-FERNS. 



185 



maturity, and by its elasticity ruptures the spore-case, 



'.vhich then discharges its 

 spores, as shown in Fig. 

 234. The leaf of the Fern, 

 then, is something more 

 than an ordinary foliage- 

 leaf, and is known as the 

 frond.. The' petiole is 

 called the stipe, -wYdlQ the 

 mid-rib is the rJiachis. 



325. A spore under 

 proper conditionsdevelopes 

 a slender thread-like cell 

 which eventually gives 

 rise to a thin, flat, green 

 expansion, resembling that 

 shown in Fig. 

 235. This is 

 called the pro- 

 thallium. From 

 the under sur- 

 face root-hairs 

 are produced as 

 shown in the 

 figure. On the 

 same surface, 

 among the root- 

 hairs, arise mi- 

 nute projections 



Fig. 231.— Root-stock and frond of Polypody. 



Fiof. 232.— Ciroinate veraation of the frond. 



Fig. 233.— Magnified view of tlie sori. 



Fig. 234. — Si-orangium discharging spores ; greatly magnified. 



Fig. 232. 



