In message , Brian
writes
Mare's tail proper is Hippuris vulgaris, and is a still or
slow-moving-water plant
~~~~~~~~~~#
And is a nut producing flowering plant. More closely related to
Chestnuts or any other FP. Brian.

Hippuris belongs to Haloragaceae, which is one of a group of families
related to Crassulaceae (stonecrops, houseleeks, etc), and is part of
the Saxifragales order, which is one of the major lineages of the core
eudicots (with rosids, asterids, Caryophyllales and some smaller
groups); the sweet, horse and water chestnuts belong to different
branches of the rosids, respectively being closish to oaks, maples and
(purple) loosestrifes.
--
Stewart Robert Hinsley