DEVELOPMENT OF PLANTS 



297 



lateral vascular bundles with the Ophioglossales that suggest the 

 derivation of the Filicales from this source. The majority of 

 ferns in temperate climates are characterized by sporangia of 

 the type shown in Fig. 224, B, and they are distinguished by 

 the form and arrangement of the sori and indusia. The shield 

 fern, Dryopteris, has a rather circular or curved sorus covered 

 by an indusium that is laterally attached at a single point (Fig. 

 222, A). The Christmas fern, Polystichum, has a circular sorus 

 with centrally attached indusium (Fig. 225, C) . The spleenwort 



Fig. 225. Common forms of the Filicales : A, the flowering fern, Os- 

 muiida, showing" below two green leaflets and above two sporangia-bearing 

 leaflets. At left a cluster of sporangia magnified. The first leaves in the 

 spring only bear sporangia; those appearing later have only green leaves. 

 B, chain fern, Woodwardia. C, Christmas fern, Polystichum. D, bladder 

 fern, Felix. E, hay-scented fern, Dennstacdtia — s, sorus enlarged. F, 

 Woodsia. — After Sprague. 



fern, Asplenium, is characterized by elongated sori arranged 

 obliquely to the midrib upon the upper side of the veinlets. 

 The indusium is attached on one side of the sorus along its entire 

 length (Fig. 222, B) . The chain fern, Woodwardia, differs 



