leafy sexless plant is conspicuous.
By overlooking the homologies with pterido phytes, great confusion has arisen among the spermatophytes in reference to sexuality and a sex terminology has been applied to certain sex less organs. In this highest group the sexual plants are so inconspicuous that they can he seen only with the special appliances of the laboratory.
All the visible organs of a flowering plant, in cluding the flowers, are sexless. Confusion has arisen because the stamens and pistils have been regarded, respectively, as male and female or gans, an idea extended by the terms ovary for a part of the pistil, and ovule for the contained structure which becomes a seed. The terms monwcious and dicecious are misapplied when used to describe plants which bear stamens and pistils respectively upon the same or distinct While the sexual structures of plants are very conspicuous, therefore, among the lower forms, they gradually become more and more inconspicu ous, until in the highest group they are beyond the reach of ordinary observation, and everything seen by the naked eye is sexless. There is thus a gradual increase in the prominence of the sexless phase, and a gradual reduction of the sexual phase. Consult: Geddes and Thompson, The Evolution of Sex (New York, 1902), where will be found further references. See METAZOA: REPRODUCTION; SEXUAL SELECTION.