Bower.—Studies in the Phytogeny of the Filieales. VII. 63 
a similar state. In the former the incompleteness of the marginal fusion- 
sorus is to be related to the deep cutting of the leaf. In Pteridium , where 
the fertile leaf is less deeply cut, the fusion-sorus is more continuous, 
and that is the condition permanently stamped on the large genus Pteris . 
Since in Paesia and Pteridium the formation of sporangia extends over the 
whole length of the commissure, a continuous fusion-receptacle has been 
formed between the indusial lips, and the lateral fusion of the sori is thus 
complete. 
Gradual steps are shown within this series illustrating the phyletic 
slide of the sorus from the marginal position, which comparison with the 
Schizaeoids and Dicksonioids indicates as primitive, to the lower surface. 
It is accompanied by degradation of the inner (abaxial) indusium. The 
marginal initiation of the receptacle together with the superficial origin of the 
double indusium has been demonstrated in Saccoloma , Linds ay a, Paesia, and. 
Pteridium , and it is seen to correspond to that in the earlier Dicksonioids ; 
there is, however, a strong curvature during development towards the lower 
surface, and a pronounced inequality of the indusia, the lower being the 
weaker (Figs. 19, 22). It is represented in some forms merely by vestigial 
hairs (Lonchitis aurita), and is entirely absent in others (L. hirsuta, Histio- 
pteris incisa , and the whole genus Pteris). With this is coupled the 
passage of the receptacle from the margin, as defined by segmentation. An 
intermediate state is seen in H. incisa (Fig. 24), where it is difficult in 
individual cases to say whether the margin does or does not give rise to the 
receptacle. But in Pteris (Figs. 29-33) it 18 clear that the margin grows 
directly on to form the upper indusium, while the first sporangia appear 
superficially. Such examples in related types indicate a transition of the 
receptacle from Its primitive position on the margin to a superficial position. 
The conclusion with regard to the inner indusium is important. When 
the whole body of comparative evidence is considered, and especially the 
habit-similarity and anatomy of such a sequence as Paesia , Pteridium , and 
Histiopteris incisa , the thesis appears to be well founded, that the inner 
indusium has become abortive. Biologically this may be held as a natural 
consequence of the downward curving of the fusion-sorus. The protective 
function of the lower indusium is thus taken over—as in the more advanced 
Dicksonioids—by the lower leaf-surface, with which the downward curved 
sorus is in juxtaposition. Morphologically all that remains is in some forms 
perhaps a few hairs, in others no vestige. 
But this conclusion is the direct converse of that of Mettenius. 1 
He developed a theory, and supported it by drawings from various 
closely-related species, that the inner indusium was an upgrade structure 
formed by the webbing together of hairs in close juxtaposition. He 
1 Farngattungen, iii, ‘ Ueber die mit einem Schleier versehenen Arten von Pteris \ Frankfurt, 
.1858. 
