56 PTEROID FERNS [CH. 



lower surface of the blade, without any markedly convex receptacle preceding 

 them; and (iv) that the sorus shows only slight signs ofa gradate sequence, and is 

 of a "mixed" type from an early stage of development. Its gradate character 

 has departed with the loss of the conical receptacle. These differences have 

 made their appearance within the circle of Ferns which have always been 

 held as closely related by the best systematists. It cannot be doubted that 

 the morphological identity of the sorus is the same throughout the series. 

 Hence the only possible conclusion is that in the course of the evolution of 

 Pteris the sorus has passed bodily from a marginal to a superficial position, 

 while at the same time the gradate sequence of the sporangia has been 

 substituted by a "mixed" condition of the sorus. The series of Pteroid Ferns 

 above described illustrates the suggested transition by very gentle steps. 

 Though they may not themselves constitute an actual line of descent, still 

 their existence supports the view that the change has actually occurred. 

 Finally, the fact that it is accompanied by the transition from dermal hairs 

 to scales, and frequently by an advance from a simpler to a more elaborate 

 vascular construction, as well as by the change from an open to a reticulate 

 venation, indicates that the progression has been parallel in vegetative and 

 in propagative characters. 



Pteroid Derivatives 



A central Pteroid type would be one with a rhizomatous or ascending habit 

 and pinnate leaves with open venation, a solenostelic or dictyostelic axis, 

 leaf-trace undivided or possibly separated into two equal straps, with dermal 

 scales as well as hairs, and a sorus of a mixed type, with narrow intra- 

 marginal receptacle, an upper indusium of marginal origin, and no lower 

 indusium. It may be seen in such a species as Pteris longifolia L. and others 

 of the section designated as En-Pteris. Though the Pteroid type does not 

 show very marked further advance, there are certain lines of specialisation 

 which may be recognised. The most frequent and obvious is the passage to 

 a reticulate venation. When partially carried out this gives the foundation 

 for the sections Heterophkbimn Fee, exemplified hy P. grandifoHa L. ; and 

 Campteria Presl, exemplified by P. bimirita L. When the anastomoses are 

 copious but without included veinlets, this is the character of the section 

 Litobrochia Presl: the extreme condition, where included veinlets are also 

 present, is the character of Hooker's section Ainphiblestria Presl, which is 

 now distinguished again as a distinct genus. We need not pursue this further 

 than to remark that anastomosis may be held to be a secondary condition, as 

 a change which brings higher physiological efficiency, and that it is apt to 

 be most pronounced where the cutting of the leaf is reduced and the segments 

 expanded, as in P. spletidens Kaulf, or in AmpJiibkstria Presl. 



Along with this, however, a widening of the receptacle may appear, so as 

 to encroach inwards upon the blade. This may be seen in some degree in 



