GROUP IV. — PHANEROGAMIA. 



451 



the four so formed divides again, so that eight nuclei are formed, 

 four at the mioropylar, and four at the chalazal pole of the 

 macrospore ; one nucleus is then conveyed from each pole to- 

 ward the centre of the macrospore, where the two nuclei meet 

 and fuse into one which is termed the definitive nucleus of the 

 macrospore or embryo-sac. Three nuclei now lie at each pole, 

 and around these aggregation of protoplasm takes place, so that 

 cells are formed : those at the chalazal pole soon acquire a cell- 

 wall, and &,ve termed antipodal 

 cells : those at the micropylar 

 end do not form any cell- wall ; 

 one of them is the female re- 

 productive cell or oosphere, 

 the other two are sterile 

 (though in rare cases they 

 are fertile), and are termed 

 the syjiergidoi, the three to- 

 gether constituting the egg- 

 appartitns. This is the extent 

 to which the development of 

 the female prothallium takes 

 place previously to fertilisa- 

 tion. In most Angiosperms 

 the structure of the prothal- 

 lium is completed by the for- 

 mation, after fertilisation has 

 taken place, of additional cel- 

 lular tissue : this process is 

 initiated by the division of 

 the definitive nucleus of the 

 macrospore, nuclear division is 

 repeated, cell-formation takes 

 place, in the manner described 

 above for the Gymnosperms, 

 and the macrospore becomes more or less completely filled with 

 cellular tissue, commonly termed endosperm. 



In a number of dicotyledonous plants {e.g. Loranthacero. Orobanchacene, 

 Labiatffi, Campanulaceje) where the embryo-sac is long and narrow, the endo- 

 sperm is developed by cell-division : the embryo-sac is divided by two or more 

 transverse septa and longitudinal divisions follow. And even when the de- 

 velopment of the endosperm begins with free cell-formation, its further develop- 

 ment is effected by the growth and dixision of the first-formed cells. 



Fig. 292.— The female prothallium of An- 

 0opperm8, shown in a longitudinal section of 

 the ovule (x 70): ai outer, ii inner, integu- 

 ment ; m micropyle ; / funicle. K Macros- 

 porangium (nucellus). E Macrospore (em- 

 bryo-sac), fc Definitive nucleus of the em- 

 bryo-sac. The female prcthallium consists 

 of the egg-apparatus at the micropylar end 

 of the macrospore, and of the group of anti- 

 podal cells at at the chalazal end. The egg- 

 apparatus consists of two synergidse s, and 

 an oosphere e. 



