SPERMATOPHYTES 
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 (fig. 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 FIGS. 587, 588. Development of megasporangium: 
of the nuclei degenerate 5 8 7> tetrad of Canna (after WIEGAND), in which the 
j xu e .LU e innermost megaspore has very much enlarged; single 
and the fourth one func- / f : 
undivided wall cell; 588, tetrad of Etchhorma (after 
tions (fig. 588). WILSON SMITH), in which no walls have appeared, but 
In other cases the thi'ee of the megaspore nuclei are degenerating; primary 
,i 11 j- -j i parietal cell divided once. 
mother cell divides only l 
once, and one of the daughter cells functions as an ordinary megaspore 
in producing a female gametophyte (as in Cypripediuni). The cell 
thus functioning is not really a megaspore, but two megaspores, which 
together 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 
523 
