Quillwort \Quill"wort`\, n. (Bot.) Any plant or species of the genus Isoetes, cryptogamous plants with a cluster of elongated four-tubed rushlike leaves, rising from a corm, and containing spores in their enlarged and excavated bases. There are about seventeen American species, usually growing in the mud under still, shallow water. So called from the shape of the shape of the leaves.

Source: Webster's Revised Unabridged Dictionary (1913)