Isoëtes
Sp. Pl. 2: 1100. 1753; Gen. Pl. ed. 5, 486, 1754.
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.
Distribution
Nearly worldwide
Discussion
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.
Species ca. 150 (24 in the flora).
Selected References
Lower Taxa
Key
| 1 | Megaspores with echinate texture. | > 2 |
| 1 | Megaspores with cristate, reticulate, rugulate, tuberculate, or smooth textures. | > 3 |
| 2 | Megaspores echinate with thin, sharp spines; girdle obscure; microspores averaging less than 30 µm; circumboreal. | Isoëtes echinospora |
| 2 | Megaspores echinate to short-cristate with stout, blunt spines and crests; girdle densely echinate; microspores averaging more than 30 µm; Alaska, British Columbia, Washington. | Isoëtes maritima |
| 3 | Plants e of Rocky Mountains. | > 4 |
| 3 | Plants of Rocky Mountains and further west. | > 20 |
| 4 | Plants submerged or emergent aquatics, of permanent lakes, ponds, streams, estuaries, and bogs, or of persistently wet soil. | > 5 |
| 4 | Plants terrestrial or becoming so, of seasonally saturated soil, temporary pools, and streams. | > 15 |
| 5 | Megaspores averaging more than 600 µm diam.; plants submerged aquatics. | Isoëtes lacustris |
| 5 | Megaspores averaging less than 600 µm diam.; plants submerged or emergent aquatics. | > 6 |
| 6 | Velum covering more than 1/2 of sporangium. | > 7 |
| 6 | Velum covering less than 1/2 of sporangium. | > 10 |
| 7 | Velum covering less than 3/4 of sporangium. | > 8 |
| 7 | Velum covering entire sporangium. | > 9 |
| 8 | Sporangium wall brown-streaked; megaspores cristate to reticulate with broad jagged ridges; plants of Georgia. | Isoëtes georgiana |
| 8 | Sporangium wall unpigmented; megaspores cristate to reticulate with broken lamellate ridges; plants of North Carolina, Tennessee, Virginia, and West Virginia. | Isoëtes caroliniana |
| 9 | Leaves dark green, rigid; plants of New Brunswick and Nova Scotia, and Maine. | Isoëtes prototypus |
| 9 | Leaves bright green, pliant; plants of Florida and Georgia. | Isoëtes flaccida |
| 10 | Megaspores averaging less than 500 µm diam.; reticulate with unbroken lamellate ridges. | Isoëtes engelmannii |
| 10 | Megaspores averaging more than 500 µm diam.; reticulate, rugulate, or cristate with isolated or broken ridges. | > 11 |
| 11 | Megaspores with densely papillate or smooth girdle; leaves olive green to reddish brown. | > 12 |
| 11 | Megaspores with obscure girdle; leaves bright green. | > 13 |
| 12 | Megaspores with densely papillate girdle, reticulate or cristate with ridges having irregular and roughened crests. | Isoëtes tuckermanii |
| 12 | Megaspores with smooth girdle, rugulate to reticulate with ridges having rounded and smooth crests. | Isoëtes acadiensis |
| 13 | Megaspores cristate, with isolated and branching, lamellate ridges; plants of ne North America and e seaboard. | Isoëtes riparia |
| 13 | Megaspores cristate to reticulate, with branched and interconnected ridges; plants of s United States. | > 14 |
| 14 | Megaspores with thick ridges; plants of Louisiana. | Isoëtes louisianensis |
| 14 | Megaspores with thin ridges or lamellae; plants of Georgia. | Isoëtes boomii |
| 15 | Velum covering entire sporangium; megaspores gray, brown, or black; plants of temporary pools on granite outcrops. | > 16 |
| 15 | Velum covering less than 3/4 of sporangium; megaspores white; plants of varied habitats. | > 18 |
| 16 | Megaspores obscurely rugulate; plants of Texas. | Isoëtes lithophila |
| 16 | Megaspores tuberculate; plants of Georgia. | > 17 |
| 17 | Leaves distichously arranged; rootstock horizontally elongated, mat-forming. | Isoëtes tegetiformans |
| 17 | Leaves spirally arranged; rootstock nearly globose, not mat-forming. | Isoëtes melanospora |
| 18 | Megaspores averaging more than 450 µm diam.; plants of calcareous soil. | Isoëtes butleri |
| 18 | Megaspores averaging less than 450 µm diam.; plants of noncalcareous soil. | > 19 |
| 19 | Megaspores obscurely rugulate; leaves pale to lustrous black toward base; plants of e, c United States. | Isoëtes melanopoda |
| 19 | Megaspores boldly tuberculate to rugulate; leaves pale to dark brown toward base; plants of se United States. | Isoëtes virginica |
| 20 | Plants submerged or emergent aquatics, of persistent lakes and pools. | > 21 |
| 20 | Plants terrestrial or becoming so, of seasonally saturated soil, temporary streams, vernal pools. | > 22 |
| 21 | Megaspores averaging less than 500 µm diam.; leaves abruptly tapering to fine tip. | Isoëtes bolanderi |
| 21 | Megaspores averaging more than 500 µm diam.; leaves gradually tapering to tip. | Isoëtes occidentalis |
| 22 | Velum covering less than 3/4 of sporangium; sporangium wall brown-streaked to completely brown. | Isoëtes howellii |
| 22 | Velum covering entire sporangium; sporangium wall unpigmented. | > 23 |
| 23 | Plants of seasonally saturated soil, temporary streams; leaves generally more than 8 cm, rigid, almost brittle; megaspores averaging more than 350 µm diam. | Isoëtes nuttallii |
| 23 | Plants of vernal pools; leaves generally less than 8 cm, pliant; megaspores averaging less than 350 µm diam. | Isoëtes orcuttii |