SPERM VTOPHYTES 



263 



the micropyle. In its growth the developing megaspore encroaches 

 upon and destroys the other megaspores and more or less adjacent 

 tissue of the nucellus, becoming a very large cell (tig. 587), which is 

 later the embryo sac. 



This account of the development of the megasporangium includes all 

 the events that ever occur, but in certain groups of angiosperms one or 

 more of these events are omitted. Among the Sympetalae, for example, 

 the hypodermal initial cell never divides into a primary wall cell and a 

 primary sporogenous cell, 

 but is itself the primary 

 sporogenous cell or 

 mother cell. This means 

 that in this great group 

 the wall tissue of the 

 megasporangium has 

 been eliminated. The 

 same condition is found 

 here and there in the 

 other groups. 



In some cases the nu- 

 cleus of the mother cell 

 divides, forming four 

 nuclei, but walls do not 

 separate them. Some- 

 times when this happens 

 (as in Eichhornia) three 

 of the nuclei degenerate 

 and the fourth one func- 

 tions (fig. 588). 



In other cases the 

 mother cell divides only 

 once, and one of the daughter cells functions as an ordinary megaspore 

 in producing a female gametophyte (as in Cypripcdiiim). The cell 

 thus functioning is not really a megaspore, but two megaspores, which 

 tcgether form the gametophyte. 



In Lilium and certain other forms a remarkable shortening of the 

 history occurs. The hypodermal initial cell does not produce a wall cell, 

 and therefore is the primary sporogenous cell or mother cell. This 

 mother cell does not divide and form a tetrad of megaspores of the usual 



Development of megasporangium : 

 587, tetrad of Canna (after Wiegand), in which the 

 innermost megaspore has very much enlarged; single 

 undivided wall cell; 588, tetrad of Eichhornia (after 

 Wilson Smith), in which no walls have appeared, but 

 three of the megaspore nuclei are degenerating; primary 

 parietal cell divided once. 



