PTERIDOPHYTES 



173 



slender stalks, and are distinctly lcptosporangiate 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 (mc) are appearing 

 in abundance. — After Miss Pfeiffer. 



