•XLV] COMPARISON 229 



-mentioned, is seen in Cliciroplcnria. Here the individual sori, as indicated by 

 their sub-receptacular conducting tracts, are greatly extended (Fig. Ji^a). 

 The limits of the individual sori are no longer recognisable superficially, 

 though they may be traced by their separate vascular supply. This extension 

 of the receptacular tracts parallel to the leaf-surface maybe held as an alterna- 

 tive to the elongation of the receptacle vertically to the leaf-surface or margin, 

 as in the Gradatae. The result is in either case increased accommodation for 

 sporangia. The course and the measure of the extension are best followed 

 in Platyccrium (Figs. 715 b, 719), and reach a climax in such sporophylls as 

 those of P. stanaria. In both of these genera the identity of the sorus is still 

 structurally maintained: but in Christopteris triaispis, and apparently also 

 in Hymenolepis spicata, the individual vascular systems of the sori may fuse 

 among themselves, thus constituting a sub-soral network, or connected tract 

 (Fig. 724). Christopteris may be held as a most complete example of the 

 diplodesiiiic sporophyll, presenting in its fullest development the Acrostichoid 

 state. 



With this soral spread there goes commonly, but not always, a "mixed" 

 condition of the sporangia. An exception is seen in Platycerium, in which the 

 sporangia arise as a rule simultaneously (P. alcicorne),w\i\i regular orientation, 

 in rows on either side of the underlying receptacular vein. This gives the 

 impression of the Platycerioid sorus as an extreme extension of the radiate- 

 uniseriate type: in fact a long drawn out representative of that in Alatonia 

 or Gleichenia. But this retention of the simple character is exceptional: in 

 Cheiropleiiria or Christopteris and others the "mixed" character of the sorus 

 is attained early, along lines already indicated in Dipteris conjugata, and it 

 is very fully developed. In fact the Dipteroid Ferns, together with the 

 Gymnogrammoids, are the most conspicuous examples of the direct transi- 

 tion from a simple to a mixed state of the sorus ; in them it arises without 

 the intervention of that gradate condition which appears in the Hymeno- 

 phyllaceae, the Dicksonioids and the Cyatheoids. Its alternative, viz. the 

 increasing soral spread, together with the development of underlying vascular 

 tracts, may be held as the leading soral feature of the Dipteroid Ferns. An 

 extension in area, and as Von Goebel has shown of the vascular supply 

 also, characterises the Pleopeltis types of Polypoditcm. These may be held as 

 pursuing a like method, though they carried it out in a less efficient manner. 



Along with such progressions among the Dipteroid Ferns go changes in the 

 sporangium and spore-output. The primitive sporangia of Gleichenia are few in 

 each sorus: each is short-stalked, with a large head, oblique annulus, median 

 dehiscence, and a large spore-output. Matonia has a non-specialised lateral 

 dehiscence, and a small spore-output of very large spores. The Mesozoic 

 Dipteroids had also few sporangia in their sori, with large heads, short stalks, 

 oblique annulus, and a large spore-output : typically 5 1 2-256 for Dictyophylbini 



