28o SUMMARY OF RESULTS [ch. 



a four-rowed stalk, a slightly oblique annulus, and lateral dehiscence. This 

 monotypic genus does not accord with other Cyatheaceae, though it has 

 habitually been included in the Family. It stands by itself. But in habit and 

 soral characters it may probably be linked with two old genera, Syngramme 

 and Elaphoglossum, which share its venation and certain other features ; they 

 differ, however, in having vein-fusions though without included free endings. 

 The sporangial stalks are three-rowed, and in ElapJioglossiim interpolated 

 sporangia lead to a "mixed" state. In Syngramme the sori are elongated, but 

 in Elaphoglossum they are fully Acrostichoid, being spread over the whole 

 leaf-surface. These comparisons are suggestive rather than demonstrational, 

 and the Metaxioid derivatives will require accurate re-investigation from 

 this point of view. 



It is well known that Ferns of Dipteroid type were prevalent in the 

 Mesozoic Period: but it has hitherto been insufficiently realised that they 

 have left behind quite a considerable number of striking descendants living 

 at the present day, such as Platycerium, Neodieiropteris, and Cheiropletiria. 



The relations of Dipteris to Matonia are certainly real : both may be held 

 as related by descent to the Gleicheniaceous stock. The Dipteroid Ferns 

 (Chapter XLV) are throughout non-indusiate, and most of the living species 

 of Dipteris, as also the related Mesozoic fossils, rank as Simplices. But 

 many of the later derivatives possess a mixed sorus. Another and more 

 effective method of increase in their spore-production was secured by ex- 

 tension of the individual receptacle, which is often accompanied by a .special 

 sub-soral vascular system running in a plane parallel to that of the blade 

 (diplodesmic). This is in fact a special feature in Ferns of Dipteroid origin. 

 They show a considerable range of progressive characters: originating from 

 a type with narrow bifurcate leaves, having one row of hemispherical sori on 

 either side of the mid-rib, after the Gleicheniaceous type. From this they have 

 passed over by webbing and expansion to broad irregularly lobed blades, 

 with the characteristic Anaxeti venation: also from protostely, through 

 solenostely, to the highly perforated polycyclic state seen in Platycerium: 

 also from dermal hairs to scales. But it is the diplodesmic soral expansion 

 that gives their most marked character: this is accompanied by a direct 

 passage from the simple to the mixed state, and from an oblique to a 

 vertical annulus. In Platyceriimi the sori that constitute the large fertile 

 patches retain something of their original identity, though greatly extended 

 in length and branched. In Neocheiroptei'is and Pleopeltis the sorus is also 

 extended, but it retains the circular or oval form. In Clieiropletcria (with 

 its strangely mixed characters), in Cliristopteris and in Hymenulepis the fertile 

 area extends over the whole surface of the blade, and the identity of the sorus 

 is lost, as in any fully Acrostichoid type (pp. 213-224). 



Comparison as detailed in Chapter XLV leaves no room for doubt that 



