FERNS. 



185 



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



which 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, while the 

 mid-rib is the rhachis. 



325. A spore under 

 proper conditions developes 

 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- 

 Fig. 231. nute projections 



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



Fig. 232. Circinate vernation of the frond. 



Fig. 233. Magnified view of the sori. 



Fig. 234. Sporangium discharging spores ; greatly magnified. 



Fig. 232. 



