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* SEX. 27 SEXTANT. example, Ulothrix, a green alga, consists of a single row of cells, each of which has ordinary vegetative powers. In some cells a few large ciliated swim- ming se.less spores are developed by cell division. Other cells produce numerous smaller similar bodies. Both sorts wlien discharged swim about and either directly form filaments, or they may fuse in pairs, thus producing a new cell, capable of developing a new vigorous individual. Since this fusing is the essence of the sexual process, botanists conclude that sexual cells have been derived from sexless swimming spores. The sexual cells (gametes) are at first alike, a condition distinguished by special terminology from that in which two sexes are di.stinet. Thus, the mother cell within which the gametes are developed is called a gamctangium ; the condi- tion of having similar pairing gametes is isog- amy; the act of fusion is conjugation, and- the resulting sexually formed spore is a zygospore or z_vgote. Only the lower algie and fungi are isog- amous. Very early in the history of the evo- lution of sex in plants the pairing gametes began to dilTerentiate. In one series the gametes be- came gradually larger and proportionately less active, until a relatively large and absolutely pas- sive cell, the female gamete, egg, or oosphere, was formed. In tlie other series activity was increased, and size perhaps diminished, resulting in the for- mation of the male gamete, sperm, antherozoid, or spermatozoid. This differentiation of sex con- tinues from the higher algte throughout the plant kingdom, with the following special terminology. The gametangium which develops the sperms is called an antberidium (q.v), and that which de- velops the usually single egg an oogonium among the algie and fungi and an archegonium in the higher groups. The condition of having dis- similar gametes is heterogamy; the process of fusion is fertilization: and the resulting sexual- ly formed spore is an oiispore or fertilized egg. Although isogamy and heterogamy may be re- garded as the normal stages in the evolution of sex among plants, there is a special form of sexuality among the red alg.Te (Rhodophycese, q.v.) that deserves mention. In this group, al- though a male cell or sperm is developed, as in cases of ordinary heterogamy, the female organ (procarp) develops no distinct egg, but is dif- ferentiated into two regions, namely a bulbous base (earpogonium) with a hair-like prolonga- tion (triehogyne) with which the male cell fuses, and thus fertilizes the earpogonium. by which, more or less directly, spores are developed. In this case, therefore, there is a sexual act involving a sperm or its equivalent, but no egg. This sexual union does not result in a distinct spore, but in the final formation of a fruit-like struc- ture (cystocarp) containing spores. This pecu- liar modification of heterogamy may be called carpogamy, which is fertilization of a earpogo- nium rather than of an egg. 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 be 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 be monn?cinus (bi- sexual or hermaphrodite). In some cases, how- VOL. XVIII,— 3. ever, antheridia and archegonia are borne upon different individuals (dioecious or unisexual). Among the pteridophytes, which is the lowest group to exhibit heterospory (q.v.), the sexual plant (prothallium), which may be either monte- eious or dioecious, is very inconspicuous, but the leafy sexless plant is conspicuous. By overlooking the homologies with pterido- phj-tes, great confusion has arisen among the spermatophytes in reference to sexuality and a sex terminolog}' has been applied to certain sex- less organs. In this highest group the sexual plants are so inconspicuous that they can be 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 montecious and dioecious are misapplied when used to describe plants which bear stamens and pistils respectively upon the same or distinct individuals. 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 (Xew York, 1902), where will be found further references. See Metazoa: Reproduction ; Sexual Selection. SEX, A.s A Factor in Evolution. As has been elsewhere stated (see Sex), the male is the more active, more variable and specialized sex, while the female is passive, conservative, and de- parts least from the normal standard. It would be a natural result that the offspring would tend to vary. Weismann goes so far as to claim that the intermingling of the sexual elements in fer- tilization is the onlj' cause of variation. Before him Treviranus. Brooks, and Galton claimed that sexual reproduction provokes variation. On the other hand, the se.xless Foraminifera are ex- posed to gi-eat variation, and we know that variation in general is due to the changed condi- tions of life, and the reproductive activities are generally acknowledged to be of secondary im- portance. Mutual sterility, by which physiological bar- riers are erected, is supposed by Romanes to result in the origination of new species. Among the higher animals, as the .social insects, birds, and mammals^ which build nests, care for their young, and where love, cooperation, self-sacrifice come into play, sex becomes increasingly impor- tant in evolution, and becomes a factor in the differentiation of sexual forms, and in social evolution. See EvonTiox. SEXAGESIMAL SYSTEM. See Scales of NoT.iTIOX. SEXTANT (from Lat. sextans, sixth part, from .9ca"fH.s.. sixth, from sex. six). An instru- ment used for measuring angles between distant objects. The sextant finds its greatest field of usefulness in navigation, but it is also employed in marine surveying. It consists of a frame ia