PTERIDOPHYTES 
173 
slender stalks, and are distinctly leptosporangiate of the more advanced 
type. 
In both kinds of sporangia sixteen mother cells are developed (eight in 
the megasporangium of Azolla), as among the most advanced of the true 
ferns. In the micro- 
sporangium sixty-four 
spores are formed ; but 
in the megasporangium 
only one megaspore 
matures, a single mother 
cell functioning and 
three spores of the 
tetrad not maturing. 
Each sorus begins by 
developing a terminal 
megasporangium (fig. 
399), and beneath this 
microsporangia begin to 
appear. The megaspo- 
rangium of Azolla de- 
velops mother cells 
and forms eight tetrads 
(32 spores). If thirty- 
one of these megaspores 
degenerate and one 
persists, the microspo- 
rangia develop no fur- 
ther, and the structure 
becomes a megasporo- 
carp containing one 
megaspore (fig. 400). 
If all thirty-two mega- 
spores degenerate, the microsporangia continue to develop (fig. 401), 
and the structure becomes a microsporocarp (fig. 402). 
The tapetal cells break down and discharge their cytoplasm into the 
sporangial cavity, forming a remarkable matrix about the spores. In 
Salvinia the microspores lie firmly embedded in hardened cytoplasm; 
while in Azolla the cytoplasm organizes into two to eight masses (mas- 
sulae), embedding the microspores. These massulae are invested by a 
mc'\- 
FIG. 401. Azolla: young microsporocarp, in which 
all the megaspores have degenerated in the single mega- 
sporangium and the microsporangia (me) are appearing 
in abundance. After Miss PFEIFFER. 
