XLix] EUSPORANGIATE FERNS 271 



tion of the leaf is seen in O. intermedium, which attains its limit in the myco- 

 rhizic O. si?nplex, where the sterile blade is absent (Vol. ll, Fig-. 345). The 

 fertile region in them all is held as of pinna-nature: the sexual organs, and 

 sometimes also the sporangia, are deeply sunk : the sporangia are largest in 

 the advanced genus Ophioglossum, a fact that may be related to the myco- 

 rhizic state. The Family is held to consist of imperfectly modernised relics 

 of an extinct Palaeozoic Flora, such as is represented by the fossil Coeno- 

 pteridaceae. 



All the sporangia of the Ophioglossaceae are distal or marginal, thus 

 retaining what may be regarded as a primitive position. The parts which 

 bear them in the Ophioglossaceae are narrow, and often branched. It is, 

 however, a wide-spread fact of experience among Ferns that where the 

 fertile blade is expanded the sori are liable to assume a position not at the 

 margin, but on the surface of the blade. Indeed it may be stated generally 

 that where this is so the position has been acquired secondarily. An illustra- 

 tion of this change may be seen in the living representatives of that ancient 

 Family, the Osmundaceae (Vol. II, Chapter XXl). Osmimda normally bears 

 its sporangia in marginal tassels on the narrow sporophyll, showing in this 

 the primitive state of the Coenopterids. Todea bears them superficially upon 

 the expanded pinnules, a state which may be held as derivative. But ab- 

 normally Osmunda may itself illustrate the transition, in leaves that are 

 described as metamorphosed (Vol. II, Fig. 420). This transition may have 

 happened early or late in descent in Ferns at large, for instance in the 

 Marattiaceae and Gleicheniaceae the sori are superficial; and Ferns of both 

 of these types, with sori already superficial, existed in Palaeozoic times. 

 Others have retained the marginal position to the present day, for instance 

 the Schizaeaceae and Hymenophyllaceae: while others again may be caught 

 in the act of transition, for instance the Pteroid Ferns. A broad distinction 

 thus exists between those which adopted the change early {Superficiales), and 

 those which retained the marginal position, or only departed from it relatively 

 late {Marginales). In certain great phyla this distinction dates back to the 

 Palaeozoic Period. 



The Marattiaceae (Vol. II, Chapter xx) are illuminating in this relation. 

 The sorus in them all is radiate-uniseriate, derived from a type like that of 

 the Coenopterid Corynepteris, which is itself a compact tassel, seated at or 

 near to the margin of a narrow leaf-segment (Vol. II, Fig. 334). A broadening 

 webbed leaf, with such sori intra-marginal and slightly elongated, would 

 give the condition seen in Angiopteris if the sporangia were still separate; 

 or oi Marattia if they were fused to form synangia (Vol. II, Fig. 392, A, C). 

 , Extension of the former along the veins would result in the state of 

 Archangiopteris, of the latter that seen in Damica; while further widening of 

 the blade now reticulate, and with the segregated sori scattered over the 



