324 



THE CYCADALES 



it has been compared to a highly modified indusium. It origi- 

 nates, however, from the base of the sporangium rather than 

 from the epidermis as in the common forms. As in Selaginella, 

 only one of the mother cells takes part in the formation of spores 

 and but one megaspore (instead of four) is formed. This mega- 

 spore does not escape from the sporangium, owing to the large 

 number of cells surrounding it and consequently it does not form 

 a thick protective coat. It has been suggested that these cells 

 enveloping the megaspore are due to the transformation of the 



5A 



m> 



FIG. 246. Section of a megasporangium of Zamia, showing the female 

 gametophyte just before fertilization: i, integument differentiating into a 

 fleshy outer and hard inner coat ; m, micropyle ; p, cavity in which three 

 microspores are shown germinating; g, female gametopfcvfa which has 

 developed from the single gainetospore. At the micropylar end of the 

 gametophyte are two archegonia opening into the archegonial chamber, ar. 

 Two neck cells lead to large female gamete, o; sp, remains of the spo- 

 rangium not yet consumed by the growth of the gametophyte. At the 

 right a few of the jacket cells which supply the female gamete with food 

 are enlarged. After Webber. 



spore mother cells, which make up the bulk of the fern sporan- 

 gium, into nourishing cells. This structure of the megasporan- 

 gium is characteristic of all the spermatophyta and the arrange- 

 ment is of decided advantage since it provides the megaspore 



