IN PLANTS. Among plants an embryo is fre quently formed by a budding outgrowth front tissues outside of the egg. and to such a phenom enon the term parthenogenesis has often been wrongly applied. The general term covering all cases of the appearance of enihryos without fer when that process is ordinarily re quired, is apogamy (q.v.). parthenogenesis being that form of apogamy in which the embryo comes from an unfertilized egg. in tracing the origin of sex among plants, it seems evident that the sexual cells (gametes) are derived from such cells as the sexless swimming spores (zoospores) of the algae. These spores have the power of producing new plants, so that when sexual cells, without pairing in the process of fertilization, in dependently produce new plants, it is simply the resumption of an ancestral power. The most primitive sexual cells show no distinction of sex, and in such cases parthenogenesis is very com mon. In the higher development of sexuality, however, the pairing gametes are very unlike, becoming the sperm and egg. and in such cases parthenogenesis is more rare, and occurs only in connection with the egg. • By the process of fertilization the egg is in duced to divide, and this is the beginning of an embryo. Certain eggs may be artificially in duced to divide by other means than fertilization, and to produce enthryos. 'lust what the stimulus is which induces the egg to divide is yet unde termined. hut it seems 'probable that natural .parthenogenesis results from the same general conditions that obtain in artificial partheno genesis.
As might be expected, parthenogenesis in plants is most common among the alga' and fun gi, there being well-known cases, as the water molds (Saprolegnia), in which the eggs are never fertilized, and parthenogenesis is the nor mal method of embryo formation. Among the
higher groups of plants, however, it is the exception, and among seed-plants (spermato phytes) probably the very rare exception. But three cases of real parthenogenesis among seed plants have been determined, though doubtless there are others. These are in certain species of Antenuaria (Compositm), of Alchemilla (Rosa cem), and of Thalietrum (Ranuncillacem).
Binuouttmeity. Saks, "Zur Entwicklungsge schichte der Mollusken mind Zoophytes," in Wiegmann's Archiv fur Naturgesehichte, i. (Ber lin, 1837) ; Steenstrup, "On the Alternation of Generations," in Transactions of the Royal So ciety (London, 1845) ; Owen. On Parthenogene sis (ib., 1849) ; id., Lecture's on the Comparative Anatomy and Physiology of Inrertchrate Ani mals (2d ed., 1855) : Ifuxley, "On the Agamic Ilem•oduction and Morphology of Aphis." in Transactions of the Linnean Society of London, vol. xxii. (London, 185S) ; Siebold, "On a True Parthenogenesis in Moths and Bees," in Trans artions of the Royal Society (ib., 1857) ; hi., Bcitriigc zur Parthenogenesis der Arthropoden ( Leipzig, 1871) ; Leuckart, "Sur l'arrhimotokie et la parthenogkese des abeilles et des antres hytmlnopteres qui vivent en socik(.." in Bulletin dr PAcademir des Sciences de Bruxelles, series ii., vol. iii. (Brussels, 1857) ; id., Zur Kedntniss des Generationsweehsels mid der Parthenogenesis bci ticn Inscktra (Frankfort, 1E4'3); Von Baer, Ucber Prof. A ir. 11 ayncrs Entd, cknny roa yen, die sick If (rin brtnins rcr waddle told cryod:4 ode Bcobachlundt um! die Piitlogutcsis iibcrlompl (Saint Peter,burg, 1865) ; Tasehenberg, dcr Lehrc von. der Peirthenogenr sis 16:931. This last named work contains a full bibliog raphy up to and including 1592.