58i INTRODUCTION TO CRYPTOGAMIC BOTANY. 



thickened above and do not reach the margin. The pinna; are 

 articulated to the rachis, as is the stipe to the rhizoma. The 

 most observable point, however, is that the New Zealand 

 species, A. tenella, has no indusium. That organ exists in the 

 two other species, which are evidently congeneric, a striking 

 proof of the comparatively small value of the indusium. Poly- 

 podium amomalum has the sori on the upper side, but the 

 plant is so like Polystiohum vestitum that it is probably a 

 monstrous form in which the indusium is not formed. Nipho- 

 bolus has several series of sori between each pair of veins, and 

 the tips of the paraphyses are remarkably stellate. 8ynam/mia, 

 finally, has oblong sori seated on the back of the lowest free 

 vein. A large portion of the species are tropical or subtropical, 

 but Polypodium vulgare is one of the commonest ferns of 

 temperate countries, and a few other species are far from 

 uncommon. Several genera and species also occur in the 

 southern hemisphere, either peculiar to it or extending from 

 warmer latitudes. Two of these are climbers. 



605. It remains only to notice the third section, which con- 

 tains Struthiopteris, one of the most remarkable genera in the 

 tribe, and which occurs both in Europe and North America. 

 The fertile and sterile fronds are dissimilar, and the membra- 

 naceous reflected margin of the former simidates an indusium. 

 The sori, it is to be observed, are really seated on the back 

 of the pinnate veins, and the margin of the frond, though 

 scarious, is turned over several sori. S. gevmanica is quite 

 hardy, and is often seen in gardens. 



9. ViTTAEiACB^, Presl. 



Sori naked, immersed in the very margin of the frond, which 

 simulates an indusium. Sporangia pedunculate. 



606. Care, of course, must be taken in estimating the species 

 which belong to this tribe, so as not to confound the edge of 

 the frond with an indusium. The sori are deeply sunk, and 

 according to the mode in which they are seated the species 

 simulate other genera. Vittaria has the sori immersed in a 

 distinct groove, at the very edge of the frond, without a trace 

 of an indusium ;* Prosaptia, in little cupshaped cavities lead- 



* Smith separates Tmniopsis from Vittaria, because the sori are 



