1 88 MORPHOLOGY AND LIFE HISTORY 



of the nucellus (apex of the ovule) is seen the 

 large 



(ti) megaspore-mother-cell (embryo-sac-mother- 

 cell), with very prominent nucleus and 

 nucleolus. 



(i) Make a diagram of all that you have observed 

 under 5, (a)-(ti), filling in the details for one 

 ovule. 



6. Development of the Megaspores. 



(a) Like the microspore-mother-cell, the megaspore- 

 mother-cell of Trillium divides twice, giving 

 rise to four megaspores (tetrads), but only 

 one of these megaspores develops a gametophyte. 

 This spore enlarges, and by three successive 

 divisions gives rise to an eight-nucleate female 

 gametophyte, the embryo-sac. Three of these 

 nuclei organize antipodal cells at the basal 

 end of the embryo-sac, and three of them 

 organize cells at the micropylar end. One of 

 the latter is the egg, the other two thesynergids. 

 The two remaining nuclei fuse near the center 

 of the embryo-sac, forming the endosperm- 

 nucleus (sometimes called definitive nucleus) , 

 but the endosperm does not develop until 

 after the fertilization of the egg. 



(b) Unlike the microspores, the megaspores, in 

 angiosperms, never become free, independent 

 cells, but always retain an intimate physiolog- 

 ical connection with the sporophyte of the 

 next preceding generation (cf. 4 GO, above). 



7. The Embryo-sac. 



(a) Using prepared slides, study the embryo-sac 

 in its two- to eight-celled stages, identifying 

 the cells mentioned in 6 above. Draw. 



