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Any of various aquatic plants of the family Podostemaceae, especially P. ceratophyllum of North America, which grows on rocks in rapidly flowing streams.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.


(Plants) a type of tropical and sub-tropical plant found growing near rivers out of hard surfaces like rock
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014


(ˈrɪv ərˌwid)

any of several submerged aquatic plants of the genus Podostemum, family Podostemaceae, growing in rapid streams by clinging to stones with the roots.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.
References in periodicals archive ?
The Yemeni minister also riverweed the difficulties in various fields faces Yemen, hailing Egypt's political and media support to the process of democratic transformation in Yemen.
My favourite, Ema Datsi (chilli and cheese), slow cooked to perfection along with ferns and riverweed topped on red rice with a side order of buckwheat dumplings can satiate any appetite!Reproduced From India Today Travel Plus.
Serviss writes that Sagitta is "interesting when viewed with an opera-glass for its row of little stars from which, as from a maypole lying horizontally, depend loops of still smaller stars looking like garlands." Or perhaps Sagitta has just snagged some riverweed along the way.