INTRODUCTION TO CRYPTOGAMIC BOTANY. 527 



sporangia crowded mstead of biseriate. Schizcem are eminently 

 peat plants, but are also found on clay banks, and sometimes 

 in white sand, under bushes ; they are common in the tropics, 

 but far rarer in the northern than in the southern hemisphere. 

 One, however, occurs in the United States, as high as New 

 Jersey. The species are very variable and widely distributed, 

 and are probably all reducible to four. If the groove of the 

 stem be observed, which indicates the upper surface, it will be 

 seen that the spikelets are resupinate. 



4. Paekeriace^, Hook, Presl. 



Aquatic. Sterile fronds membranous, with a thick vesicu- 

 lar footstalk, floating; fertile, repeatedly divided, divisions 

 linear. Sporangia nearly ringless, or furnished with a broad 

 incomplete ring. 



594. This small tribe consists of but two genera, of which 

 Ceratopteris has received half a dozen generic names. The 

 sterile leaves float on the surface of the water like Trapa natans. 

 They are singularly proliferous, short and membranous, taper- 

 ing into a thick vesicular footstalk. The fertile fronds, which 

 are not in general perfected till after the sterile fronds have 

 decayed, are, on the contrary, perpendicular, repeatedly divided 

 and forked ; the divisions linear. The nearly ringless, irregu- 

 larly bursting sporangia are arranged along the margin, like 

 those of a Pteris, and have a distinct indusium. Ceratopteris 

 has likewise continuous sori, occupying the longitudinal veins. 

 The sporangia have a very broad incomplete ring, and thus 

 connect the exannulate with the annulate ferns. The spores 

 are triangular, marked with three sets of concentric ridges. 

 Parkeria scarcely difiers, except in the more incomplete ring. 

 They are inhabitants of tropical countries. Parkeria is fre- 

 quently cultivated in our stoves. 



5. ACEOSTICHACE^E, Pr. 



Sori naked, occupying the whole inferior surface, and in a 

 few cases the upper surface ; sporangia seated on the veins, 

 veinlets, and parenchym. 



595. We commence here with those ferns which have been 

 included under the common name of GathetogyratcB by 

 Bernhardi, or of Polypodece by Endlicher and others, with 



