BOTRYOPTERIDEAE 341 



found in Osmundaceae, where the annulus is represented 

 by a unilateral group of thickened cells, the areola, 

 resembling the Botryopteris arrangement in a reduced 

 form. 



On the other hand no annulus is differentiated in 

 Diplolabis or Stauropteris, genera which were certainly 

 allied to the Botryopterideae, though this character 

 would exclude them from the family as defined by 

 Renault. 1 The annulus evidently afforded less constant 

 characters among Palaeozoic Ferns than is now the 

 case. 



The position of the sporangia in Zygopteris, Botryo- 

 pteris, and Stauropteris, where they are terminal on 

 ultimate branches of the rachis, is not very closely 

 paralleled among recent Ferns, but finds its nearest 

 analogy in Helminthostachys and Botrychium among 

 the Ophioglossaceae, 2 the comparison being closest 

 with the case of Botryopteris (Fig. 123). 



The dimorphism of the fronds, though so striking a 

 character, is not of the first importance, for we know 

 that it recurs in almost every group of living Ferns, 

 and that within the same genus some species may be 

 dimorphic, while in others there is no differentiation 

 between the sporophylls and the ordinary foliage. 3 



At the same time the great modification of the 

 fertile frond in Zygopteris, Botryopteris, Stauropteris, 



1 Flore fossile oTAutun et d ' Efinac, t. ii. p. 33, 1896. 



2 See Bower, "Spore-producing Members. II. Ophioglossaceae," Plate 

 v. Fig. 81. 



3 The word dimorphism is used to include both the differentiation of 

 sterile and fertile leaves, and that of sterile and fertile parts of the same 

 leaf. In the case of fossil Ferns it is usually impossible to say which was 

 the case, and among recent Ferns the distinction is not always constant, 

 even in the same species. 



