640 GENERAL COMPARISON OF THE FILICALES 
archaic types the annulus was vertical or oblique, and the dehiscence 
was mainly, though not exclusively,! in a plane including the axis of the 
sporangium; this is seen in all the surviving Simplices, excepting the 
Matonineae, and also in Zoxsoma. But in the Gradatae the annulus was 
oblique and the dehiscence lateral, while in the Mixtae the annulus is 
again vertical, but the dehiscence transverse. If we contemplate 
a derivation by descent of Ferns with a lateral dehiscence from 
those with median dehiscence, we shall have to enquire whether 
there are any evidences of shifting of the annulus itself as well as 
of the point of dehiscence. One material point is that the formation 
of the annulus in Ferns at large does not stand in any constant relation 
to the segmentation of the sporangium-mother-cell, though that segmentation 
itself shows so singular a constancy. This fact leaves the question of 
a shifting of the annulus more open than it would otherwise appear. The 
more primitive type of complete annulus is that seen in the Gleicheniaceae 
and Schizaeaceae, with oblique position and median dehiscence: Loxsoma 
maintained the oblique position and median dehiscence, but part of its 
annulus is incompletely indurated. In others, while the complete oblique 
ring was maintained, the point of dehiscence was shifted laterally, the 
result being as in the Cyatheaceae, Hymenophyllaceae, and others with 
a basipetal sorus. With the transition from the basipetal sorus to the 
mixed came also a further change of the annulus: maintaining the 
lateral dehiscence, the annulus became vertical, stopping short on either 
side of the stalk, which interrupts it. But in many cases a slight 
obliquity was retained, as seen in Dennstaedtia apitfolia (Fig. 333 C) 
and Diacalfe, the two sides being so far dissimilar that it is possible 
still to distinguish the “central” from the “peripheral” face: this is 
also the case in Davallia, Lindsaya, Nephrodium (Fig. 6), and many 
others. But there are other outstanding cases of an oblique annulus among 
Polypodiaceae which have been the subject of discussion, and have even 
been considered a sufficient reason for rejecting the oblique or vertical 
positions of the annulus as characters which are not dependable: for 
instance, the genus Zomaria, in which the § Plagiogyria has a well-marked 
continuously oblique. ring of the annulus, somewhat similar to that seen 
in the Dicksonieae. So far from looking upon such cases as these as 
being subversive of views based on the character of the annulus, they are 
exactly what might have been anticipated if the types with a vertical annulus 
were derived from forms in which the annulus was oblique: it is hardly 
to be expected that the transition would be carried out completely in all 
cases: these exceptions may be regarded as being occasional survivals of 
the earlier oblique type. 
It would appear thus probable that the simple annulus of the Leptosporan- 
giates is prefigured by the vertical, many-rowed hoop of the Eusporangiates. 
That in the course of descent, as the bulk of the sporangium was reduced, 
1 Scott quotes a porous dehiscence for Stauropteris (Progressus Rei Bot., i., p. 186). 
