Bower. — Studies in the Phytogeny of the Fi lie ales. 427 
originated from separate sori of such a type as those of Mattcuccia inter- 
media , arranged in linear sequence. That they were gradate sori, like 
those of Matteuccia , is indicated by the prevalence of a basipetal sequence 
of their sporangia in the simpler forms of Blechnum , and even in many that 
are more advanced. Accordingly, the commissure is a secondary formation, 
as it is also in Pteris and in Saccoloma ; if this be so, the term ‘ fusion- 
sorus * is correctly applied. When this fusion-sorus is interrupted, as it is 
in Woodwardia , or Scolopendrium , or Asplenium , the interruptions do not 
always coincide with the number or position, or the limits of the original 
sori. Consequently, each isolated portion of the fusion-sorus cannot be 
held to be the exact equivalent of the sorus Matteuccia. It would doubtless 
be inconvenient to mark this discrepancy by any change of terminology. 
The * sorus ’ of Ferns must still be so designated in a general sense, as it 
always has been. But in using the term in the case of these genera, 
the discrepancy which it covers and obscures should be clearly borne 
in mind. 
Another and a distinct line of modification of the Blechnoid sorus is 
that which leads to ‘ Acrostichoid ’ developments. By this is meant the 
extension of the soral area, so as to spread over an enlarged surface, which 
is not necessarily restricted to the position above the veins. The fusion- 
sorus is itself an example of such lateral spread, i. e. transversely to the 
veins. But in other types the spread may be along the course of the veins, 
and in the area between them. Examples in a minor degree have been 
given in B. Pater soni, penna-marina , and filiforme (pp. 387-90). It only 
requires an extension of such developments as are there seen to produce the 
condition of Stenochlaena , or of Brainea. These show the full features 
of the old genus Acrosticlium. In such cases the receptacle becomes 
flattened, and the basipetal sequence of the sporangia is lost, while the 
numerous sporangia appear to be more independent of protection while 
young. In Stenochlaena there is reason to believe that the £ flange * has 
been reduced, while the indusial leaf-margin is flattened, and even everted 
as maturity comes on. In Brainea , on the other hand, it is the c indusium ’ 
that is aborted, and the spread of sporangia is outwards upon the ‘ flange ’, 
following especially the course of its veins, which correspond in position 
and character to those of Eu-Blechnum. It thus seems probable that these 
two Acrostichoid derivatives of Blechnum have arisen independently, as 
parallel developments. They illustrate what is becoming abundantly plain, 
that Acrostichum is not a genus in the phyletic sense, any more than 
Polypodium is a genus. Both are states or conditions of soral construction, 
which may have been arrived at along several distinct phyletic lines. 
They both show a common tendency of the more advanced Leptosporangiate 
Ferns to become less dependent upon protection of their young sori than the 
relatively primitive types from which they sprang. 
