PTERIDOPIIYTES 



331 



In the ordinary Ferns all the spores in the sporangia 

 are alike, and when they germinate each spore produces a 

 prothallium upon which both antheridia and archegonia 

 appear. 



In some Pteridophytes, however, there is a decided dif- 

 ference in the size of the spores, some being quite small and 





Fig. 301. A series showing the dehiscence of a fern sporangium, the rupture of the 

 wall, the straightening and bending back of the annulus, and the recoil.— After 

 Atkinson. 



others relatively large, the small ones producing male game- 

 tophytes (prothallia with antheridia), and the large ones 

 female gametophytes (prothallia with archegonia). When 

 asexual spores differ thus permanently in size, and give rise 



