ATJSTEALIAN FEENS 39 



stances, we should pay attention to the venation, for not merely 

 barren fronds but sometimes those which have been in a state of 

 fructification and lost their indusium, are difficult to determine. 

 The absence of an indusium being one of the principal characters of 

 the genus Polypodium, it sometimes happens that species are re- 

 ferred to that genus simply from the want of an organ which has 

 disappeared. Sir William Hooker felt peculiar difficulty in deal- 

 ing with the species P. rugosulum, and perhaps he may have 

 included some species of Cheilanthes or Hypolepis under that name 

 from the impossibility of discovering any trace of an involucre in 

 certain stages of the plant's growth. Again, in describing the 

 Pern which he refers to Poly podium (P. Beckleri), a doubt is ex- 

 pressed as to the possibility of some mistake ill the genus, for 

 from the specimens forwarded to Europe, it was uncertain whether 

 the species had any indusium or not. (This Fern is now referred 

 by Dr. Mueller to Aspidium obliteratum). In the various forms 

 also which are grouped together under NepTirodium decompositum, 

 (which according to Sir W. Hooker, comprises N. microsorum, of 

 Norfolk Island, Lastrea acuminata, N. glabellum, (All. Cunn.), and 

 I suppose Brown's N. tenerum), a similar doubt may arise, unless 

 due attention be paid to the position of the sori on the veinlets, 

 and the existence of an indusium, which, in the specimens from 

 some localities, it is difficult to find. Dr. Hooker has remarked 

 that JV. decomposition is a very variable species, and, as far as my 

 limited observation has extended, I should fully concur in the 

 opinion, for I have frequently collected fronds on which no trace 

 of an indusium appeared, and which differed very much from the 

 type of the species in the texture of the frond and the comparative 

 size of the sori. The venation, however, is much more uniform, 

 and this circumstance probably led to the conclusion that many 

 varying forms might conveniently be referred to the same species. 

 In a new country, in which many parts remain to be explored, 

 there may exist forms which hereafter will unite some species 

 now deemed distinct ; and, on the other hand, reasons may arise 

 for separating Ferns which are now placed together. It has 

 been remarked by a late writer on British Ferns " In some few 

 cases, even among our British Ferns, it is hardly possible to 

 decide whether a plant should be regarded as a species or a 

 variety, while their classification cannot be considered as even 



