180 



PHENOGAMS AND CRYPTOGAMS 



332. Fruiting frond of Christ- 

 mas fern. Sori at a. One 

 sorus with its indusium, at b. 



sporangia are exposed. Not all ferns have indusia. The 

 polypode (Figs. 333, 334) does not: the sori are naked. In 

 the brake (Fig. 335) and maiden- 

 hair (Fig. 336) the edge of the frond 

 turns over and forms an indusium. 

 In some ferns (Fig. 337) an entire 

 frond becomes contracted to cover 

 the sporangia. In other cases the 

 indusium is a sac-like covering, 

 which splits. (Fig. 338.) 



347. The sporangium or spore- 

 case of a fern is a more or less globu- 

 lar body and usually with a stalk. 

 (Fig. 334.) It contains the spores. 

 When ripe, it bursts and the spores 

 are set free. Lay a mature fruiting 

 frond of any fern on white paper, top side up, and allow it 

 to remain in a dry, warm place. The spores will discharge 

 on the paper. 



348. In a moist, warm place the spores germinate. They 

 produce a small, flat, thin, green, more or less heart-shaped 

 membrane. (Fig. 339.) This is the prothallus. Sometimes 



the prothallus is an inch or 

 more across, but oftener it 

 is less than one-fourth this 

 size. It is com- 

 / monly un- 

 known except 

 to botanists. 

 Prothalli may 

 often be found 

 334. Sori and sporan- in greenhouses 



gium of polypode. where ferng ^ 



grown. Look on the moist stone or brick walls, or on the 

 firm soil of undisturbed pots and beds. 



333. Common polypode fern. 

 Polypodium vulgare. 



