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Key to Isoetes

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1 Plants with narrow (0.5-1.0 mm wide), stiff leaves 3-20 cm long; leaves ± erect; emergent in sand and fine gravel of shallow, ephemeral bedrock pools and seeps in woodland glades or on open outcrops.
  2 Leaves grass-green; megaspore surface granular or unornamented, > 525 μm in diameter; [in basic (limestone) substrates]; [2n: n. AL, nw. GA and northward]
  2 Leaves gray-green to bright-green; megaspore surface ridged or tuberculate, < 525 μm in diameter; [in acidic substrates].
    3 Plants 10-15 cm tall, individual plants apparent; leaf bases often with shiny black phyllopods; velum coverage < 30%; megaspores white to light gray, 400-525 μm in diameter.
      4 Megaspores typically 440-510 μm in diameter; ornamentation pattern wrinkled to low tuberculate; [plants on granite outcrops]; [4n: Piedmont of e. AL (and w. GA?)]
      4 Megaspores typically < 450 μm in diameter, ornamentation pattern sparsely wrinkled, short-ridged (some with pseudo-reticulate configuration) or densely tuberculate; [plants on granite or sandstone outcrops]; [2X: Piedmont of e. AL to s. VA; 4n populations reported in s. GA and se. VA]
    3 Plants 2-10 cm tall; individuals densely clustered or mat-forming, often not readily discernable; leaf bases pale (black phyllopods absent); velum coverage > 70% (as little as 25% in Isoetes species 1); megaspores light gray to black, < 400 μm in diameter.
        5 Plants 3-8 (-10) cm tall; velum coverage 40-60% (occasionally as little as 25%); megaspores olive-gray to brown; [2n: Lancaster County, Piedmont of SC]
        5 Plants 2-4 (-7) cm tall; velum coverage 70-100%; megaspores pale gray to black.
          6 Plants in large, sometimes densely clustered colonies, 3-5 (-7) cm tall, with globose rootstocks (some with elongated rootstalks); megaspores ± dull, densely ornamented with fine, often low and/ or obscure tubercles; [2n: Piedmont of GA]
          6 Plants in dense, continuous mats; frequently joined, 2-3 (-4) cm tall, with elongated rootstalks (no globose rootstocks); megaspores ± shiny, unornamented or obscurely wrinkled; [2n: Piedmont of GA]
1 Plants with broad (1.25-2.25 mm wide), ± flaccid leaves > 15 cm long; submerged or emergent in mineral or organic substrate of swamps, marshes, ponds, rivers, shores or open graminoid swales.
             7 Megaspores > 575 μm in diameter, with reticulate ornamentation.
               8 Velum coverage < 30%; plants with leaves 15-20 cm long in fast-flowing open streams; [Mountains of TN or VA and northwards].
                 9 Megaspore muri unevenly tall in congested, broken-reticulate pattern; [10n: n. VA, n. NJ northwards]
                 9 Megaspore muri evenly tall in uncongested, evenly-reticulate pattern; [8n: se. TN]
               8 Velum coverage > 30%; plants with leaves 20-45 cm long in slow-flowing woodland stream channels; [Coastal Plain of GA, FL, and AL].
                   10 Megaspore ornamentation congested with thinner but still moderately thick muri; velum coverage ± 30%; [6n: s. GA, ne. FL, se. AL]
                   10 Megaspore ornamentation open with thick muri; velum coverage ± 60%; [6n: s. GA]
             7 Megaspores < 575 μm in diameter; with conspicuously echinate (spiny), reticulate, nearly spiny, low tuberculate and/or vermiform (worm-like) ornamentation.
                     11 Velum coverage > 40% (uncommonly to ± 30% in Isoetes valida).
                       12 Plants with broad (1.25-2.25 mm wide), light yellow-green, ± firm, reflexed leaves; velum coverage 45-70% (uncommonly only 30%); megaspores with tall, uneven, thin muri in ragged-reticulate pattern; [2n: found in woodland seepages (less commonly in pools); [primarily Mountains, uncommonly of Piedmont and Coastal Plain, PA to s. AL; also ne. MS]
                       12 Plants with narrow (1.0-1.5 mm wide), dark-green, straggly to widely reflexed leaves; velum coverage 80-100%; megaspores with low tubercles or broad ridges (mounds) of even height in broken to evenly reticulate pattern; [Coastal Plain].
                          13 Megaspores mostly psilate with only occasional tubercles or low muri; plants of Texas coastal prairies [2n: Calhoun and Aransas Cos., TX]
                          13 Megaspores tuberculate; plants of streams and swamps [nearly endemic to FL].
                            14 Megaspores ca. 500 μm in diameter, with dense ornamentation pattern of small tubercles; leaves 1.25-1.5 mm wide; [2n: Jackson Co., FL]
                            14 Megaspores < 450 μm in diameter, with coarse ornamentation pattern of broad tubercles or loosely interconnected vermiform mounds; leaves ca. 1.0 mm wide; [2n: FL, sw. GA, se. AL Coastal Plain]
                              15 Megaspore ornamentation of low vermiform muri in semi-reticulate pattern or with low tubercles.
                                16 Megaspore ornamentation of semi-reticulate pattern or with (usually obscure) tubercles; megaspores 440-480 (-520) μm in diameter (tetraploids).
