SPORE-PRODUCING MEMBERS 633 



At first sight it might seem probable that the non-soral state was 

 primitive, since it is seen apparently in such early forms as Botryopteris 

 and Myriotheca, and in Osmunda. But there are objections to this as a 

 generalisation ; for, in the first place, definitely soral types, such as the 

 Marattiaceae, are quite as well represented in the primary rocks as any 

 non-soral forms : secondly, while Botryopteris itself appears to be non-soral, 

 there is a distinct indication of a disposition of the sporangia around a 

 central point in Zygopteris (Fig. 272, p. 503), while in Corynepteris there 

 are very definite sori : thirdly, there are among living Ferns clear indica- 

 tions that the soral may pass into the non-soral state : such a progression is 

 suggested in the species of Dipteris (p. 620), while the condition of Acrosti- 

 chum and of Platy cerium can hardly have been produced in any other way 

 than by spreading of the sporangia of some soral type over an enlarged 

 surface, as is indeed suggested by such genera as Gymnogramme and 

 HemionitiS) etc. This is exactly what would be expected as a consequence of 

 indefinite multiplication of parts closely aggregated together, in cases where 

 no biological check determined their exact position. Thus it would seem 

 probable that the soral state is the original condition and the non-soral 

 the derivative, notwithstanding its early appearance. 



But the sorus, whether marginal or superficial, does not always maintain 

 its identity, even in those cases where it is habitually circumscribed. In 

 many Ferns, and especially in those in which there is an enlarged leaf-area, 

 sori may be found of unusual size, elongated, and constricted in the middle ; 

 and from these it is a slight step to complete fission, two smaller sori being 

 then seated close together (Figs. 281, 310). In such changes from the normal 

 as these there lies a capacity for increase in number of sori, and there is 

 hardly room for doubt that in such cases as Kaulfussia and Dipteris, as 

 well as in many of the broader-leaved Polypodiaceae, where the sori con- 

 stitute more than a single row on either side of the midrib, the more complex 

 condition has been brought about in this way ; in fact, the statement seems 

 fully justified that the primitive disposition of the sori was in a single 

 marginal or intra-marginal row : all more complex arrangements in Ferns 

 are secondary and derivative. 



A further matter for disgussion is the position which the sorus holds 

 relative to the leaf which bears it. Two positions are common, marginal 

 and superficial, the latter almost always on the lower surface of the leaf: 

 both of these are of very early occurrence, the superficial being characteristic 

 of the Marattiaceae and Gleicheniaceae, and the marginal of the Botryo- 

 pterideae and Schizaeaceae, while both types are continued upwards into the 

 Gradatae and Mixtae. It would be important to know which of these 

 positions was the more primitive in Ferns. Here, again, an indirect 

 indication may be obtained by comparison of more recent types : among 

 the Leptosporangiate Ferns there is ample evidence to show that the 

 marginal sorus has shifted by gradual steps to the lower surface. This 

 is clearly proved by comparison within the Dennstaedtia-Davallia series : 



