PTERIDOPHYTA— FERNS 315 



Gleicheniaceae, some tropical ferns of few species ; the Schizeaceae represented 

 by the Small Curly Grass (Schisaea pusilla), Climbing Fern, {Lygodium palmat- 

 um), sporangia ovoid or sessile provided with an apical ring, a family contain- 

 ing about 100 species ; the Polypodiaceae the largest sub-order with 200 genera 

 and 3000 species ; Marsiliaceae containing Marsilia a common aquatic or semi- 

 aquatic plant ; represented by Salvinia, Azolla, also aquatic. The order Ma- 

 rattiales, contains the Marattia, tropical. 



It is not at all strange that the ferns should be poisonous since Greshoff 

 and others have reported the presence of hydrocyanic acid in these plants. 

 Greshoff says the odor of oil of bitter almonds is especially intense in the young 

 leaves of Cystopteris fragilis Bernh; and there is also a trace of HCN in the 

 spores. He also calls attention to the presence of the same substance in the 

 common brake (Pteris aquilina), and states that several tropical ferns namely 

 Davallia brasiliensis, and other species are cyanogenetic, and that one fern, the 

 D. pentaphylla, forms a large amount of this substance, especially the cultivated 

 form elegantissinia. Several species of the Glcichenia contain saponin. 



POLYPODIACEAE 



Perennial with horizontal erect, short or elongated rootstocks ; leaves 

 various, entire, pinnate, pinnatifid, or decompound, vernation circinate (coiled) ; 

 sori on the margins of the leaf or on the lower side, generally without an 

 indusium (covering) ; sporangia with a vertical many celled incomplete ring, 

 which on straightening out ruptures and discharges the spores. A few of the 

 ferns, as Male Shield fern Aspidium Filix-mas, and the A. marginale, are used 

 in medicine. 



KEY FOR THE GENERA 



Indusium absent Polypodium 



Indusium present, evident. 

 Sori marginal. 



Indusium with margin of frond rolled over. 

 Sporangia borne on a continuous marginal vein-like receptacle. 



Stipe light colored Pteris 



Sporangia on the ends of the veins. Stipe black Adiantum 



Sori on back with special indusium covering the same. 



Sori linear or oblong Asplenium 



Sori roundish on the back or rarely the apex of the vein. 

 Stipe not articulated. 



Indusium flat or slightly convex or round reniform, fixed by 



the center, opening all round the margin Aspidium 



Indusium convex, fixed by a broad base, commonly reflexed 



as the sporangia ripen Cystopteris 



Indusium obscure, leaves closely rolled together with necklace-like seg- 

 ments Onoclea 



Polypodium. L. Polypody 



Simple or pinnate fronds from horizontal rootstocks ; stipes articulated 

 to the rootstocks; sori (fruit dots) round, naked on the back of the frond 

 in one or more rows each side of the midrib or scattered ; indusium wanting. 

 About 350 species, mostly tropical. The species in the Northern United States 



