APOGAMY. In all plants which have reached the level of sexuality, a new individual is initiated by the union of two cells, called gametes. In exceptional cases the ability to reproduce sexually is lost, either temporarily or permanently. Plants which have lost this power, after having attained it, are said to be apogamous and the condition is apogamy, the word meaning that the union of gametes has been left out. The new individual may be de veloped from any part of the gametophyte, or gamete-bearing plant. When the new individ ual arises from an egg, without fertilization, this kind of apogamy is called Parthenogenesis (q.v.). When the new individual arises from any other part of the gametophyte except the egg, some prefer to call the phenomenon vege tative apogamy. A familiar case of vegetative apogamy, is found in some common ferns, es pecially in highly cultivated greenhouse vari eties. The beautiful fern, Nephrodium molle, which can be found in almost any greenhouse, is the most thoroughly investigated illustration. The prothallium, or gametophyte (see ALTER NATION OF GENERATIONS) becomes thickened along its median portion, making it look like a very small prostrate leaf with a midrib. This thickened region grows out beyond the border of the prothallium, curves upward, then be comes entirely erect and develops leaves, stem and roots. This plant, in every way, looks just like one arising from a fertilized egg. The essential feature of the phenomenon is the transition from the gametophyte to the sporo phyte generation without any fusion of gametes. The thickened rib-like region of the
prothallium, viewed from the under side, is shown m Fig. 1, s. A later stage, after the young apogamous sporophyte has devel oped leaves, is shown in Fig. 2, in which g is the gametophyte and s the sporophyte. Vege tative apogamy has not been found in Thallo phytes,liverworts, mosses, lycopods or equisetums; but a score of cases have been reported in the true ferns and water ferns. No case has been established in Gymnosperms. In the Angiosperms several cases are well known. In the common onion (Allium Cepa), embryos have been found developing from the antipodal cells and from synergids. (See ALTERNATION OF GENERATIONS). In the fern (Nephrodium molle) the number of chromosomes in the nuclei of the new sporophyte is the same as the number of nuclei of the gametophyte. In the flowering plants the gametophyte which gives rise to the apogamous embryo has the sporophyte, or 2x, number of chro mosomes. Some persist in using the phe nomenon of apogasny as an argument for or against certain theories of alternation of gene rations; but in our opinion, apogamy is a sec ondary, highly specialized condition which has no bearing upon the subject.
Lang, W. H., On Apog amy and the Development of Sporangia upon Fern Prothallia> (Philosophical Translations, Royal Society of London, Vol. 190, p. 189, 1898); Yamanouchi, S., in Nephro din& (Botanical Vol. XLV, p. 289, 1908); consult also any modern textbook of botany.