GROUP III. PTERIDOPHYTA : F1LICINJE. 



361 



of the lamina (Acrostichese, such as Polybotrya, Chry sodium) : in 

 the former case they occur in groups, termed son, which are 

 commonly isolated, but occasionally (e.g. Pteris) a continuous 

 marginal sorus is formed. 



The sorus generally consists of a large number of sporangia : in 

 the Gleicheniacese, however, the number is small (2-8); and in 

 some cases (e.g. Lygodium) there is only a single sporangium. 

 The sporangia of the sorus are borne on a projection of tissue, the 

 placenta or receptacle, which presents various forms : it may be a 

 slight rounded elevation (e.g. Aspidium) ; or more elongated and 

 conical (e.g. Cyathea, Hymenophyllum) : or very long and filiform, 

 bearing sporangia only at its base (e.g. species of Trichomanes, 

 Tig. 214 ^4) ; or a ridge (e.g. Pteris, Blechnum). 



FIG. 215. A dehisced spor- 

 angium of Asiridium Fillx-mas 

 (xOO): a the stalk, with a 

 glandular hair p ; r tho annu- 

 lus ; the stomium. 



FIG. 216. Sections of young sporangia; A of a 

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

 turn; a s archesporium. 



The sorus is quite bare in many forms (Gleicheniacese ; 

 Osmundacese ; Alsophila among Cyatheacese ; Schizseaceae, except 

 Lygodium ; Polypodiese) ; 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, it is somewhat cup- 

 shaped : thus it is urceolate and entire in Trichomanes (Fig. 214 

 A), Lygodium, Cyathea, Davallia. When it springs from the apex 

 of the placenta, above the sorus, the indusium has the general 

 appearance of a peltate scale, either orbicular in outline (Aspidium) 

 or reniform (Nephrodium, Fig. 214 D). When it is developed on 

 one side of the sorus, the indusium is a long narrow scale, attached 



