Quillwort n. : 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.

Example:
The current population of the Louisiana Quillwort?

Taxonomy
Quillwort, species, Isoetes, Louisiana Quillwort, order Isoetales, free encyclopedia, Spring Quillwort, Land Quillwort, Stylites, genus of plants, Isoetopsida, South American species, Mat-forming, Several species, search, Kingdom, Plantae, Family.

Defination from the web:
Quillwort leaves are hollow and quill-like, arising from a central corm. ... Quillwort species are very difficult to distinguish by general appearance. The best way to identify them is by examining the megaspores under a microscope. ...