332 



BOTANY: PRINCIPLES AND PROBLEMS 



rupturing the sporangium wall and forcibly ejecting the spores. 

 The shape and position of the sorus and indusium, as well as the 

 type of annulus, vary greatly among the different groups of ferns 

 and serve as useful characters by which to distinguish genera and 

 families. 



The spores germinate into a thin, small, thallus-like gameto- 

 phyte or prothaUus (Fig. 202) which possesses chlorophyll and is 



Fig. 205. — Graphic representation of the life-history of a fern. 1, the fern 

 plant or sporophyte, bearing sori, or clusters of sporangia, on its leaves. 2, a 

 sporangium. 3, a tetrad of young spores. 4, the four mature spores which have 

 come from the tetrad shown in 3. 5, a spore germinating into a young gameto- 

 phyte. 6, mature gametophyte, bearing sexual organs. 7, archegonium. 7a, 

 antheridium. 8, egg cell or female gamete. 8a, sperm, or male gamete. 9, 

 fertilized egg. 10, young sporophyte growing out of a fertilized egg, the whole 

 still attached to the remains of the gametophyte. 



somewhat heart-shaped in outline, though its form varies con- 

 siderably. It is rarely more than a few millimeters in diameter 

 and lies flat upon the surface of the soil, to which it is attached 

 by delicate rhizoids growing from the under surface. Plentiful 

 moisture and a partially shaded situation are necessary for the 

 successful growth of a fern prothallus. The sexual organs (Fig. 

 203) are produced on the under surface, the archegonia near the 

 "notch" and the antheridia farther back, among the rhizoids. 

 The archegonia are much smaller than those of bryophytes and 

 only their necks project above the surrounding tissue. They 



