Bower. — Studies in the Phytogeny of the Filicales. 4 1 7 
a question whether this is a trustworthy basis for any wide comparison, 
however valuable it may be within narrow circles of affinity. It was chiefly 
on this character that the allocation of Plagiogyria in relation to Blechnum 
was based by the earlier writers. But a better knowledge of the details of 
anatomy and of the soral characters tends to loosen the comparison rather 
than to draw it closer. Whether or not there be a real relationship to 
Plagiogyria , the heterophylly of Matteuccia and of Onoclea , combined as it 
is with other features of correspondence, is a valid point establishing their 
relation with Blechnum , and suggesting what is the starting-point for our 
comparison of Blechnoid Ferns ; viz. that in them heterophylly was a rela- 
tively primitive state. The lines of advance which emerge by comparison 
of the facts stated above illustrate various ways in which these Ferns have 
broken loose from the physiological disability of heterophylly. The essential 
point is the securing of an equal assimilative area in their sporophylls to 
that of their sterile leaves (compare p. 382, above). The several ways in 
which this has been accomplished will be best treated in relation to the 
sorus, as exemplified in Woodwardia and Doodia , in the Krebsii var. of 
B. punctulatum, and in the genera Scolopendrium and Asp lenium. We note, 
meanwhile, the broad fact that the majority of those types which we regard 
as Blechnoid derivatives appear more nearly homophyllous than are the 
relatively primitive Matteuccia and the primitive Lomar.ia section of 
Blechnum. Speaking generally for this phylum, the progression has been 
from heterophylly to homophylly ; and not only comparison of the detailed 
facts, but also the line of physiological probability above indicated, shows 
that the progression has been as thus stated. 
One of the most remarkable modifications of form seen in this phylum 
is that related to the climbing habit. The transition from a ground-growing 
to a climbing state is easily grasped. Comparison of specimens of B. attenua- 
tum in its native habitat provides the intermediate steps ; elongated inter- 
nodes, a supply of grasping roots, and slight structural alteration of the 
axis combine to fit a creeping type to a climbing habit. In B. attenuatum 
the usual type of heterophylly is seen. But in other climbing types more 
pronounced modifications appear. For instance, in B. ( Salpichlaena ) volubile 
we see a climbing, doubly pinnate type, in which the leaves themselves are 
prehensile and may show a pronounced heterophylly ( Lomaria type), or the 
sporophylls may closely resemble the sterile leaves (. Blechnum type). In 
others, however, a still higher degree of heterophylly is seen, and these pro- 
vide the most pronounced examples of heterophylly in the Filicales : for in 
B . filiforme and Stenochlaena sorbifolia2i trimorphism of the leaves is seen 
combined with a high degree of variability of the sterile leaves. The lower 
parts of the plant are rooted in the soil and bear sterile leaves of the 
‘ Pimpinelloid ’ type. In the climbing region the sterile leaves are of the 
‘Blechnoid’ type, while the sporophylls are narrow, but with a tendency to 
