430 A TEXTBOOK OF GENERAL BOTANY 
while the tip of the sporophyll is modified as a stigma for the 
reception of the microspores. Frequently two or more mega- 
sporophylls are united to form a compound ovary which may 
have either one or more cavities (Fig. 205). In this case the tips 
may form separate stigmas or only a single stigma. The megaspo- 
rangia (ovules) are inclosed in the ovary by the megasporophylls. 
These microsporo- 
phylls of the angio- 
sperms are also known 
as stamens (Fig. 206), 
while the microsporan- 
gia are the pollen sacs. 
These megasporo- 
phylls and microsporo- 
phylls are borne in 
flowers, and in com- 
Fig.486. Formation of male gametophytes in plete flowers are sur- 
Elodea rounded by two sets 
Left, a tetrad of microspores of an angiosperm of modified leaves, the 
(Llodea) (x 550); upper right, microspore nucleus petals and the sepals. 
has divided, resulting in the formation of a large The torus is ante 
tube cell and a small generative cell, which in the 
figure is situated in the lower left portion of thetube Stem, and as it bears 
cell; lower right, the generative cell has divided to sporophylls the flower 
form two male cells (x 205) 

is a modified strobilus. 
Megasporangium and female gametophyte. In the angio- 
sperms the megasporangium consists of an oval structure, the 
nucellus, oY Sporangium proper, surrounded by one or two in- 
teguments. At the apex there is a small opening, the micropyle, 
which extends through the integuments (Fig. 227). 
In the nucellus there is usually found a single megaspore 
mother cell (Fig. 484), which in the majority of cases divides 
to form a row of four megaspores (Fig. 484). The three mega-— 
spores situated toward the apex of the nucellus degenerate 
(Fig. 485). The basal megaspore is also known as the embryo 
sac, and within it the female gametophyte, or prothallus, is 
formed. First the embryo sac increases considerably in size and 
