RESULTS, PHYLETIC AND MORPHOLOGICAL 715 
this indicates that they are themselves essentially strobiloid types which 
have progressed to a condition of megaphylly. That is also the conclusion 
which comparison of their external morphology with that of other phyla 
suggests, while the absence of differentiation of the sterile and fertile regions 
is the same as is seen in the “ Se/ago” condition of the strobiloid types. On 
the general biological ground that in homosporous forms there is direct 
advantage in enlarged spore-output, there is reason to regard amplification 
as probable. The amplification of the appendages has been more extensive 
here than in any other phylum, but there are many points of similarity with 
what is seen in certain of the strobiloid Pteridophytes, and especially in the 
Sphenophyllales and Ophioglossales. Accordingly, it is held that the Filicales 
were ultimately of strobiloid origin, but have undergone amplification of their 
leaves analogous to, but phyletically quite distinct from what ts seen in other 
. Pteridophytes, and carried to a higher degree. 
One chief reason for regarding the lines of the Filicales and Ophioglos- 
sales as distinct lies in the difference of position of the spore-producing 
members. It has been argued above (p. 633) that the soral condition was 
primitive for Ferns, and that the sorus ts a body similar in kind to the 
sporangtophore, the two being alike in function, in structure, and in capacity 
Sor fission and extension (p. 699): the number and position are points of 
difference. An increase in number of sporangiophores (or sori) is a natural con- 
comitant of increase in size and nutritive capacity of the leaves; in the Ferns 
a process of fission similar to that suggested in the Sphenophyllales probably 
played a part, rather than elaboration of the single sporangiophore as seen 
in the Ophioglossales. The disposition of the numerous sori upon the leaf 
in Ferns differs from that in other Pteridophytes: but it must be remembered 
that in large-leaved forms this necessarily became a matter of biological 
adaptation in the absence of the protection afforded by a compact strobilus. 
The Filicales are thus a phylum showing fundamentally the strobiloid 
characters, but secondarily modified in relation to their pronounced 
megaphyllous habit. This was adopted very early by them, as the fossil 
story as well as their general morphology clearly show. Accordingly, the 
Filicales appear as the most divergent phylum of homosporous Pteridophytes. 
The prevalence of a whorled arrangement of the leaves has already 
been noted among early strobiloid types, but it was seen to have been 
departed from in many of the Lycopods, and in the modern Psilotaceae. 
In the Filicales, however, as also in the Ophioglossales, alternate leaf- 
arrangement is the rule. This difference from early strobiloid types is a 
very natural one in megaphyllous shoots: for the whorled arrangement is 
mechanically inconvenient where the leaves are large. The alternate 
leaf-arrangement in the megaphyllous types may be held as a natural 
though not an inevitable consequence of the large size of the appendages. 
If this is itself secondary in the Filicales it is quite possible that their 
alternate arrangement was also secondary in descent. But on this point 
there is no clear evidence. 
