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Rh mixing with the ovula, 508; opinion of Professor Amici that they come into contact with the ovulum, 508-9; traced into the aperture of the ovulum, 510; the author not so far advanced, 509; origin of the pollen tubes, 509; their growth, 509, 510; a vital action, which may be excited by the stigma of another species, 509-510; their nutriment, whence derived, 510; effect of action of pollen on stigma, 510; subsequent changes in the ovulum, ibid.; formation of the embryo, ibid.; cellular thread passing from its apex to the apex of the original nucleus, 510, 511; changes in the embryo, 511; seed and its funiculus entirely without vessels, 511; areola, or nucleus of the cell, its position, &c. 511-514; spirally striated cells in, 514, 515, 548; raphides in, 515; mucous tubes in various species, ibid.; structure of fibrillæ, constituting the pubescence of the aerial roots in, 548

Orchideæ and Asclepiadeæ, analogy between the two orders, 530; in the presence of an additional part, consisting of a modification or production of the stigma, 530-1; in the frequency and necessity of the agency of insects in fecundation, 531; penetration of pollen tubes into the cavity of the ovarium, 531-2; function of the granules within the pollen tubes in reference to impregnation, 532

Orchideæ and Asclepiadeæ, supplementary observations on the fecundation of, 545—551

Orchis fusca, cellular thread of ovulum in, 547

Orchis Morio, mucous tubes entering the aperture of ovulum in, 540; cellular thread of ovulum in, 547

Orchis ustulata, cellular thread of ovulum in, 547

Orobancheæ, Mr. Lindley's idea of the placentation of, controverted, 562

Oudney, Dr. Walter, plants collected by in Central Africa, 257, 259, 200, 272

Oudneya, characters of the genus, and observations on its structure, 272

Ovarium, vascularity of, compared with that of the leaf, 379 note; theoretical view of the origin or formation of, 555; exceptions to the ordinary condition of, 555-6; origin of various forms of compound, 457—563; supposed exceptions in Orchideæ and Orobancheæ considered, 557—502

Ovulum, structure of the unimpregnated, in Cephalotus, indicates the position of the radicle of the future embryo, 77, 440; structure and development of in Rafflesia, and in Phænogamous plants generally, 406-7; in Hydnora and Cytinus, 407; structure of the uninipregnated, 439—452; in Kingia, 439; in Cephalotus, 77, 440; generally consists of two coats enclosing a nucleus, 440; origin and relation of the coats, and of the future embryo, 440-1; structure of ovulum, as described by Grew, Malpighi, Camerarius, Morland, Etienne Francois Geoffroy, Claude Joseph Geoffroy, Vaillant, Needham, Adanson, Spallanzani, Gærtner, Turpin, Richard, Mirbel, Auguste de Saint-Hilaire, Ludolf Christian Treviranus, Dutrochet, and Link, 442-7; in what respects the account given differs from all those observers, 447; exceptions to the general structure in Compositæ and Lemna, 448; importance of distinguishing between the coats of the ovulum and those of the ripe seed, 449; how determinable, 449—452; structure and development of unimpregnated, in Orchideæ, 504-5; in Asclepias, 549-550; the ovulum naked in Cycadeæ, Coniferæ, Ephedra, and Gnetum, 453; arguments in favour of this view. 454; objections to it, 454-5; origin of the ovulum in Cycadeæe and Coniferæ, 458-9

Ovules, their marginal insertion in the simple pistilla, and in the component parts of the compound ovarium, shown in various instances. 379 note; cases of apparent