ON GENERA AND SPECIES. 19 



Accessory indusia are formed of the entire margin of the 

 frond, or a poi'tion of it being changed in texture and form 

 and inflexed, its margin conniving with the free margin 

 of the interiorly attached special indusium, the two thus 

 combined forming a continuous or interrupted groove, 

 opening exteriorly. When the special indusium is pro- 

 duced on a punctiform receptacle, an urceolate, bilabiate, or 

 tubular cyst is formed, which contains the sporangia, some 

 being exerted beyond the margin (pedicellate) ; while in 

 some cases the cyst is antemarginal, and is then termed 

 cucullate, in all these cases opening exteriorly, but often 

 more or less turned inwards, and then having the appearance 

 of being- on the under side of the marg-iu. 



Universal indusia occurs when the segments of the fertile 

 fronds are contracted. They consist of the margins of the 

 segments being more or less changed in texture, becoming 

 membranous, and folded inwards (involute*), the two 

 opposite marg'ins generally meeting, and thus enclosing all 

 the sori upon the segment, whatever may be their form or 

 position upon the veins, the segments thus assuming the 

 form of a pod or siliqua {siliquiform) as in Struthiopteris ; 

 or of a berry (bacciform) as in Onoclea. 



In some Polypodece the sori are furnished with scales of 

 various forms ; in Pleopeltis they consist of numerous 

 orbicular peltate scales, imbricate over one another, and 

 covering the sporang-ia of each sori ; in Hymenolepis they 

 are very thin and membranous ; in Lopliolepis they consist 

 of numerous slender scales, longer than the sporangia ; in 

 Scliellolepis they are irregularly laciniated ; club like and 

 other forms are also common to the genera Vittaria and 

 Tcenitis. Such bodies are considered to be imperfect 



* That is, the margins turned or rolled inwards, so as to face and 

 meet one another on the under side of the frond or segment. 