                                  17 Leaves firm, ± stiffly erect (Juncus-like); sporangium heavily brown-streaked and with 20-40% velum coverage; megaspore surface smooth, prominently ornamented with broad, often vermiform muri; [4n: s. GA Coastal Plain]
                                  17 Leaves reflexed to loosely erect; sporangium ± unmarked and with ca. 10% velum coverage; megaspore surface mealy, sparsely to (uncommonly) densely ornamented with ± obscure tubercles and/or narrow muri; [4n: Piedmont, s. VA and n. NC]
                                16 Megaspore ornamentation plain or with irregular pattern of distinct, often numerous tubercles; megaspores < 440 μm in diameter (diploids).
                                    18 Megaspores < 360 μm in diameter; megaspores ornamented with dense pattern of ± narrow, short muri; velum coverage 15-35%.
                                    18 Megaspores > 360 μm in diameter; megaspores plain or ornamented with vermiform muri and/or broad tubercles; velum coverage 5-15%; [ephemeral inland freshwater pools and swales].
                                         20 Broad (± 1.3 mm wide) light green leaves, bases never becoming black-brown and hardened; megaspores ± 410 (to 440) μm in diameter in diameter; conspicuously ornamented with numerous low tubercles or ridges; [2n: woodland pools of Piedmont of VA to AL and se. MS
                                         20 Narrow (± 1 mm wide) gray-green leaves with shiny, black-brown bases when spores are mature; megaspores ± 380 μm in diameter in diameter; typically with plain and mealy surface or obscurely ornamented with scattered tubercles and ridges; [2n: wet prairies and open graminoid swales of s. MS, w. AL and westward
                              15 Megaspore ornamentation in distinctly reticulate (‘honeycomb’) pattern or with thin sharp spines or otherwise dense pattern of tall, short-crested, almost echinate muri.
                                           21 Plants emergent or aquatic; megaspores ± 460 (occasionally to 500) μm in diameter; evenly-reticulate ornamentation pattern of even-topped muri continuous to the equatorial ridge without an equatorial band; [2n: Atlantic Coastal Plain, Appalachian Mts and irregularly northward, south to c GA, ne AL]
                                           21 Plants primarily emergent or amphibious; megaspores > 520 (rarely to over 600) μm in diameter; unevenly-reticulate and interrupted ornamentation pattern of ragged-topped muri, with an equatorial band of few to numerous spines; [4n or 6n].
                                             22 Megaspores primarily reticulate, often bearing tubercles or some equatorial spines but not densely echinate throughout; [collectively widespread]
                                               23 Leaves narrow (< 2.0 mm wide), blackish-green to olive green; megaspore ornamentation broken-reticulate to almost spiny with numerous tubercles often with short, mostly stand-alone muri or with broken ridges, usually with an equatorial band of numerous spines.
                                                 24 Leaves of mature plants 20-45 cm long, 1.0-1.5 mm wide, slender, wiry, dark (blackish) green; velum coverage 10-20%; moderately congested, almost echinate megaspore ornamentation pattern; aquatic in blackwater streams; [4n: Coastal Plain, VA to AL]
                                                 24 Leaves of mature plants 10-20 cm long, 1.25-2.0 mm wide, olive-green; velum coverage 25-30%; densely congested (uncommonly uncongested and low-reticulate) megaspore ornamentation pattern; emergent on tidal beaches.
                                                    25 Megaspore ornamentation densely congested with tubercles and moderately tall, narrow, short muri with uneven crests; [4n: Coastal Plain, VA northward]
                                                    25 Megaspore ornamentation open with few tubercles and with low, broad muri with even crests; [4n: Coastal Plain, s. VA to DC].
                                               23 Leaves broad (> 2.0 mm wide), dark green to bright green; megaspore ornamentation moderately to evenly reticulate with few or no standalone short muri or tubercles, usually with equatorial band absent or a plain band with few spines.
                                                        27 Irregularly reticulate megaspore ornamentation pattern of both interconnected, longer muri and short, stand-alone muri; velum coverage ± 30% (occasionally less); plants usually deeply rooted (> 20% of leaf length) in clay or clayey-sand; [4n: Coastal Plain; LA, MS, AL]
                                                        27 Regularly reticulate megaspore ornamentation pattern of interconnected muri with few or no stand-alone muri; velum coverage 10-25%; plants usually shallowly rooted (10-20% of leaf length) in sand or silty-sand substrate.
                                                          28 Velum coverage 20-25%; megaspores with ± open, reticulate ornamentation pattern; [4n: Appalachian Mts and VA to AL Coastal Plain]
                                                          28 Velum coverage 6-10 (-20)%; megaspores with ± congested, densely reticulate ornamentation pattern; [6n: NC Coastal Plain]
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