This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Leaves crowded in verticils, linear or filiform, spinulose-serrulate, forked. Sterile flowers with 10-20 stamens, the anthers about as long as the involucre. Fertile and sterile flowers generally at different nodes, but sometimes in opposite axils at the same node. Ovary and fruit somewhat longer than the involucre, the fruit beaked with the long persistent style.
One or possibly two species, widely distributed in fresh water, the following typical.
Ceratophyllum demersum L. Sp. Pl. 992. 1753.
Stem 2°-8° long, according to the depth of water. Leaves in verticils of 5's-12's, linear, 2-3 times forked, the ends of the segments capillary and rigid, 4"-12" long; ripe fruit oval, 2"-3" long with a straight or curved spine-like beak 2"-4" long, smooth and spurless or with a long basal spur on each side, or tuberculate and with narrowly winged spiny margins or broadly winged without spines.
In ponds and streams, throughout North America except the extreme north. Cuba. Several species and varieties have been proposed, based on the spurs, spines or wings of the fruit, but none of them seem to be more than races. Morass-weed. June-July.