XLixJ SUPERFICIAL DERIVATIVES 275 



open venation to reticulate: from marginal sori to superficial: frombi-indusiate 

 to uni-indusiate, and from gradate sori to mixed: from a convex receptacle 

 to one flattened and extended over the whole of the lower leaf-surface. 

 Comparison of habit in the earlier terms of the series leaves no room for 

 doubt that the Pteroids are of Dennstaedtioid affinity, and the series leads 

 finally to that cosmopolitan Fern, which, with two others, remain the sole 

 claimants to the old generic name oi Acrostichuui (Vol. Ill, p. 58). 



Two features present in the primitive state are retained by the Marginales 

 with peculiar tenacity, but finally relinquished: viz. the marginal position 

 of the sorus, and dermal hairs. It is a fact of diagnostic interest that the 

 dendroid Dicksoniaceae retain both, while the dendroid Cyatheaceae have 

 relinquished both. This is not the sole ground for separating the former 

 phyletically from the latter: but either of these points would suffice at a 

 glance to distinguish a Dicksonioid Fern from any true Cyatheoid, the 

 latter having superficial sori, and plentiful chaffy scales. 



Superficial Derivatives 



There is ample evidence of the existence of Ferns in Palaeozoic time 

 with sori seated on the lower surface of the flattened blade. With the 

 Coenopteridaceae before us to point to the distal or marginal position as 

 primal, and the fact that more than one series of living Ferns may be seen 

 in course of transfer of the sorus from the margin to the surface, it is 

 legitimate to conclude that the superficial types of Palaeozoic time had also 

 acquired the superficial position secondarily, though at a very early period. 

 This conclusion is not, however, essential to our argument; the undoubted 

 fact of their early existence is sufficient. Oligocarpia (Vol. II, p. 211) gives 

 an example, from the Coal Measures, of a Fern with a superficial sorus of 

 Gleicheniaceous type. The oldest authentic records of Gleicheniaceae are 

 however, from the Keuper of Switzerland : but they are plentiful throughout 

 the Mesozoic Period, those early records running parallel with but separate 

 from those of the marginal Schizaeaceae, as they do to-day. 



The Gleicheniaceae (Vol. II, Chapter XXIV) are a Family of uniform habit, 

 with bifurcating usually protostelic rhizome, and leaves often possessing 

 unlimited apical growth and of Pecopterid type, with open venation: their 

 radial unprotected sori consist of few large sporangia, with high spore-output, 

 and longitudinal dehiscence. All these characters, seen particularly in Dicra- 

 nopteris, mark them as relatively primitive (Chapter XXIV). Eu-Gleichenia, 

 Stromatopteris and Platyzoma are specialised xerophytes. A peculiar phyletic 

 interest attaches to the sub-genus Eii-Dicranopteris, which bears hairs only, 

 while scales are present in other Gleichenias. Solenostely is characteristic of 

 D. {Gl.) pectinata (Willd.) Pr., while this species and D. {Gl.) linearis (Burm.) 

 Clarke have in each sorus 6 to 1 2 smaller sporangia, with a lower spore-output, 



