CAPSELLA BURSA-PASTORIS. 241 



surface of the stigma, develops a pollen-tube and pene- 

 trates the tissues of the style. The rate of descent of 

 the pollen-tube is quite various in different plants. In 

 the style and walls of the ovary there is usually a 

 region of least resistance to penetration, furnished by 

 the delicate " conducting tissue," or the style is 

 frequently tubular (as in Viola). In Capsella, very 

 soon after pollination, an abundance of pollen-tubes is 

 found in the ovarian cavity. Some of them may be 

 seen to have entered the micropyles of the ovules and 

 penetrated to the nucellus. 



The preparation of the ovule for fertilization has 

 been the development, at the apex of the nucellus, of 

 the embryo sac (macrospore), at the micropylar end of 

 which lies the oosphere (embryonal vesicle), accompa- 

 nied usually by two similar masses, the synergidae. At 

 the base of the embryo sac appear three or more free 

 cells, the antipodal cells " of Hofmeister. The six 

 cells which differentiate into the antipodal cells, 

 oosphere and synergidae, constitute a very rudimentary 

 prothallium, 17 which is far more reduced than in gym- 

 nosperms, but corresponds to the primary endosperm 

 of these plants. The endosperm (of most text-books), 

 more properly secondary endosperm, is produced by 

 cell-formation around the nuclei arising from division 

 of the definitive nucleus of the embryo sac. 18 When 



16 Strasburger, Bot. Pract., p. 522, et seq.; Prantl and Vines, Text- 

 book, p. 205. 



17 Sachs, Text-book, 2nd Eng. ed., p. 582, where a fuller account 

 of the changes preliminary to fertilization in angiosperms may be 

 found. 



18 Sachs, Text-book, 2nd Eng. ed., p. 585. 



