COMPARISON OF EXTERNAL CHARACTERS 625 
5 
dominant in our comparisons; and the burden of the argument must still 
rest upon the facts derived from the sporophyte generation. We shall then, 
excepting for an occasional reference, leave the gametophyte aside in the 
present discussion, and review the characters of the Fern-plant in its relation 
to the general theory of the sporophyte. 
EXTERNAL CHARACTERS, 
The Ferns are the characteristic megaphyllous members of the Pterido- 
phyta, and thus differ markedly in habit from the smaller-leaved strobiloid 
types. It is necessary first to inquire what are their probable relations to 
these series. In point of time the distinction of habit dates back as far as 
the earliest known fossils, and accordingly it is only by comparison that any 
opinion can be formed as to their origin by descent, and then only as a 
probability, not as a demonstration. The similarity of life-history shows, 
however, that the sporophyte of the Fern as a whole corresponds to that 
of the strobiloid types: the further question will then be as to the 
correspondence of the parts, especially the axis and leaf. 
The chief difference lies in the proportion of leaf to axis, and.in the 
branching of the leaf, not in the fundamental relations of those parts as 
regards origin or position: this is specially obvious in upright growing 
species, with radial symmetry of the shoot. In the Ferns, as in other 
Pteridophytes, there is reason to regard the radial type of the shoot as 
primitive, notwithstanding the fact that a very large proportion of living 
Ferns are dorsiventral. Among the Ferns of the Primary Rocks no dorsi- 
ventral type of shoot has been described, unless it be the Permo-Carboniferous 
genus Glossopteris, the relation of which to the true Ferns is still a matter 
for discussion. It is possible that a creeping rhizome may have existed as 
the base of insertion of some of the unattached fronds, but still in the 
absence of demonstration of this the evidence points to the radial type as 
having been prevalent. This is the case with the various stems designated 
Caulopteris, in many of which the leaf-arrangement is on a spiral plan: 
even those designated Megaphytum, where the leaves are distichous, were 
of radial character, and all evidence indicates that their position was upright. 
Among the best known of the early forms are the Botryopterideae, which 
had relatively thin axes with leaves in some cases closely aggregated, in 
others more laxly disposed: both types are of radial construction. Thus 
the evidence, so far as it goes, indicates that the radial type of shoot was 
prevalent, if not indeed exclusive, for the early Ferns.. It is exemplified 
by the Botryopterideae, the Marattiaceae, and the Osmundaceae, all early 
types. 
‘That large-leaved forims would be mechanically unstable structures is 
obvious, especially where the stem is thin and the internodes of appreciable 
length. There is an inherent probability that such axes should become 
oblique or prone, with a dorsiventral development as a natural consequence. 
2R 
