The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

quillwort
    n 1: any of several spore-bearing aquatic or marsh plants having
         short rhizomes and leaves resembling quills; worldwide
         except Polynesia