pillwort

from The Century Dictionary.

• noun A plant of the genus Pilularia; especially, P. globulifera, the creeping pillwort of Europe: named from the pellet-like involucres containing the fruit.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) Any plant of the genus Pilularia; minute aquatic cryptograms, with small pill-shaped fruit; -- sometimes called peppergrass.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun botany Any plant of the genus Pilularia; a minute aquatic cryptogram with small pill-shaped fruit.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun European water fern found around margins of bodies of water or in wet acid soil having small globose sporocarps