422 Bower .—Studies in the Phytogeny of the Filicales. 
facts that in the latter the sori form a regular line on each side of the midrib 
of the narrow pinna, and that they are protected by the strongly deflexed 
margins which curl over them ; in fact, that it is dimorphic. But in neither 
are there any vascular commissures connecting the sori laterally. The 
pinna of M. intermedia is, in fact, of the Cyatheoid type, but modified in 
slight degree for protection of the sori which it bears. The Blechnoid 
pinna, however, shows with high constancy an advance on this, in the 
presence of a vascular commissure which links together the line of sori into 
what has here been called the ‘ fusion-sorus 
(a) The Fusion-sorus. 
It has been amply shown, in many species of the genus Blechnmn , 
especially in the least-modified of them belonging to § Lomaria , and in the 
early stages of their development, that the sori are, like those of Matteuccia , 
basipetal. This sequence may be departed from as the individual grows 
older, and markedly so in the more advanced Eu-Blechnum types, while the 
basipetal sequence may also disappear in those Ferns which have become 
‘ Acrostichoid ’ : also in such ultimate derivatives as W oodwardia and 
Doodia , or Scolopendrium and Asplenium , the ‘ mixed 5 type of sorus is pre- 
valent. These facts very greatly support the position adopted here ; for 
the progression from a simple or a basipetal sorus to the mixed type has 
been traced now in so many sequences, that it may be held as a general 
conclusion that the latter is relatively late, and derivative from the former. 
Applying this in the present case, Matteuccia and the Lomarioid Blechnums 
will be held as relatively primitive, and Eu-Blechnum , Woodwardia ) Doodia , 
Scolopendrium , and Asplenium as relatively derivative types. The transi- 
tion from the one to the other is, however, by only slight degrees, and no 
sharp line can be actually drawn ; this in itself suggests that there has been 
a progression. 
Occasionally the vascular commissure may be incomplete in the normal 
pinnae of simple Blechnums, as in B. discolor (Fig. 3, h). But very instruc- 
tive conditions are found in pinnae of various species which are intermediate 
between the sterile and the fertile states (compare Mettenius, Fil. Hort. Lips., 
Taf. IV, Fig. 21, also PI. XXIX, Fig. 22). In these the commissures are 
variously interrupted at the transitional point, and the details strongly sug- 
gest that the origin of the commissure has been by a lateral widening of the 
vascular supply of the receptacle right and left. In fact, that its origin has 
been just the same as is seen in Saccoloma or Lindsay a (compare Ann. 
of Bot., vol xxvii, PL XXXIV, Figs. 20, 21), or in the series of the 
Ptericleae as explained by Prantl (Engler’s Bot. Jahrb., iii, p. 403,), where 
Pellaea has separate sori, but in Pteris they are connected laterally, and 
have a vascular commissure. It thus appears that in three phyla, for which 
there is no reason to assume any common evolutionary advance in this 
