This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Submerged, amphibious or uliginous plants with a cluster of elongated awl-shaped leaves rising from a more or less 2-3-lobed fleshy short stem, the leaves with or without peripheral bast-bundles, with or without stomata, bearing a small membranous organ (ligule) above the base. Sporanges sessile in the excavated bases of the leaves, orbicular or ovoid, the sides more or less covered with a fold of the inner side of the leaf-base (velum). The sporanges of the outer leaves usually contain spherical, mostly sculptured macrospores, those of the inner ones contain minute powdery usually oblong microspores. [Name Greek, taken from Pliny, apparently referring to the persistent green leaves.]
The family consists of the following genus only.
About 60 species, widely distributed. Besides the following 2 are known from the southern United States, 7 from the Pacific Coast and 2 from Mexico. Owing to their aquatic habitat and apparently local distribution, these plants are popularly little known. The spores mature in summer and autumn. Type species: Isoetes lacustris L.
Leaves without peripheral bast bundles.
Leaves without stomata; plants submerged.
Leaves stiff and erect.
Leaves slender and mostly recurved.
Leaves about 1/2" in diameter.
Leaves at least 1 1/2" in diameter.
Leaves with stomata; plants partially submerged, or emersed.
Macrospores armed with spines.
Macrospores without spines, merely crested or warted.
Leaves 2'-3' long; macrospores less than 55ou in diameter.
Leaves 4'-8' long; macrospores about 600 u in diameter.
Leaves reddish, or rarely olive green.
Leaves with stomata and bast bundles.
Aquatic, but usually inhabiting the water's edge, sometimes completely emersed.
Leaves from 12'-30' long; macrospores with convolute labyrinthine ridges.
Leaves shorter; macrospores otherwise marked.
Bast bundles only 4, except in some forms of /. Engelmanni,
Macrospores coarsely crested.
Macrospores merely reticulated.
Polygamous; microspores rare.
Bast bundles 4, with extra ones near periphery.
Terrestrial, or rarely growing near water's edge.