GROUP III. PTERIDOPHYTA : FILICINJE ; LEPTOSPORANGIATJE. 391 



filiform, bearing sporangia only at its base (e.g. species of /Tricho- 

 manes) ; or a ridge (e.g. Pteris, Blechnum). 



The sorus is quite bare in many forms (Gleicheniaceae ; 

 Osmundaceae ; Alsophila among Cyatheaceae ; Schizaeaceae, except 

 Lygodium ; Polypodieae) ; in others it is more or less covered, by a 

 protective membrane, the indusium, which is an outgrowth of the 

 tissue of the leaf, generally of the epidermis alone. When it 

 springs from the placenta below the sorus (indusium inferum), it 

 is somewhat cup-shaped : in the extreme case of Diacalpe 

 (Polypodiaceae) the indusium forms a completely closed globular 

 investment to the sorus; it is urceolate and entire in Trichomanes 

 (Fig. 257 A), Lygodium, Cyathea, Davallia : it is bilabiate in Hy- 

 menophyllum; two-valved in Cibotium, Dicksonia; or it consists of 



FIG. 258. A dehisced spor- 

 angium of Aspidium Filix-mas 

 ( x 90) : a the stalk, with a 

 glandular hair p ; r the annu- 

 lus ; s the stomium. 



B. 



FIG. 259. Sections of young sporangia; A of a 

 Fern (Mohria), B of Equisetum ( x 150) : w wall ; t tape- 

 turn ; a s archesporium. 



a single lateral valve, as in Hemitelia and Cystopteris : in Woodsia 

 the calyciform indusium is laciniate, the laciniae frequently ending 

 in long hairs. When it springs from the apex of the placenta, 

 above the sorus (indusium superum), the indusium has the general 

 appearance of a peltate scale, either orbicular in outline (Aspid- 

 ium) or reniform (Nephrodium, Fig. 257 D). When it is developed 

 on one side of the sorus (indusium laterale), the indusium is a long 

 narrow scale, attached along its length, and overlying the sorus 

 (e.g. Asplenium [Fig. 257 E], Blechnum, Scolopendrium). In some 

 cases, where the sori are near the margin, they are protected by 

 a false indusium, which is merely the incurved margin of the leaf 

 (e.g. Cheilanthes, Adiantum, some species of Pteris). In Pteris 



