82 FERNS 



shaped depression. An archegonium is flask-shaped. The 

 neck is composed of a single layer of wall cells inclosing one 

 layer of neck cells which later form mucilage. The enlarged 

 basal portion, or venter is sunken in the tissues and contains 

 the egg. 



Fertilization. In the presence of moisture the multiciliate 

 sperms enter the neck of the archegonium, swim to the egg 

 cell and unite with it to form the beginning of the sporophyte. 



Development of the Sporophyte. The fertilized egg under- 

 goes a series of divisions to form the four-celled embryo. 

 One of these cells by repeated division forms a stem; one, the 

 embryonic leaf or cotyledon; one develops a root, while the 

 fourth develops a foot. The foot grows down into the tissues 

 of the prothallus and absorbs food to nourish the growing 

 sporophyte. The root of the young sporophyte develops 

 first, then the leaves and finally the stem. The young sporo- 

 phyte remains a parasite upon the prothallus until about 

 four leaves are formed, then the prothallus or gametophyte 

 generation dies and disappears. The sporophyte because of 

 the presence of roots, a well developed vascular system and 

 chlorophyll continues to live from year to year or is perennial. 



Alternation of Generations is as sharply defined in the ferns 

 and other pteridophytes as in the liverworts and mosses, but 

 the gametophyte or sexual generation in inconspicuous and 

 temporary, while the sporophyte or asexual generation is 

 conspicuous and permanent. 



SUMMARY. 



Alternation of Generations occurs. 



Gametophyte. The gametophytes are of temporary dura- 

 tion. The prothallus produces antheridia and archegonia. 

 The antheridium is sessile and mostly spherical; the sperms 



