XXXVIII] 



PTERIS 



51 



Fig. 616. Vertical section through a sorus of Histiopteris incisa well advanced 

 towards maturity. ( x 75.) 



Pteris L. 

 This large genus includes 157 species as stated in Christensen's Index, 

 The wide synonymy that surrounds it is a witness to the divergences of view 

 to which its treatment has been subjected in the past. Some authors have 

 narrowed the limits of the genus, giving separate generic rank to various 

 outstanding types, while others have included all which have " marginal 

 linear continuous sori, occupying a slender filiform receptacle," under the 

 generic title {Syti. Fil. p. 153). From the evolutionary point of view the 

 Pteroid Ferns as a whole are those which share a certain biologically 

 successful innovation; this consists in the linking together of numerous 

 marginal two-lipped sori into a linear series. That series once so constructed 

 has been subject to certain modifications, such as the slide of the resulting 

 coenosorus towards a superficial position, the elimination of its lower 

 indusial lip, and finally the spread of the linear receptacle as a broadening 

 area over the lower surface of the leaf-blade. Roughly speaking the generic 

 title of Pteris is applied to all those Ferns in which the lateral fusion of the 

 marginal sori and the elimination of the lower indusium are complete. It is 

 a large and cosmopolitan genus, including plants of almost every kind of 

 leaf-form and venation: in fact the institution of the Pteroid-sorus was an 

 evolutionary success. But compared with the Dicksonioid source, from which 

 comparison shows these Ferns to have originated, they may be expected to 



4-2 



