Azolla caroliniana
Azolla caroliniana Willd.
(of Carolina, a region which historically included much of the se U.S.; early botanists gave many plants this epithet; in this case the type description says “Habitat in aquis Carolinae”)
Local names: mosquito fern, water fern, Carolina mosquito fern, eastern mosquito fern
Plant ranging from green to with red margins or dark red; largest hairs on upper leaf lobe with 2 or 3 cells, the broad basal cell often 1⁄2 or more height of hair, the short apical cell curved, with tip nearly parallel to leaf surface; megaspores without bumps or pits, but with dense covering of tangled filaments, the megaspores, however, usually not present since plants are rarely fertile; 2n = usually 44, rarely 66 (Stergianou & Fowler 1990). Quiet water of ponds, lakes, or slow-moving streams or stranded on mud; Austin (S.G. Jones 7807), Brazos, Cherokee, Colorado, Dallas, Franklin, Harrison, Hays, Liberty, Madison, Montgomery (A.K. Neill 1997), Newton, Nueces, Tarrant, Travis cos. (all BRIT, specimens annotated by J. Peck), and Jim Wells (M.C. Johnston 542139, TEX-LL, identified V.M. Bates using SEM) cos.; sporadically but widely scattered in e 1⁄2 of TX; B.C., Ont., and e U.S. from NH s to FL w to SD and TX. Sporulating rarely summer–fall (Fertile specimens are rarely collected—Nauman et al. 2000). This species has long been considered to be the only Azolla in the e portion of TX, and it and A. microphylla are often nearly impossible to distinguish when sterile. Therefore, numerous herbarium and literature records of A. caroliniana may be the result of confusion with A. microphylla—these records of uncertain identity are mapped as open circles on the county distribution map; black dots on the county map indicate specimens annotated as A. caroliniana by J. Peck (pteridologist with expertise in Azolla) or by SEM identification (see further discussion under A. microphylla). Although there is “ample morphological and molecular evidence” to recognize A. caroliniana as a separate species [from A. microphylla], there is a nomenclatural problem—the type specimen is sterile and is thought by some to actually be of a different species (Reid et al. 2006). As a result, the species currently going by the name A. caroliniana “may require a nomenclatural change pending the satisfactory identification of the current type.” Scanning electron micrographs of the megaspore apparatus of this species and the related A. microphylla (as A. mexicana) can be seen in Perkins et al. (1985) and Lumpkin (1993). Where found, A. caroliniana is often abundant, and huge numbers of individuals can at certain times of the summer turn the surface of ponds a striking red color (G. Diggs, pers. obs.). According to Correll (1956), “Its occurrence in remote locations is doubtless due to its dissemination, in part, by water-fowl.” The species is commonly cultivated as an aquatic ornamental in aquaria and decorative pools (Lellinger 1985). It is often found growing unintentionally at nurseries and water garden stores where aquatic plants (e.g., waterlilies) are sold in TX; it grows easily in decorative “ponds” or water gardens in TX and overwinters without problems in such situations (e.g., Grayson Co. near the TX-OK border, G. Diggs, pers. obs.) even in temperatures reaching 10° F (-12° C).