Abstract
Morphological, phytogeographic, ecological, preliminary genetic, and C-value evidence indicates that populations of Isoetes confined to freshwater tidal marsh habitats along the St. Lawrence River in Quebec, Canada, represent an undescribed endemic species, proposed here as Isoetes laurentiana sp. nov. The combination of short velum coverage (ca. 10%) over a plain (unstreaked) sporangium, broad (ca. 1.0 mm) erect leaves, small megaspores (ca. 459 μm) with more densely convoluted-reticulate, almost echinate ornamentation, and its occurrence within an exceptional habitat, readily distinguish I. laurentiana from I. tuckermanii A.Braun with which it was formerly combined. Isoetes laurentiana is one of a series of endemic taxa of the floristically extraordinary St. Lawrence River estuary. Some populations of I. laurentiana are immense; in cultivation it demonstrates the capacity for prolific production of new individuals, which is presumably necessary to sustain such local abundance. Isoetes laurentiana is of national and global phytogeographic significance, but is not considered to represent a species at risk according to standards from the International Union for Conservation of Nature.
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