398 Bower. — Studies in the Phytogeny of the Filicales . 
condition, the anatomy, and the dermal appendages — is clearly Blechnoid, 
and ultimately Matteuccioid. But in assuming the soral character known 
as ‘ Acrostichoid ' it has probably proceeded along a line independent 
from that of Stenochlaena , while the appearance shown in Fig. 18 suggests 
that it may have passed through a stage with interrupted sori, of the 
type seen in W oodwardia and Doodia. It will, indeed, be seen below 
how the indusium in W. areolata may be strongly reduced (PI. XXIX, 
Fig. 25) ; it only requires the complete absence of the indusial flap in such 
a pinna to give a source from which Brainea might readily have arisen. 
From the observations thus detailed, it is seen that Blechnum has 
given rise to a number of derivative forms showing the soral characters 
of the old collective genus Acrostichum . Further, there is reason to think 
that these have not all followed one phyletic line of origin ; more probably 
the change of character has appeared more than once within the Blechninae. 
iii. The Disintegration of the Fusion-Sorus of the Blechnoid Type. 
From the comparative study of Matteuccia and Plagiogyria, and then 
of a number of species of Blechnum , it has been seen that probably from an 
original type with a simple , flattened pinna, or pinnule, bearing isolated sori 
upon the veins, and protecting them by in-rolling of the reduced leaf- 
margins,, which formed flaps thinning off in an e indusioid ’ manner, a more 
complex structure than the original one has arisen. Vascular commissures 
appeared, linking the veins laterally together ; the soral development accom- 
panied these extensions, so that the originally isolated sori became threaded 
together to form continuous lines on either side of the midrib. They may 
be styled the ‘ fusion-sori of the Blechnum type. Further, the comparison 
of numerous species of Blechnum has made it appear probable that a con- 
comitant, or early consequence of the fusion, was a ‘ phyletic slide * of the 
protective flap from the margin to the lower surface of the pinna. It is 
still regarded as the phyletic margin, though its insertion comes to be more 
or less distinctly superficial. But meanwhile the activity of growth and 
segmentation was continued at the margin itself, resulting in the formation 
of the ‘ flange *, which assuming ultimately large dimensions, became the 
chief assimilating surface of the Eu-Blechnum type of fertile pinna. 
It is clear that the soral lines thus resulting from fusion are not them- 
selves sori in the sense of the term as it is used in the Cyatheoid or Onocleoid 
Ferns. In point of origin they may be compared with what is seen in the 
Saccoloma- Lindsay a series (see ‘ Studies in Phylogeny, III,’ Ann. of Bot., 
1913, p. 459), and a similar condition appears also to have arisen in the 
Pterideae. 
But a further derivative state is arrived at by the subsequent breaking 
up of these ‘ fusion-sori ’ into short isolated portions, a phenomenon which 
has frequently been the subject of observation and remark in Blechnum 
