PTERIDOPHYTA 



285 



may be formed by the inroUed leaf margin. Such a "false indusium" is 

 present in the Schizaeaceae, Hymenophyllaceae, Dicksoniaceae, and in 

 such well-known genera of Polypodiaceae as Pteris, Pteridium, Adiantum, 

 Pellaea, Cheilanthes, and Notholaena (Fig. 240B, D). This condition 



K^\\^^ 



N 



S \ S \ \ \ \ \ \ \ "^v \ \ \ \ 



C E 



Fig. 240. Portion of the leaflets of five common ferns, illustrating differences in sori and 

 indusia, X4. A, Woodwardia; B, Adiantum; C, Dryopteris; D, Pteridium; E, Poly podium. 

 In B and D a false indusium is seen, while in E there is no indusium. 



may be regarded as primitive. In a number of true ferns the sori lose 

 their individuality by a spreading of the sporangia over the leaf surface or 

 along the leaf margin. Such "confluent sori" are seen in Pityrogramma, 

 Elaphoglossum, Pteris, and Pteridium. 



With respect to the order of appearance of the sporangia within the 

 sorus, three conditions are recognized, as follows: (1) The simple sorus, 



