400 Bower . — Studies in the Phytogeny of the Fi tic ales. 
left of the vein ; and finally, as minute processes of such tracheides rising 
from the vein into the minute receptacle. The appearance is here as though 
the fusion-sorus were resolved into a series of isolated sori, somewhat like 
those of Matteuccia , from which it may be supposed to have sprung. But 
here each is still covered by a persistent portion of the indusial flap, 
a structure not represented in Matteuccia. 
It may thus be that the fusion-sorus may be disintegrated into portions 
corresponding in position at least to its original constituents. But even 
here the correspondence is not very exact ; and in other cases there may 
be greater irregularity. This is seen in the transitional forms so well known 
in B. spicaut. Examples have been noted by Luerssen (Rab. Krypt.-Fl., 
p. 1 1 2, Fig. lxxxvi). The apical part of such a type is shown in PL XXIX, 
Fig. 23. The distal part is sterile, with pinnate venation, while the lower veins 
bifurcate. By various transitional steps it is seen how, in the lower fertile 
tract, these furcate veins are linked together by commissures, which in the 
normal pinna constitute with them continuous vascular connexions running 
parallel to the midrib. The 
constitution of these is thus 
in part from deflexion of the 
anadromic branch of the furcate 
vein, partly from the com- 
missure of storage tracheides. 
The relation of these two con- 
stituents is suggested by the 
diagram, Text-fig. 14. The 
two constituents may run parallel, or independently. This is already 
indicated by the diagram, but it is made clearer by a drawing from 
the leaf of Blechnum fraxineum (Fig. 20, f)> in which it is seen that 
beneath the greater part of the receptacle both the conducting xylem 
of the veins and the storage xylem of the commissure are present. 
But the veins on the one hand may take a separate course into the 
assimilating flange, and on the other hand the storage xylem of the com- 
missure may form a bridge of its own, even with an independent process 
in a plane below that in which the conducting veins run. This accords 
entirely with the condition seen in B. Fraseri (PI. XXIX, Fig. 21). It is 
important to realize the existence of these two allied but distinct systems 
of vascular tissue. They appear to find an analogy in the state of the 
fertile regions of Platycerium, as already noted by Mettenius (Fil. Hort. 
Lips., PL IV, Figs. 1-3). 
We may return now to the case of Blechnum boreale, and the disintegra- 
tion of the fusion-sorus which it shows (PI. XXIX, Fig. 23). At first sight 
it would appear that the result of the disintegration is the more or less 
regular resolution into the original sori. But though there is a relation 
Text-fig. 14. Scheme of venation of Blechnum, 
showing the midrib, and forked veins arising from it. 
The dotted line indicates the commissure. 
