STRUCTUKE OF FERNS 



173 



indusium. This indusium separates from the frond at 

 its edges, and the sporangia are exposed. Not all ferns 

 have indusia. The polypode (Figs. 

 306, 307) does not: the sori are 

 naked. In the brake (Fig. 308) and 

 maiden -hair (Fig. 309) the edge of 

 the frond turns over and forms an 

 indusium. In some ferns (Fig. 310) 

 an entire frond becomes contracted 

 to cover the sporangia. In other 

 cases the indusium is a sac -like cov- 

 ering, which splits (Fig. 311). 



319. The sporangium or spore - 

 case of a fern is a more or less 



IT, IT J 1, .,1 305. Fruiting frond of Christ- 



globular body and usually with a „,as fern. Sori at a. One 



stalk (Fig. 307). If contains fJte somswitii its indusium, at t. 

 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. 



320. In a moist, warm place the spores germmate. 

 They produce a small, flat, thin, green, more or less 



heart-shaped membrane (Fig. 



312) . This is the prothallus. 



Sometimes the prothallus is 



an inch or more 



/ across, but oft- 



1 ener it is less 



than one-fourth 



this size. It is 



commonly un- 



307. Sori and sporan- knOWU CXCCpt 

 gium of polypode. ^q botauists. 



Prothalli may often be found in greenhouses where ferns 

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



306. Common polypode fern.- 

 Polypodium vulgare. 



