Key to the Isoetes. 



I. — Plants normally submerged 



Spores with low tubercles or warts. /. Bolanderi 



Spores with slender spines . . /. echinospora 



Spores with short rounded crests . /. Macouni 



Spores with worm-like crests . /. Jiivroglyphica 



Spores with thin crests sparingly anastomosing 



/. lacustris 

 Spores with many anastomosing ridges 



/. Tuckerniani 

 II. — Plants normally exposed in summer 

 Indusium complete 



Spores with fine warts . . /. melnnospora 



Spores with heavy tubercles . I.flaccida 



Indusium incomplete 



Spores with faint crests and warts /. Howelli 

 Spores with numerous truncate spines /. Gravesii 

 Spores with spines and short crests 



Mostly spinulose . . /. saccJiaraia 



Crests often anastomosing /. riparia 



Spores reticulated 



Ridges sub-continuous . /. Canadensis 

 Ridges anastomosing 



Ridges thin . . /. Engelina?im' 



Ridges moderately thick /. vah'da 



Ridges very thick . /. fcweolata 

 III. — Plants seldom if ever submerged 



Spores srnooth . . . /. Orcu/ti 



Spores faintly warty 



Warts few . . . /. Nitttallii 



Warts many 



Commissures wide . /. melanopoila 



Commissures narrow . /. Buileri 



Spores with short spines . . /. minima 



