Isoetes
See list of 8 species in this genusThe size and ornamentation of mature megaspores, found in the expanded leaf bases, are crucial for confident identification of most species. Collections of Isoetes from most of the growing season will possess only fragile, yellow-white, and largely unornamented megaspores. For most New England taxa, collections from August and later will yield identifiable material. It is recommended that collectors check the condition of the megaspores while in the field to insure that mature spores are available (they should be spherical in orthospecies and bone white upon drying). Hybrids are to be expected where two or more species are found in the same water body. Hybrid quillworts can be detected by their polymorphic megaspores that range in shape from spherical to pyramidal and flattened. References: Eaton (1900), Kott and Britton (1983), Taylor et al. (1993).
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1a. Megaspores echinate with thin, sharp spines; leaves deciduous
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1b. Megaspores variously ornamented, but without a spiny texture; leaves persisting more than 1 year
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2a. Megaspores 0.4–0.56 mm in diameter, averaging less than 0.5 mm, patterned with an unbroken reticulum [Fig. 4]
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2b. Megaspores 0.4–0.75 mm in diameter, averaging more than 0.5 mm, cristate, rugulate, or reticulate with broken or anastomosing ridges
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3a. Leaves with abundant stomates, bright green; plants aquatic to amphibious, occasionally emergent; girdle of megaspores obscure
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3b. Leaves with few or no stomates, dark green to red-green or red-brown; plants aquatic, usually submerged; girdle evident (obscure in I. prototypus)
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4a. Megaspores 0.55–0.75 mm in diameter, averaging more than 0.6 mm; leaves abruptly tapering to the tip; plants typically submerged 1–3 (–5) m
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5a. Megaspores with a papillate girdle, cristate to reticulate with sharp or roughened crests [Fig. 5]
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5b. Megaspores with a smooth girdle, reticulate with low, rounded ridges
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4b. Megaspores 0.4–0.65 mm in diameter, averaging less than 0.6 mm; leaves gradually tapering to the tip; plants submerged to 1 (–3) m
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6a. Velum covering the entire sporangium [Fig. 6]; leaves very rigid and straight to the tip; sporangium wall unpigmented; girdle of the megaspore obscure
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6b. Velum covering less than half the sporangium [Fig. 7]; leaves pliant and curling at the tip; sporangium wall brown-streaked; girdle evident
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7a. Megaspores with a papillate girdle, cristate to reticulate with irregular, roughened crests
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7b. Megaspores with a smooth girdle, reticulate with smooth, rounded crests [Fig. 3]
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Each photo represents one species in this genus.