260 THE CONTINUITY OF THE RACE 



Spore formation. On the backs of the fronds of many ferns, one may 

 often observe small fruit dots, or sori. These are composed of numerous 

 minute spore cases, or sporangia, in which are contained a number of 

 spore mother cells. These cells undergo reduction and division to form 



Fig. 17.6. A fertile frond of the marginal shield fern, Dryopteris marginalis, showing "fruit 

 dots," or sori, on the under surface. The sori of this species have a cover (indusium), which 

 has been broken in some of those to the left, exposing the sporangia. (Courtesy American 

 Museum of Natural History.) 



spores that, when ripe, are thrown into the air by the violent bursting of 

 the sporangium. 



The gametophyte generation. Such of the spores as chance to fall 

 in favorable situations soon germinate. The spore wall cracks open, and 

 the protoplasmic contents undergo repeated cell divisions that result 



