314 



FORMS OF FILICALES 



fern, Filix, the partly inferior indusium covers the circular sorus 

 like a hood (Fig. 225, D), while it is wholly inferior in the hay- 

 scented fern, Dennstaedtia, forming a cup (Fig. 225, E) and in 

 Woodsia the indusium is roundish or star-like (Fig. 225, 7*"). 

 Several ferns are distinguished by false indusia that are formed 

 by the more or less modified margins of the leaf. In the bracken, 

 Pteridium, the entire membranous margin of the leaf curves over 

 the closely crowded sori (Fig. 226, A), and in the maiden-hair 



Fig. 226. Ferns without indusia or possessing false ones: A, leaflet of 

 bracken fern, Pteridium. B, maiden-hair fern, Adiantum. C, polypod fern, 

 Polypodium. — After Sprague. 



fern, Adiantum, the sporangia are at the ends of the veins and 

 covered by reflexed portions of the leaf (Fig. 226, B). The 

 indusia are lacking in some forms, as in the beech fern, Phegoteris, 

 and in the polypody, Polypodium (Fig. 226, C). These genera 

 are distinguished by the fact, among others, that the leaves of 



