THEORIES OF EMBRYOLOGY. 247 



more cells or germinal vesicles are produced from cytoblasts ; that the 

 pollen-tubes pass down the style to the ovary into the foramen of the 

 ovule, and come into contact with the embryo-sac, either at its apex 

 or a little below it; then an imbibition of fluids takes place, the 

 embryo-sac begins to increase, and the embryo is produced. The 

 chief point to be determined, is the existence or not of the germinal 

 vesicle before impregnation. 



514. The formation of the process called the suspensor is variously 

 accounted for. Schleiden considers it as a part of the pollen-tube ; 

 Amici thinks that it is part of the embryo-sac prolonged upwards, 

 forming a confervoid filament (fig. 435 e); Mohl, Mirbel, and Spach 

 maintain that the suspensor is produced from the germinal vesicle, and 

 therefore intimately united to the embryo, which is developed from 

 the lowest cells of that vesicle ; Dickie thinks that the suspensor may 

 be a cord-like process sent out from the cellular embryo, reaching a 

 certain degree of development, and sometimes sending off tubular 

 prolongations or filaments, as in Euphrasia and Orchidacea?. The 

 suspensor is usually directed to the apex of the nucleus or the micro- 

 pyle, and it is sometimes of great length. In Draba verna (fig. 485, 2), 

 Dickie says he observed in an embryo, ^n f an mcn ^ on g' a suspensor 

 three times that length. In Gnetum, Griffith mentions a tortuous 

 suspensor 3 J to 5 inches long, the whole seed being only one inch long. 



515. Taking a comprehensive view of the whole subject, it may be 

 said that the union of two kinds of cells appears to be necessary for 

 fertilization. In Cryptogamic plants this has been traced, particularly 

 in certain cases of conjugation ; where the two cells come into contact, 

 a tube is formed between them, and the contents of the one unites 

 with those of the other, giving rise to a germinating body. In Phane- 

 rogamic plants, also, there are two cells with different contents the 

 pollen-grain with its granular fovilla, and the ovule with its muci- 

 laginous fluid. These are brought into connection by means of the 

 pollen-tube, formed from the intine, which either enters the embryo- 

 sac, or comes into contact with it, the union taking place either 

 directly by its extremity, or indirectly by cellular prolongations from 

 the conducting tissue, or from the ovule. By this means the forma- 

 tion of the embryo is determined, which commences as a cellular body 

 or germinal vesicle, in the interior of which other cells are subse- 

 quently formed in a definite order of succession.* 



* For opinions as to Embryology, see Schleiden's paper in Nova acta Academ. C:vsar. Leppold- 

 Corol. Naturae Curios.; Brown on the Fecundation of Orchidaceie and Asclepiadaceze, in the 

 Transactions of the Linnaean Society for 18:J3 ; Brongniart on the same subject, in Annales des 

 Sciences Natnrelles, 1st series, torn, xxiv., 1831; Meyen on the Act of Impregnation, translated in 

 Taylor's Scientific Memoirs, vol. iii. ; Amici sur la Fe"eondation des Orchide'es, in Annales des 

 Sciences Naturelles, 3d series, torn. vii. ; Wilson on Tropseolum majus, and Hairs of Campanula, 

 London Journal of Botany, voL ii. ; Giraud on Embryology, in the Transactions of the Edinburgh 

 Botanical Society, and in the Annals of Natural History, vol. v. ; and on the Ovule of Tropa:o- 

 lum, in Linnseau Transactions, vol. xlx. ; Henfrey and Dickie on Embryology, in the Annals of 

 Natural History for 1848; Griffith on the Ovule of Santalum, Osyris, &c., in Linnsean Trans- 

 actions, vol. xix. ; also Notulae ad Plantas Asiaticas, Calcutta, 1847. 



