ISOETACEsE. 143 



microspores. Name from Gr. zero?, equal, and eroS, year. Con- 

 tains about 50 species, of which sixteen are found within our 

 limits. 



NOTE. The measurements of the spores are given in millimetres; mm. = .03937 

 inch. 



I . Submerged, rarely above water in driest seasons ; leaves 

 quadrangular without peripheral bast bundles ; velum incom- 

 plete. 



* Stomata absent. 



1. I. lacustris L. Leaves 10 25, stout, rather rig?d, ob- 

 tusely quadrangular, acute but scarcely tapering, dark or olive- 

 green, 2' 6' long ; sporangia orbicular broadly-elliptic, with a 

 narrow velum ; ligula triangular, short or somewhat elongate ; 

 macrospores 0.50 0.80 mm. in diameter, marked all over with 

 distinct or somewhat confluent crests ; microspores smooth, 

 0.035 0.046 mm. long. Var. pauper cula Engelm. has fewer, 

 thinner and shorter leaves and smaller spores, the microspores 

 somewhat granulate, 0.026 0.036 mm. long. (/. macrospora 

 Durieu.) Catskill Mountains, New York (Schweinitz), Echo 

 Lake, New Hampshire (Tuckerman), Fresh Pond, near Cam- 

 bridge, Massachusetts (W. Boott), Uxbridge, Massachusetts 

 (Robbins), Brattleborough, Vermont (Frost}, Lake Superior 

 (Porter). The variety from Grand Lake, Middle Park, Colo- 

 rado (Engelmami) and Castle Lake near Mt. Shasta, California 

 (Pringle). 



2. I. pygrnaea Engelm. Leaves 5 10, stout, rigid, bright- 

 green, ' i' long, abruptly tapering to a fine point, with very 

 short often almost square epidermal cells; sporangia orbicular 

 with a narrow velum ; macrospores 0.36 0.50 mm. thick, 

 marked with minute, rather regular, distinct or rarely confluent 

 warts; microspores brown, almost smooth, 0.024 0.029 mm. 

 long. Mono Pass, California (Bolander). 



3. 1. Tuckermani A. Br. Leaves 10 30, very slender, 

 tapering, olive-green, 2' 3' long, the outer recurved ; sporangia 

 mostly oblong, white or rarely brown-spotted, the upper third 

 covered by the velum; macrospores 0.44 0.56 mm. thick, the 

 upper segments marked with prominent, somewhat parallel and 

 branching ridges, the lower half reticulate ; microspores smooth 



