Isoetes microvela (Isoetaceae), a new quillwort from the coastal plain of the southeastern United States

Isoetes microvela, sp. nov., is described from cytologically confirmed hexaploid populations from the coastal plain of North Carolina. At least one population grows over thinly-buried calcareous bedrock, a rare condition on the predominantly acidic southeastern coastal plain. It is a large quillwort of periodically inundated and scoured stream banks and shallow water. It is characterized by a densely short-crested to reticulate-tuberculate megaspore ornamentation pattern intermediate in appearance between that of I. appalachiana and I. hyemalis, an exceptionally small velum covering ± 10% of the heavily brown-streaked sporangium, and by obscurely tuberculate microspores. A key to the Isoetes of the southeastern coastal plain is presented. Isoetes microvela is suspected to represent an allopolyploid derived from the doubling of a sterile triploid hybrid, most likely I. ×bruntonii (= I. engelmannii × I. hyemalis) or I. appalachiana × I. engelmannii. Isoetes microvela is a rare species, presently known from only two populations.