NAJADALES

also called Potamogetonales the pondweed order of monocotyledonous flowering plants, comprising 10 families, with 19 genera of aquatic and marsh-dwelling herbs. The plants are so diverse in appearance, structure, and technical character that they are unified as an order mostly by their aquatic habit and lack of endosperm (nutrient tissues in the seed for the developing embryo). A further diagnostic character is the presence in most genera of small scales near the leaf bases. (However, some members of the order Arales also exhibit this feature.) The order is considered to be closely related to the water-plantain order (Alismales) and probably evolved from it. The families Scheuchzeriaceae and Juncaginaceae are composed of emergent plants of marshes and bogs. The two species of the genus Scheuchzeria, which constitute the family, grow in wet sphagnum bogs. The arrow-grass family (Juncaginaceae), with four genera and about 18 species, includes the flowering quillwort (Lilaea subulata), Triglochin (15 species), and two more single-species genera. The Aponogetonaceae (the lattice plant family) and Potamogetonaceae (the pondweed family) are composed mainly of freshwater aquatic plants with submerged or floating leaves but frequently with emergent flowering shoots. Aponogeton, the only genus of its family, contains 30 species, mostly from Africa, of which A. distachys is often grown ornamentally in aquariums and pools. The lattice plant of Madagascar (A. fenestralis) has unusual underwater leaves that consist of veins only. The pondweed family contains two genera (Potamogeton, 100 species; and Groenlandia, one species) of widely distributed plants important as food for waterfowl and cover for fishes. The Ruppiaceae (widgeon grass family), with one genus, Ruppia, and two species; Zannichelliaceae, with three genera, including the horned pondweed, Zannichellia; and Najadaceae (the naiad family), with one genus, Najas, and 50 species, are submerged aquatics of freshwater or brackish water. The Zosteraceae (the eelgrass family), with two genera, Phyllospadix and Zostera, and 12 species, consists entirely of submerged marine plants in temperate regions of the world. The family Cymodoceaceae (four genera and 21 species) comprises plants that grow in shallow saltwater bays and brackish streams, but it is principally found in warmer coastal waters such as Florida. Its largest genera are Cymodocea (10 species) and Halodule (7 species). The family Posidoniaceae, with one genus, Posidonia, and two species, is a Mediterranean and Australian group similar to the eelgrass.