XXVII] COMPARISON 249 



facts will bear, but at least they serve to connect the more definite!}^ hygro- 

 philous filmy Ferns with the latter in this structural detail. 



The marginal position of the sorus is shared with the Schizaeaceae, 

 Botryopterideae, and Osmunda. But the H\'menoph}'Ilaceae differ in the 

 basipetal sequence of the sporangia, the elongated receptacle, and the cup- 

 like indusium. The basipetal sequence may be held as a secondary condition, 

 advantageous in spreading the drain of spore-production over a longer period. 

 The intercalary elongation of the receptacle is an almost necessary condition 

 of its adoption. The basal indusinm is foreshadowed in principle though 

 not in detail by the Schizaeaceae, and it serves an obvious biological purpose. 

 It is thus possible to conceive the origin of the Hymenophyllaceous sorus 

 from some Fern-type with marginal sporangia, or perhaps tassels of sporangia, 

 by initiation of a basipetal sequence and the establishment of a protective 

 indusium. The type from which they might have originated would probabl}' 

 be found among protostelic types with large sporangia marginally produced, 

 of which the Botryopterideae, Osmundaceae, and Schizaeaceae are the known 

 representatives. 



A comparison of the sporangia themselves is naturally directed to the 

 Simplices rather than to Leptosporangiate Ferns. Among the Schizaeaceae 

 it is Anemia that corresponds most closely, for there the distal face is of 

 considerable size though flat (see Fig. 45 i, F, G). If this were enlarged and 

 convex, the basal face reduced, the annul us oblique instead of transverse, 

 and the line of dehiscence swung obliquely to one side but maintaining the 

 indefinite stomium, the sporangium of Hymenophyllinn would be the result, 

 while the typical spore-number (128) is the same. It is not suggested that 

 Anemia was an actual progenitor of HymenopJiyllum: what is suggested is 

 that the two sporangia have essential points of similarity. But in spore- 

 number certain Hymenophyllaceae approach still more nearly to the earlier 

 Simplices. H. tunbridgense with 256-512, T. reniforme and sericeum with 

 256, show figures which find no correlative among ordinary Leptosporangi- 

 ates, a fact which strongly supports the suggested relation with the Simplices. 

 On the other hand the low spore-output of certain species of Trichomanes 

 is held to be evidence of their advanced hygrophytic adaptation. 



The comparison of the gametophyte of the family with that of ScJiizaca 

 is striking, though some may still doubt whether it has not some homoplastic 

 origin. But Heim's comparison of the antheridia of the Hymenophyllaceae 

 with those of the Gleicheniaceae {Flora, p. 363, 1896) again directs attention 

 in a matter of detail to the Simplices. 



The result of a general comparison of the Hymenophyllaceae with other 

 Ferns is to recognise that the\- approach most nearly to certain Sim- 

 plices, with which they agree in man\- characters both of the sj)oroph}^te 

 and of the gamctoi)h>-te. Those of the sporoph>-te are the more distinctive: 



