ALISMATALES

also called Alismales, the water-plantain order of flowering plants, belonging to the class Liliopsida (monocotyledons; characterized by a single seed leaf). Most species are aquatic and grow submerged or partially exposed to the air in marshes and other freshwater habitats. Many are aquatic weeds that hinder irrigation and navigation, though others provide important habitats for fish and help stabilize shorelines. Although of little economic importance, these herbaceous (nonwoody) plants, by forming complex communities with other aquatic and emergent stream-bank plants, bring about vegetational succession; a pond may fill up with plant parts and silt to become a biological landfill, which eventually supports land plants. Arrowhead (Sagittaria). The order Alismatales comprises three families. The Butomaceae, consisting of a single genus and species (Butomus umbellatus), is used in landscaped water gardens. The three genera of Limnocharitaceae are tropical, and some species grow well in tropical fish aquariums. The Alismataceae, with 14 genera and about 90 species, is found in the North Temperate Zone and the tropics. The members of Alismataceae are notable for developing latex channels (containing milky juice) in their stems and leaves. Water plantain (Alisma plantago-aquatica), arrowhead (Sagittaria; see photograph), and burhead (Echinodorus) are of frequent occurrence. Most members of the Alismatales order possess short stems and are rooted in mud with only the leaves visible above water level. In some species even the leaves are submerged and fail to differentiate into the familiar lamina (blade) and petiole (leaf stalk) of land plants. Emergent leaf blades are often triangular or arrowhead-shaped, and some attractive types have been reproduced in artistic designs. Alisma, which gives the order its name, produces heart-shaped leaves noted for contoured venation and delicate crossveins. Succulence is characteristic of Alismatales, and air spaces may occupy 80 percent of the volume of internal tissues of the leaf and root. The vascular system, which consists of the xylem (water-conducting tissues) and phloem (food-conducting tissues), is rather inconspicuous and plays only a minor role in supporting the plant. A potential flower cluster (inflorescence) arises as a specialized stem from the flattened vegetative stem at the juncture of a leaf base. As it elongates above water, a succession of flower buds forms and develops in a sequence of sepals and petals. In the Alismatales, the small petals attract beetles, butterflies, and flies, which are the main pollinators. After pollination and fertilization, an ovule becomes the seed. In some species, seeds are shed when the ovary matures into a fruit and eventually ruptures, as in Butomus. In other groups, such as the Alismataceae, seeds are liberated only after the ovary wall disintegrates in the water.