STKUCTURE OF FERNS 



173 



sorus with its indusium < at b - 



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) a 



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 

 globular body and usually with * 

 stalk (Fig. 307). 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. 



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

 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- 

 | ener it is less 

 than one-fourth 

 this size. It is 

 commonly un- 



306. Common polypode fern. 307. Sori and sporan- known CXCCpt 

 Polypodium vulgare. gium of polypode. Q botanists. 



Prothalli may often be found in greenhouses where ferns 

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



