HYDROCHARITALES

the frog's-bit order of monocotyledonous flowering plants comprising the family Hydrocharitaceae, with 16 genera of submerged and emergent, freshwater and saltwater aquatic herbs, mainly of warmer regions. The plants are characterized by radially symmetrical, usually separate male and female flowers (rarely are they bisexual) that are produced on separate plants. The female flowers have an inferior ovary (i.e., the ovule-bearing parts are positioned below the attachment point of the sepals and petals). The leaves are produced in whorls or clusters at numerous points along the stems, which may be horizontal, rootlike structures (rhizomes) or leafy erect or floating stems. The largest genera are Ottelia (40 species), Lagarosiphon (15 species), Elodea (including Anacharis, 12 species), Blyxa (10 species), Halophila (10 species), and Vallisneria (8 to 10 species). Thalassia (2 species) and Halophila are marine. The rest are freshwater plants. The order is considered to have evolved from common ancestors with the order Alismales. The order is notable for the unique pollination mechanism of some genera (e.g., Elodea, Enhalus, Hydrilla, and Vallisneria). The male flowers become detached and float about until they encounter and transfer pollen to a female flower, which has reached the surface of the water by means of an elongated stalk. After pollination the developing fruit is drawn under the water, where it ripens. Other genera are pollinated by wind, insects, or water (by means of floating pollen in the last case). Many produce special stems with buds that drop off and spend the winter in the bottom mud (Hydrocharis, Stratiotes, and Elodea). Several members of the order are cultivated or are otherwise economically important. Elodea (q.v.), for example, is used in aquariums as an ornamental plant and in schools as an experimental plant. The frog's-bit, from which the order receives its common name, is Hydrocharis morsus-ranae, a rootless water plant with round or heart-shaped floating leaves and small, attractive, three-petaled white flowers. The water soldier (Stratiotes aloides) bears floating rosettes of tough, sharp-edged leaves that float in summer but sink and decay in the autumn. The tape grass, or eelgrass (Vallisneria), includes an important aquarium ornamental plant. Turtle grass (Thalassia) is often washed ashore in such quantities following storms at sea that it is collected and used as a fertilizer.