INTRODUCTION TO CRTPTOGAMIC BOTANY. 541 



widely distributed. With the exception of Geterach, the other 

 genera are tropical. The old Asplenium Nidus, familiar as 

 an object of cultivation and one of the noblest of ferns, belongs 

 to the genus Neottopteris. 



616. Blechnum is the typical genus of the fourth section- 

 Athyrium, which comprises our Filix foemina, though in 

 some measure approaching the next tribe, is distinguished by 

 the decidedly linear junction of the indusium with the frond. 

 In A. latifoUum, however, it is often curved so as to make the 

 indusium reniform. The straight sori are, indeed, the rarer of 

 the two. The indusium is at length reflected by the growth 

 of the sporangia. Doodia and Woodwardia are two closely 

 allied genera. In both the sori are short and parallel to the 

 costa on transverse veins. In Doodia, however, the indusium 

 is flat, and in Woodwardia immersed and convex. In the 

 former the veins and veinlets are raised beneath and more 

 distinct from the costa. Blechnum has the sori more or less 

 confluent, with the free side of the indusium towards the costa. 

 The Brazilian Salpichlcena is distinguished by its climbing 

 habit, and by the venules being combined by a transverse 

 slight infra-marginal vein. The indusium, moreover, is very 

 long and cylindrical, and bears part of the sporangia upon its 

 line of attachment. Blechnum is easily confounded with 

 Lomaria, La which the indusium is formed from the altered 

 margin of the frond, while in Blechnum it is perfectly distinct. 

 Actiniopteris (Fig. 113, a) appears to be a true ally oi Asple- 

 nium, though its marginal indusia appear at first to indicate 

 a different afiinity.* The genera are indifferently tropical or 

 temperate. Doodia extends to New Zealand, and both that 

 genus and Woodwardia occur in the United States. Athy- 

 rium and Blechnum are both tropical and extratropical, 

 varying much in size. 



617. The section of which Cystopteris is typical is some- 

 what analogous to Cyatheaceos. The indusium of Cystopteris, 

 a well-known European genus, though not confined to the 

 northern hemisphere, is suborbicular, and fixed by a lateral 



* The origin of the indusium from the vein is not properly expressed 

 in the woodcut as it was in the original drawing. 



