288 Seaton : Embryo-sac of Nymphaea advena 



except that it lacks the surrounding regular-shaped mass of cyto- 

 plasm. The endosperm-nucleus in the constriction of the sac some- 

 times divides at this time but usually later. The lower endosperm- 

 nucleus in the chalazal end of the long tube-like portion of the 

 sac is very large and conspicuous with a reticulated surface and a 

 large dense nucleolus (figures 14, ip). 



The young embryo is nearly spherical, lying against the wall 

 at the micropylar end of the sac, and is nearly surrounded by en- 

 dosperm. This endosperm tissue is surrounded by perisperm 

 containing a rich food supply. The endosperm tissue never extends 

 into the tube-like base of the embryo-sac but the nucellus closing in 

 from the sides obliterates it, often leaving only the cavity at the 



extreme chalazal end containing its persisting nucleus (figures 

 20, 21). 



Summary 



1. The hypodermal cell can be distinguished before the integu- 

 ments begin to develop. 



2. The integuments begin to develop at the time of the division 

 of the hypodermal cell. 



3. By the division of the parietal cell the megaspore mother- 

 cell is buried four cells deep. 



4. The order in which the four cells of the axial row arise 

 varies, but the lowest one always functions. 



5. By simultaneous division of the epidermal and the parietal 

 tissue the embryo-sac is buried six to ten cells below the micro- 

 pylar end of the ovule. 



6. The functionless megaspores degenerate so that the em- 

 bryo-sac mother-cell lengthens toward the micropyle. 



7. The embryo-sac expands until within eight, six, or even 

 four cells of the micropylar end of the ovule, but never to the 

 superficial row of cells as Cook found to be the case in Castalia 

 odorata. ~ 



8. The nucellar tissue between the upper end of the sac and 

 the micropyle assumes a characteristic appearance ; cells crowded, 

 small, dense, in regular rows and stored with food. 



9. The eight-nucleate embryo-sac develops a tube-like pro- 

 longation toward the chalazal end of the ovule. This tube is always 

 narrower where it joins the broadly elliptical upper part of the 

 sac than throughout the rest of its extent. 



