Most aquatic plants have flowers that grow into the air, where they are pollinated by insects. Plants that are water-pollinated usually have small, inconspicuous male flowers that release lots of pollen grains that drift in the water where they are caught by the large, feathery stigmas of female flowers. Some species growing in shallow pools have long, noodle-like pollen that sticks to female flowers. Many varieties of seaweed are water-pollinated; some invasive water-pollinated plants can choke ponds and slow-moving streams.
Common waterweed (Egeria densa), also called Brazilian elodea, is originally native to the coasts of Argentina, Brazil and Uruguay, and is now found throughout North America. Its leaves are about 1 1/2 inches long and 1/4-inch wide and grow in whorls along stout stems that are several feet long.
Pond weed (Potamogeton crispus L) has stems that float on slow-moving creeks, streams, rivers, lakes and ponds. The horizontal floating stems have nodes from which vertical stems grow. It produces thick, leathery, oblong leaves that also float. The stems and leaves often come together to form dense mats on the surface of the water.
Eel-grass (Zostera L.) roots on the bottom of tidal bays and inlets both in North America and Europe. Eel-grass has distinctive tubular leaves, hence its name. Once common, in the 1930s it began dying out from the ravages of a slime mold, Labyrinthula zosterae. In recent decades it has been making a comeback.
Hydrilla (Hydrilla verticillata) grows below the water or on the surface, where it can form dense mats. Its slender, branched stems grow up to 25 feet long, with whorls of 4 to 8 saw-toothed leaves. Hydrilla roots, which have potato-like tubers, grow in the mud. Hydrilla can grow in a few inches of water or more than 20 feet deep in freshwater ditches, marshes, lakes, springs, streams and rivers.
Coontail ,sometimes called hornwort (Ceratophyllum demersumis), is a free-floating plant that has no roots. It grows in lakes, ponds, sluggish water and streams throughout most of Canada and the U.S. . Its feathery, fan-shaped leaves, growing in whorls on the stem, resemble the tail of a raccoon, hence its name. The middle rib of the leaves has several small teeth.
There are several species of the genus Vallisneria, a seaweed native to tropical waters. Vallisneria are distinguished by their leaves, which have seven veins; these leaves are sometimes a striking red that change to green in intense light. The main species is the Vallisneria gigantea, that grows to more than 6 feet in length. Others are Vallissneria asiatica, which has leaves in the form of ribbons , and Vallisneria neotropicdalis, a colorful seaweed that grows up to 5 feet long. Vallisneria are often grown in sub-tropical ponds or large aquariums.