This may be the genus Sparganium you are talking about. AKA Burreeds.
There are several species native to the Northeast and Central US. They
typically grow as emergent plants but the leaves often sprawl or float
in deeper waters. One species - the Floating Burreed - S. fluctuans
always grows in this fashion. I collected it in Wisconsin with Ray Wolff
last summer - or at least I think it was that one. Closer to home there
is a cold, clear stream impounded by beavers where our locally common
one - probably Eastern Burreed - S. americanum grows emerged and on wet
shores and underwater both floating and as submerged plants in deeper
water that look very much like a bed of Val. or Sagittaria. Indeed it
took me a while to figure the difference between the groups.
As a rule, Burreeds have pale green leaves that are spongy in texture.
Underwater leaves are ribbon like but emergeny leaves are triangular in
cross sections. Seeds occur in bur like clusters. Burreeds spread from a
creeping root stock that runs thru the mud.
Vallisnaria leaves are ribbon like and are fine veined with a darker
center stripe. The flowers are on long thread like stalks that twist
like a corkscrew to pull the fruit underwater to ripen. Val spreads by
runners in similar fashion to the strawberry plant (over the substrate
as opposed to below) and seems to like a clean sandy substrate whereas
burreeds often grow in mud.
Sagittaria has a stiffer more unuformly colored leaf and white flowers
with three petals above water. It runs over the substrate like Val and
depending on the species - grows on mud or sand or both. Many species
have diverse underwater and above water morphs and change with the
seasons and also may have peculiar morphs when growing in tidal waters
that inundate and emmerse the plants over the course of a day.
Best references are of course Common Marsh, Underwater & Floating-leaved
Plants of the US & Canada by Neil Hotchkiss and Aquatic and Wetland
Plants of the Southeastern US : Vol I Monocotyledons.
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