What plant is that?

Click on photo below to view larger image

River-weedRiver-weed with thumb for scale
River-weed growing in natural habitat
Top two photos: Doug Goldman, hosted by the USDA-NRCS PLANTS Database
Lower photo: jillllybean, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>,
via Wikimedia Commons

The mystery plant for November 2020 is a small, hard to find aquatic plant of rivers and streams which grows in the eastern United States.  Prior to 2007, this plant had not been reported from Rhode Island since 1890 (see RI Botanical Survey Checklist by Gil George). Two botanists from the University of New Hampshire came to Rhode Island, about 1990, in search of the plant.  Unfortunately they were unable to find it. One hundred-seventeen years after the last sighting, in the fall of 2007, and with the help of Norm Boyer, we were able to find the plant growing in the shallow waters of the Pawtuxet River.  This plant inhabits river and streams and attaches itself to submerged ledge and rocks.  It has been described as having the appearance of sea weed.  It belongs to a mostly tropical plant family and is rare in New England.

It’s. . .

Botanical Name:
 
Podostemum ceratophyllum
Common Name:
  River-weed
Family:
  Podostemaceae (Riverweed family)
Habitat:
  River, streams, and waterfalls