318 



FERTILIZATION. 



embryo has attained its full development. A monocotyledonous 

 embryo has this end undivided. In the polycotyledonous embryo 



of Pines, &c., the budding apex divides successively as it grows 

 into four, six, or more lobes, each of which becomes a cotyledon.* 



* The principal points of discordance in the later investigations are con- 

 nected with the embryonal vesicle. According to Mohl, Henfrey, &c., in 

 Orchis Morio, as many as three germinal vesicles exist within the apex of the 

 embryo-sac, anterior to fertilization, as minute free cells, formed from so many 

 nuclei ; one (or sometimes more than one) of them enlarges soon after the pol- 

 len-tube has reached the embryo-sac, and developes in the manner above de- 

 scribed. According to Hoffmeister, also, in CEnothera, two or three germinal 

 vesicles appear a long time before fertilization, from free cell-nuclei, forming so 

 many delicate, free cells, one of which being fertilized developes into the 

 embryo in the manner already described, while the others perish. The results 

 of the more recent researches of Tulasne (upon the embryogeny of Scrophu- 

 lariaceae, Campanulacese, and Cruciferae) principally differ in this; that he 

 was unable to detect any embryonal vesicle before the pollen-tube had pene- 

 trated to the embryo-sac ; and afterwards he finds only one, mostly of an elon- 

 gated form, and always from the first attached by one end to the inside of the 

 wall of the embryo-sac, at a point near that to which the pollen-tube is applied 

 externally. He is led to conclude that the embryonal vesicle originates at 



FIG. 427. Plan of a vertical section of the pistil of a Polygonum, and of the erect orthotro- 

 pous ovule it contains, at the period of fertilization : the grains of pollen resting on the stigma 

 have sent their tubes down the style to the mouth of the ovule : and the nascent embryo-sac is 

 seen at the apex of the nucleus. 428. A pollen-grain detached, with its tube. 429. Plan of the 

 vertical section of the ovule more magnified, and-at a later period: the nascent embryo with its 

 suspensor seen in the embryo-sac. 430. The nascent embryo with its suspensor, more magni- 

 fied. 431 -433. Views of the successive development of the embryo. 434. The embryo as it 

 exists in the seed. 



