EMBRYOGENY IN ANGIOSPEEMS. 



295 



agency of wind or insects. The viscid fluid secreted by the stigmatic 

 cells {pi) causes a rupture of the extine, and the intine passes out in 

 the form of a tubular prolongation, -which gradually elongates (tp, tp) 

 as it proceeds down the loose conduct- 

 ing tissue (tc, te) of the style tiU it 

 reaches the ovule. The length attained 

 by the pollen-tube is sometimes very 

 great. In Cereus grandiflorus, Morren 

 estimated that the tubes, when they 

 reached the ovary, extended as far as 

 1150 times the diameter of the pollen- 

 grain ; in Orinum amabile, Hassall says 

 that they reach 1875 times the diameter 

 of the grain; in Oleome speciosa, 2719 

 times ; in Oxyanthus speciosus, 4489 , 

 times ; and in Oolchicum autumnale, 

 9000 times. The length of time which 

 the poUen-tabe takes to traverse the 

 conducting tissues of the style in Anglo- 

 sperms varies. 



On reaching the ovule the pollen- 

 tube enters the foramen, and finally 

 comes into contact with the embryo-sac pj 527 



(fig. 528 e). In the interior of this 



sac one or more nucleated germ-vesicles are produced before impregna- 

 tion in the midst of the endospermal cells and protoplasmic matter 

 (fig. 530 e). In fig. 529 an anatropal ovule is represented with the 

 raphe r, the opening in the primine and secundine ra, en, the nucleus 

 n, the embryo-sac es, and the pollen-tube pt, in contact with the 

 germ-vesicle e. , 



After the contact of the poUen-tube, one of the embryonal vesicles 

 becomes enlarged, and is then divided by septa into two, the upper 

 division growing out in a filamentous form, constituting the suspensor 

 (fig. 530 s, 531 h), while the lower portion enlarges and divides re- 

 peatedly so as to form a cellular globule — the embryo (fig. 530 s, 

 531 c). The parts of the embryo being finally differentiated into 

 cotyledonary and radicular portions, as shown in fig. 532, 1-4. 



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 

 fertilisation. In Oiyptbgamic plants this has been traced, particularly 



Fig. 527. Portion of the stigma of Antiixhinum majus at the time of fecundation, ^s, 

 j)s, Superficial cells forming the papillae, tc, te. Deep elongated cylindrical cells forming 

 the conducting tissue, g'p, Graiils of pollen attached to the surface of the stigma, the 

 extine having been ruptured, and the intine protruded in the form of tubes, tp, tp, which 

 pierce the interstices between the superficial stigmatic cells. 



