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 " Selago " 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 is 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 is a body similar in kind to the 

 sporangiophore, the two being alike in function, in structure, and in capacity 

 for fission and extension (p. 699) : the number and position are poi?its 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 homosporotts 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. 



