ALISMACEAE (from the Gr. ἄλισμα, a water-plant mentioned by Dioscorides), in botany, a natural order of monocotyledons belonging to the series Helobieae, and represented in Britain by the water plantain, Alisma Plantago, the arrow-head, Sagittaria, the star-fruit, Damasonium, and flowering rush, Butomus (from the Gr. βοῦς, ox, τέμνειν, to cut, in allusion to leaves cutting the tongues of oxen feeding on them). They are marsh- or water-plants with generally a stout stem (rhizome) creeping in the mud, radical leaves and a large, much branched inflorescence. The leaves show a great variety in shape, often
on the same plant, according to their position in, on or above the water. The submerged leaves are long and grass-like, the floating leaves oblong or rounded, while the aerial leaves are borne on long, thin stalks above the water, and are often heart- or arrow-shaped at the base. The flower-bearing stem is tall; the flowers are borne in whorls on the axis as in arrow-head, on whorled branchlets as in water plantain or in an umbel as in Butomus (fig. 1). The flowers are regular and rather showy, generally with three greenish sepals, followed in regular succession by three white or purplish petals, six to indefinite stamens and six to indefinite free carpels. The floral arrangement thus recalls that of a buttercup, a resemblance which extends to the fruit, which is a head of achenes or follicles.
The flowers contain honey, and attract flies, short-lipped bees or other small insects by the agency of which pollination is effected. The fruit of Butomus is of interest in having the seeds borne over the inner face of the wall of the leathery pod (follicle). Damasonium derives its popular name, star-fruit, from the fruits spreading when ripe in the form of a star. It is a western Mediterranean plant which spreads to the south of England, where it is sometimes found in gravelly ditches and pools. The order contains about fifty species in fourteen genera, and is widely distributed in temperate and warm zones. Alisma Plantago (fig. 2), a common plant in Britain (except in the north) in ditches and edges of streams, is widely distributed in the north temperate zone, and is found in the Himalayas, on the mountains of tropical Africa and in Australia.