76 OUR NATIVE PTERIDOPHYTA. 



late, one-celled, encircled by a more or less complete, jointed 

 elastic ring, collected in clusters of various forms on the under 

 surface of the frond, with or without an indusium or covering ; 

 or panicled, or spiked and naked ; or borne on receptacles of 

 various kinds. Spores of various forms, minute. Prothallium 

 above ground, green, monoecious or dioecious. Contains six 

 well-marked sub-orders, four of which are represented with us. 

 Genera, seventy,* of which we have representatives of thirty. 



SUB-ORDER I.-POLYPODIACE>E Presl. 



Sporangia pedicelled, surrounded more or less completely by 

 a jointed, vertical, and elastic ring, bursting transversely. Sori 

 dorsal or marginal, borne on a leafy frond, with or without 

 indusia. 



TRIBE I. ACROSTICHES. Sporangia spread in a stra- 

 tum over the under surface, or rarely over both surfaces of the 

 frond. Indusia wanting. 



I. Acrostichum L. Sori covering the entire surface of 

 the upper pinnae. 



TRIBE II. POLYPODIES. Sori dorsal, borne at or near 

 the ends of the veinlets, without indusia. 



II. Polypodium L. Possessing characters of the tribe. 

 TRIBE III. GRAMMITIDES. Sori dorsal, variously 



arising from the veins, usually linear. Indusia wanting. 



III. Gymnogramme Desv. Sori oblong or linear, follow- 

 ing the course of the veinlets. 



IV. Notholaena R. Br. Sori on the veins or near their 

 extremities, roundish or oblong, soon confluent into a narrow 

 marginal band. 



V. Tsenitis Swz. Sori linear, central, or submarginal. 

 TRIBE IV. VITTARIES. Sporangia borne in a continu- 

 ous marginal or intra- marginal furrow. 



VI. Vittaria Sm. Fronds simple, linear, grass-like. 

 TRIBE V. PTERIDES. Sori marginal or intra-marginal, 



provided with an indusium formed of the reflexed margin of 

 the frond, and opening inwardly. 



* This number is based on Hooker's classification. Other authors, narrow- 

 ing the limits of generic characters, recognize a greater number. Smith, for 

 example, publishes 220 and Presl 230, yet the tendency among most botanists 

 is to restrict the number. 



