PTEKIDOPHYTES 
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 
