Water ferns and a note on Quillwort

Ferns (Pteropsidae). Several water ferns are indispensible in the aquarium, two forms of Java Fern (Microsorum pteropus), Bolbitis Fern (Bolbitis heudelotii) and the floating Water Sprite (Ceratopteris cornuta).
 
Quillworts (Lycopsidae). Isoëtes flaccida (Soft Quillwort). Quillwort grows in a clump of very slender quill-like leaves that are square in section; if you roll one gently between your fingers you'll feel its corners. The leaves have a sporecase embedded at their base that reminds me a little of chives. Quillwort isn't related to chives at all; in fact it's an unusual survivor of a just-about-extinct class of plants, the Lycopodiopsids, which evolved even before the ferns. The handful of other surviving Lycopodiopsids include club mosses, but the real Lycopodiopsid heyday was in the Paleozoic, say 300 million years ago. Then giant tufty-headed Lycopsid "trees" fell over in stagnant swamps and started turning to peat — and eventually to coal.
 
American aquarists get to use Quillwort more than Europeans, because the European species tend to flag when exposed to the warm waters of tropical aquaria, and they're even more brittle than the American Isoetes, almost too brittle to use. The Isoetes species from the American southeast is named "flaccida" precisely because it will bend without breaking.
 
Quillwort is a little demanding about light levels, and it is better when temperatures don't get much over 75°F. Quillwort will stand out of low-growing gravel-covering plants like a tuft of grass. It's useful for this, since no other plant is quite like it. Perhaps I've grown it partly for its strangeness.