DEVELOPMENT OF PLANTS 



341 



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 





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 gametophyte which has de- 

 veloped from the single megaspore. At the micropylar end of the gameto- 

 phyte are two archegonia opening into the archegonial chamber, ar. Two 

 neck cells lead to large female gamete, o; sp, remains of the sporangium 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 



