Publication: Bull. Torrey Bot. Club. 27: 358. 1900.
selected vouchers: WTU
Notes: FNA2: "Megaspores of Isoetes 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 Isoetes occidentalis and its tough, dark green leaves suggested to early workers an affinity with I. lacustris.
Isoetes occidentalis hybridizes with I. bolanderi, I. echinospora, and I. maritima [ = I. x truncata (A. A. Eaton) Clute]."