Quillwort, isoète [Greek isos, equal, and etos, year, referring to evergreen habit of some species]

Rootstock 2(--3)-lobed, nearly globose to horizontally spindle-shaped and proliferous, corky. Leaves several to many, erect to spreading, straight to recurved, 1--100 cm; ligule deltate to cordiform, 1--6 mm, membranous. Sporangia ovoid to ellipsoid or oblong, 3--15 mm, walls unpigmented or brown-streaked to completely brown, traversed internally by trabeculae (internal partitions). Megaspores white, gray, or black, globose, mostly 300--700 µm diam., trilete, each with equatorial ridge and 3 converging proximal ridges, smooth or with spines, tubercles, or ridges. Microspores grayish or brownish in mass, reniform, mostly 20--50 µm, monolete, smooth or textured with spines, tubercles, or ridges. x = 11.

Species ca. 150 (24 in the flora): nearly worldwide.

Geography, habitat, megaspore texture, spore size, and velum provide features that will distinguish Isoëtes taxa. The northeastern, southeastern, and western regions of North America contain endemic species. Within these geographic regions, habitats are important for identification. Some species are submerged or emergent aquatics growing in permanent lakes, ponds, streams, estuaries, or bogs, or on persistently wet soil. Others are temporary aquatics that pass into dormancy as the pools and streams that they inhabit dry. A few are seasonal terrestrials, actively growing in spring when the soil is saturated.

Texture and size of mature, dry megaspores are usually required for identification. A 10× hand lens will adequately resolve megaspore textures of some species, but magnification of 30× or more is required for others. A compound microscope fitted with an ocular micrometer is necessary to determine spore size. Twenty spores should be measured to determine their average size.

Normally, megaspores are globose and marked with four bold ridges: an equatorial ridge encircling the spore and three radial ridges converging at the proximal pole of the spore. Between these ridges, megaspore textures may be echinate, cristate, reticulate, rugulate, tuberculate, or nearly smooth. A zone around the spore along the distal side of the equatorial ridge is called the girdle. The girdle is obscure when it is textured like the rest of the spore or it is distinguishable when it is textured differently.

Plants collected early in the growing season possess small, fragile, yellowish white, immature megaspores that have an underdeveloped smooth or mealy surface. Such specimens are difficult to identify and have been a source of taxonomic confusion. Mature spores may be found in decaying leaf bases or in soil around the rootstock throughout the year, but plants collected soon after their growing season are easiest to identify because spores are well developed and still within sporangia.

The velum, a thin flap of tissue completely or partially covering the adaxial wall of the sporangium, also has diagnostic value. Seven taxa in this flora have a velum covering the entire sporangium wall (see illustration of Isoëtes nuttallii leaf base). The others here have a velum covering less than three-fourths of the sporangium wall (see illustration of I . howellii leaf base). Although the velum is usually membranous, it is reasonably durable and its coverage can be determined by using forceps to lift it off the sporangium wall.

Interspecific hybrids, which are frequent, have confused clear distinctions between species. These hybrids, recognized by often flattened, malformed spores of variable size, shape, and texture, may be expected where two or more species occur together.

Species of Isoëtes appear to have evolved in two ways, either by ecological isolation and genetic divergence as taxa adapted to terrestrial or aquatic habitats or through interspecific hybridization and chromosome doubling as divergent species migrated, possibly via waterfowl, into the same aquatic habitats. Recognition of primary diploid species, interspecific hybrids, and allopolyploids reduces confusion in the identification of Isoëtes species.

SELECTED REFERENCES

Boom, B. M. 1982. Synopsis of Isoëtes in the southeastern United States. Castanea 47: 38--59. Cody, W. J. and D. M. Britton. 1989. Ferns and Fern Allies of Canada. Ottawa. Eaton, A. A. 1900. The genus Isoëtes in New England. Fernwort Pap. 2: 1--16. Engelmann, G. 1882. The genus Isoëtes in North America. Trans. Acad. Sci. St. Louis 4: 358--390. Hickey, R. J. 1986. Isoëtes megaspore surface morphology: Nomenclature, variation, and systematic importance. Amer. Fern J. 76: 1--16. Hickey, R. J., W. C. Taylor, and N. T. Luebke. 1989. The species concept in Pteridophyta with special reference to Isoëtes. Amer. Fern J. 79: 78--89. Kott, L. S. and D. M. Britton. 1983. Spore morphology and taxonomy of Isoëtes in northeastern North America. Canad. J. Bot. 61: 3140--3163. Reed, C. F. 1965. Isoëtes in southeastern United States. Phytologia 12: 369--400. Soper, J. H. and S. Rao. 1958. Isoëtes in eastern Canada. Amer. Fern J. 48: 97--102. Taylor, T. M. C. 1970. Pacific Northwest Ferns and Their Allies. Toronto. Taylor, W. C. and R. J. Hickey. 1992. Habitat, evolution, and speciation of Isoëtes. Ann. Missouri Bot. Gard. 79: 613--622. Taylor, W. C., N. T. Luebke, and M. B. Smith. 1985. Speciation and hybridization in North American quillworts. Proc. Roy. Soc. Edinburgh, B 86: 259--263.