FERNS 



365 



terior) end. Among these are the sperm cases (antheridia) . 

 The sperm case is in the form of a rounded protuberance from 



Fig. 328. 



Prothallium of fern, underside, showing rhizoids, antheridia scattered among and near 

 them, and the archegonia near the sinus. 



the underside of the prothallium. The wall consists of a single 

 layer of thin cells. The central portion develops a number 



of 



sperms 



The 



sperms are coiled in 

 the form of a screw, 

 with numerous deli- 

 cate cilia over the 

 smaller end, by the 

 vibration of which 

 the sperms swim 

 rapidly for a half 

 hour or so in the wa- 

 ter like a moving screw after they escape from the sperm case 

 (paragraph 520). The egg cases (archegonia) are borne also 

 * The motile sperms of plants are sometimes called " anther ozoids." 



Fig. 329. 



Section of antheridia, showing sperm cells, and sperms 

 in the one at the left. 



