Bower. — Studies in the Phytogeny of the Filicales . 423 
respect, a lateral fusion of sori is seen, resulting in each case in long con- 
tinuous receptacles, with a connected vascular supply beneath them. The 
identity of the original sorus of the Cyatheoid type, seen in Matteuccia , 
is thus lost in Blechnum , just as that of the Dicksonioid sorus is lost 
in Sctccoloma or Lindsay a, or the Pellaea- type of sorus is lost in P ter is. 
In each of these cases it must be remembered that in strict terminology 
‘ sorus } should be applied to the original type. The derivative state might 
properly be styled a * fusion-sorus \ 
The protection of the fusion-sorus of Blechnum is by the recurved 
margin of the pinna. That this is the real nature of the so-called 1 false 
indusium ’ is shown by comparison of Matteuccia or Plagiogyria , not only 
in the mature state, but also as regards early development. Further, the 
structure shows in its different forms various stages of the reduction in 
complexity, from that of an ordinary leaf-margin to a single layer of cells 
in extreme cases. It cannot be doubted, from comparison of the genera 
named with the simplest of the Blechnums, that the morphology of the 
organ in question is the same throughout. It is usually styled an * indusium 
but in the use of the word a reservation must be made. It must not 
be held to connote a morphological entity for Ferns at large, but be used in 
a general sense to cover any membranous structure which protects young 
sporangia of Ferns. For already several types of indusium, phyletically 
quite distinct from one another and different -in their origin, are known. 
For instance, there is the umbrella-like covering of the sorus in Matonia , 
which arises from the distal end of the receptacle ; the scale-like indusium 
of Hemitelia and Nep hr odium ; the basal cups (often two-lipped) of the 
Dicksonioid Ferns and Hymenophyllaceae, which are surface growths from 
the leaf below the marginal receptacle ; the marginal flaps of the Pterids, 
and of the Blechnoids. It is impossible, from what we know of the onto- 
geny of these different organs, and the difference of their positions, to hold 
that they are morphologically identical. They appear to be independent 
responses along distinct phyletic lines to the same requirement for protection 
of the young sporangia. 
Another question of morphological interest is, what is the real nature of 
that ‘flange’ which makes its appearance in the Blechnoid series? Its 
origin has been traced in a number of species, from those which are 
Lomarioid, in which it is small or even absent, to those which are Eu- 
Blechnoid, in which it is of considerable size. They illustrate its inception 
as a consequence of a ‘ phyletic slide 5 of the indusial margin from the 
genetic margin to the lower surface of the pinna. The indusial flap has 
been held to be the ‘ phyletic margin ’, because its identity is maintained 
throughout the series, though its position alters. What then is the flange ? 
It must be of the nature of a new formation, emerging by gradual steps 
which the series of species above described sufficiently demonstrates. This 
