426 Bower. — Studies in the Phytogeny of the Filicales. 
(2) An outward arching, with further disintegration at the apex of the 
arch, so that each is represented by two parts. This is seen in Blechnum 
punctulatum, var. Krebsii , and becomes a permanent feature in Scolopendrium 
vulgare. 
(3) Additional branching of the veins in the region between the limbs 
of their bifurcation, which, as it corresponds to a convex sinuosity of margin 
subtending the primary vein, is held to represent a suppressed pinnation. 
This is seen in B. ptmctulatum , var. Krebsii , and becomes a regular feature 
in Scolopendrium vulgare. 
(4) Formation of additional sori upon these veins ; their position is such 
as to face the central line of the suppressed pinna, thus corresponding to that 
of the Blechnoid sorus throughout. But the consequence is that they are 
back to back with the relatively prior sori. This is seen in B. punctulatum , 
var. Krebsii , and in Camptosorus. 
(5) Continuation of these sori downwards towards the midrib, so as 
to extend below the point of branching of the vein. Thus they are placed 
back to back with the prior sori, and on the same vein. This is the 
condition characteristic of Diplazinm^ and may be seen occasionally in 
Camptosorus. 
These factors, in various combinations, may be held to provide an 
explanation of the several types of Scolopendrium and Asplenium along 
phyletic lines, as derivatives from a Blechnoid source. The leaves in which 
they may be traced have probably had an origin from a more highly 
branched ancestry, and traces of suppressed pinnation have been noted 
above, especially in 5 colop endrium and in the var. Krebsii. A further 
biological consideration has probably been a progressive increase of the 
assimilating area. The Lomarioid type, from which all of these Ferns 
appear to have been derived, had excessively narrow fertile pinnae with 
recurved flaps ; in fact, assimilation was sacrificed to protection of the sori. 
The material for nourishing the developing sori had then to be brought to 
them via the stock, since the narrow sporophylls could not suffice for 
forming it on the spot. Obviously, an increase of the assimilating area of 
the sporophylls themselves would then be an advantage. And this was 
obtained, either by the formation of the flange, as in Eu-Blechnum , or by 
expansions which involved the soral lines themselves, and led to their 
sinuosities and their ultimate disruption, as in the var. Krebsii , in Scolo- 
pendrium , and Asplenium ; or to their interruption without marked curvature, 
as in W oodwardia and Doodia . 
In contemplating thus the breaking up of the fusion-sorus from the 
bio-evolutionary point of view, it is necessary to consider it also morpho- 
logically, and to see how the soral unit fares in the changes in question. 
There is good reason to believe that the fusion-sorus of Blechnum , with its 
continuous line of receptacle and its underlying vascular commissure, 
