386 Bower . — Studies in the Phytogeny of the Filicales. 
a single-layered, protective flap. But there is no ‘ true ’ indusium. This 
state is seen in Matteuccia , and also in Plagiogyria, both of which genera 
have been closely related by the older systematists with Blechnum itself. 
In the former genus there is a convex receptacle in each isolated sorus, and 
a clearly marked gradation in the appearance of the sporangia ; but in 
Plagiogyria the receptacle is flattened, and there are indications of a ‘ mixed ’ 
state. An irregularity in thickness of the lateral wings of the pinna has been 
noted already in Plagiogyria : it becomes more marked in various species 
of Blechnum, and it contributes to the peculiarity of the outline as seen 
in section. 
In the first-named species of Blechnum , such as B. discolor , tabulare, 
and lanceolatum , the pinnae as seen in section conform very nearly to the 
type of Matteuccia or Plagiogyria. But indications are already seen that 
the protective flap does not always coincide with the margin, as defined by 
segmentation ; the divergence appears most marked at the apex and base 
of the pinna. Proceeding through the series of species described, this 
divergence increases, both in time and place of origin of the ‘ indusial flap ’, 
till the Eu-Blechnum condition is arrived at, as seen in B. brasiliense. The 
series suggests that the true margin by descent is the iudusial flap , and that 
it has undergone a ‘ phyletic slide ’ from its original marginal position to a posi- 
tion on the lower surface ; consequently it is the ‘ flange ’ that now originates by 
direct continuance of the marginal segmentation. If this be the true history, 
then the ‘ flange ’ is really a new structure, of secondary origin by descent, 
but taking a prior place in the ontogeny in accordance with its importance 
as an assimilating organ. For by its formation the assimilating tissue of 
the fertile pinna is largely increased, spongy parenchyma and stomata being 
produced upon it. The consequence of its appearance is that the dimorphism 
is obliterated, and all the leaves, sterile or fertile, take a similar form. The 
biological advantage of better nutrition of the sporophylls thus gained is 
too obvious to need insistence. This is a brief and bare statement of the 
comparative conclusion. It might of course be possible to invert the thesis, 
and to suggest that the series illustrates an abortion of a broad lamina, and 
that the Lomarioid state is really the derivative. But this view would 
present various comparative difficulties, especially in relation to the sorus 
and indusial flap. 
On the view stated above, the Blechnoid fusion-sorus arose from a gradate 
type with isolated sori like those seen in Matteuccia , or possibly in Plagio- 
gyria. These were seated on distinct veins. The formation of commissures 
connecting the veins, and the consequent running together of the separate 
sori into the Blechnoid fusion-sorus, is a step easily understood when starting 
from a type like Matteuccia . Also the progression from the gradate to the 
mixed condition, illustrated in the ontogeny of the species described, would 
readily follow with the other changes. It does not appear, if the thesis be 
