128 



DRYOPTEROID FERNS 



[CH. 

 ;roup 



reticulation (Fig. 659). Particularly is this so in Sagcnia Presl 

 largely represented in the Southern Hemisphere. 



The prevalence of the two larger genera is itself a witness to the success 

 of the superficial sorus with its orbicular or peltate covering, the ''indusium 

 superu7n" of authors (Fig. 660). This type of sorus is clearly secondary and 

 derivative. The source from which it probably arose was in the first instance a 

 type with a basal indusium, and radical receptacle, as seen in the Cyatheaceae. 



Fig. 6^^9. Aspidittin Plninierii Pr., whole leaf, reduced, showing the broad 

 reticulate blade. A small portion is shown magnified, with sori. (After Christ.) 



Already the first step towards lop-sidedness has been recognised in Heinitelia : 

 this led to the horse-shoe-shaped receptacle, prefigured in Paraneina with its 

 lateral pouches, and realised in Dryopteris. It is a minor step from the horse- 

 shoe-shaped receptacle to the completion of the circle of the receptacle 

 round the indusial stalk, as it appears in Aspidium or Polystichuni. An equal 

 development of the covering all round would then result in the orbicular 

 indusium. The close similarity in habit and structure of those genera that 



