Isoëtes occidentalis L. F. Henderson, Bull. Torrey Bot. Club. 27: 358. 1900.
Isoëtes flettii (A. A. Eaton) N. E. Pfeiffer; I. lacustris Linnaeus var. paupercula Engelmann; I. paupercula (Engelmann) A. A. Eaton; I. piperi A. A. Eaton
Plants aquatic. Rootstock nearly globose, 2-lobed. Leaves evergreen, dark green, pale toward base, spirally arranged, to 20 cm, rigid, gradually tapering to tip. Velum covering less than 1/2 sporangium. Sporangium wall unpigmented. Megaspores 500--700 μm diam., cristate, tuberculate, rugulate, or echinate with ridges, tubercles, or spines; girdle smooth. Microspores brown in mass, 35--45 μm, papillose to spinulose. 2n = 66.
Spores mature in late summer. Lakes; B.C.; Alaska, Calif., Colo., Idaho, Mont., Oreg., Utah, Wash., Wyo.
Megaspores of Isoëtes occidentalis are variable in wall pattern. Populations exist with rugulate or tuberculate megaspores and other population with cristate to echinate megaspores. Plants with thin-walled megaspores that crack easily have been called I. paupercula. Populations in which megaspores have short ridges and tubercles in a band along the equator have been called I. flettii. Populations with broad-based tubercles on the megaspores have been called I. piperi. The variation in megaspore pattern may indicate multiple allopolyploid origins for I. occidentalis.
The general aspect of Isoëtes occidentalis and its tough, dark green leaves suggested to early workers an affinity with I. lacustris.
Isoëtes occidentalis hybridizes with I. bolanderi, I. echinospora, and I. maritima [ = I. x truncata (A. A. Eaton) Clute].