Bower . — Studies in the Phytogeny of the Filicales . 305 
the name D. (?) nodosa , Hook. In the ‘ Synopsis Filicum \ however, the query 
is dropped, and it appears (p. 93) as Davallia ( Leucostegia ) nodosa , Hk. Diels 
(Engler u. Prantl, i. 4, p. 164) upholds its generic distinctness, and names 
it Acrophorns nodosus , (Bl.) Presl, giving it a place in close relation to 
Cystopteris. Christ (Farnkrauter, p. 285) also upholds the genus, but 
places it in near relation to Struthiopteris . 
It is clear from these diverse opinions that this isolated species shows 
characters apparently in common with two widely different affinities, both 
characterized by superficial sori, protected by an indusial structure attached 
on the side of it nearest the midrib. The Davallia-ty^e is, however, 
a derivative of the Dicksonioid stock, with marginal sori, while the affinities 
of Cystopteris and Struthiopteris , as long recognized and as specially 
indicated on the widened basis of comparison in the above pages, are with 
the Cyatheoid stock with superficial sori. The former approaches the latter 
in appearance by a gradual phyletic shifting of the sorus from the margin 
to the surface of the leaf, a process clearly indicated by the comparative 
study of Davallia. Consequently the position of the sorus, supported 
merely by the comparison of habit, will not serve to determine the true 
affinity in any critical case. A careful examination of the anatomy, the 
superficial appendages, and especially of the structure and development of 
the sorus and sporangia will be required for a decision in any given example. 
This seems specially necessary in the case of Aerophones , and in the 
absence of material for detailed study I am not prepared to express an 
opinion whether its relation is truly with Cystopteris or with Davallia. 
There is, however, one feature which seems to make the former affinity 
doubtful. In all the Gleicheniaceae and Cyatheaceae, as well as in the 
other allied genera with basal indusium, the sorus is seated on a vascular 
strand, which is continued beyond its insertion. In Acrophorns , as its very 
name implies, the vascular strand stops at the sorus, which is described as 
being on its tip. This is the usual, though not invariable character of Ferns 
with marginal sori (Hymenophyllaceae, Dicksonieae, &c.), and is mostly 
retained, at least in cases with relatively simple venation, even where the 
sorus has become superficial. This fact appears to strengthen the Davallioid 
alliance of Acrophorns , but no definite expression of opinion on the point 
is here given. 
This is an example of many like questions which must necessarily 
arise in the phyletic treatment of the more advanced Ferns ; for they, 
perhaps as frequently as any well-represented group of plants, show con- 
verging lines of specialization. So that the most advanced representatives 
of stocks quite distinct in origin assimilate closely to one another, and can 
only be disentangled by minute comparative study, bringing all possible 
criteria which may serve to guide a final decision. 
