CHARACTERS OF TRIBES AND GENERA. 273 



Tribe 19. ADIAKTBJB. (Plate 19.) 



Vernation fasciculate or sarmentose, generally hypogeous, 

 and becoming caespitose. Sori marginal, transverse, ob- 

 long, linear, or punctiform. Indusium lateral, exterior 

 attached, bearing the sporangia on its under surface. 



OBS. This is truly a natural tribe of Ferns, having the 

 smooth, polished, and flaccid character of Lindscea, but 

 differing in the indusium being attached exteriorly, and its 

 under surface venose, and bearing the sporangia, which 

 latter character distinguishes it from the tribe Pteridece ; 

 it is formed of a portion of the laminea of the margin of 

 the frond, which is changed in texture, its form depending 

 on the ultimate segments being entire, or more or less 

 divided, thus varying from reniform, linear, to oblong, 

 continuous, occupying the whole margin of the segments. 

 Those with reniform indusia are distinguished from 

 Cheilanthes not only by the indusium being sporangiferous 

 on its disk, but also by its being produced on the con- 

 vergent apices of two or more venules, whereas the sori of 

 Cheilanthes are seated on the axis of a single venule, ter- 

 minating in the axis of the indusium. 



In the " Species Filicum " 109 species are described, but 

 in the " Synopsis Filicum " the number is reduced to 62 ; 

 the first number was arrived at by Sir W. Hooker, after 

 more than half a century's study of Ferns, and the latter 

 number by the editor of the " Synopsis " after three years' 

 study of Sir W. Hooker's herbarium. This is a striking 

 example of the different views taken by Pteridologists as 

 to what constitutes species.* 



* In the Appendix to the " Synopsis Filictim " nineteen additional 

 species are described. 



