PTEROPSIDA 167 



'fusion sori' near the margin, much as in Pteris. Adiantum 

 has the sporangia restricted to the under side of special 

 reflexed marginal flaps of the lamina (Fig. 25S). The 

 majority of the members of the family inhabit fairly dry 

 regions and some are markedly xeromorphic, e.g. Cheilanthes 

 and Pellaea. However, at the other extreme, Ceratopteris is 

 a floating, or rooted, aquatic plant, now widespread in 

 tropical countries, where it chokes up canals and slow moving 

 rivers. Anogramma leptophylla is interesting, in having a 

 subterranean perennial prothallus, from which arise deUcate 

 annual sporophytes. 



Polypodiaceae 



Within this family are placed a number of genera of ferns, 

 all of which completely lack any kind of indusium. There are 

 about 1,000 species in the family, almost all tropical in 

 distribution (but note that Polypodium vulgare occurs in the 

 British flora), and most are epiphytic. Many have highly 

 complex anastomosing venation and some are acrostichoid, 

 e.g. Platycerium. This genus is markedly dimorphic, with 

 'nest leaves' appressed to the tree trunk on which it is grow- 

 ing, while the fertile fronds are quite diff'erent in shape and 

 give rise to the name 'Stag's horn fern'. 



It will be clear, from this brief survey of the Filicales, that 

 there is much scope for disagreement among pteridologists 

 as to the relationships and detailed phylogeny of the group, 

 and that much more research is necessary before final con- 

 clusions can be reached. The areas of affinity indicated in 

 Fig. 23 must, therefore, be regarded as only tentative. On 

 the evidence so far available, it would seem that the group 

 might well be diphyletic, with two evolutionary starting 

 points, one with marginal and the other with superficial 

 sori. Furthermore, it seems clear that even among those with 

 marginal origins there has been a trend towards the super- 

 ficial condition. Should the Filicales prove to have been 



