HYDROCHARIDEAE  , in

botany, a natural order of Mono- cotyledons, belonging to the series Helobieae . They are water- plants, represented in Britain by frog-bit (Hydrocharis Morsus- ranae) and water-soldier (Stratiotes alaides) . The order contains about fifty species in fifteen genera, twelve of which occur in fresh water while three are marine: and includes both floating and submerged forms . Hydrocharis floats on the surface of still water, andhas rosettes of kidney-shaped leaves, from among which spring the flower-stalks; stolons bearing new leaf- rosettes are sent out on all sides, the plant thus propagating itself in the same way as the strawberry . Straliotes abides has a rosette of stiff sword- like leaves, which when the plant is in flower project above the surface; it is , also stoloniferous, the young rosettes sinking to. the bottom at the beginning of winter and rising again to the surface in the spring . Vallisneria (eel-grass) contains two species, one native of tropical ,Asia, the other . in- habiting the warmer parts of both hemi- spheres and reaching as far north as south Frog-bit—male plant. the mud at the bottom I, Female flower. of fresh water, and the 2, Stamens, enlarged. short stem bears a 3, Barren pistil of male flower, enlarged. cluster of long, 4, Pistil of female flower. g, narrow 5, Fruit. grass-like. leaves; new 6, Fruit cut transversely. plants are formed at 7, mod' the end of horizontal 8, 9, Floral diagrams of male and female flowers respectively. runners . Another type s, Rudimentary stamens. is represented by Elodea canadensis or water-thyme,which has been introduced into the British Isles from North America . It is a small, submerged plant with long, slender branching stems bearing whorls of narrow toothed leaves; the flowers appear at the surface when mature . Halophila, Enhalus and Thalassic are submerged maritime plants found on tropical coasts, mainly in the Indian and Pacific oceans; Halophila has an elongated stem rooting,at the nodes; Enhalus a short, thick rhizome, clothed with black threads resembling horse-hair, the persistent hard-bast strands of the leaves; Thalassic has a creeping rooting stem with upright branches bearing crowded strap-shaped leaves in two rows . The flowers spring from, or are enclosed in, a spathe, and are unisexual and regular, with generally a calyx and corolla, each of three members; the stamens are in whorls of three, the inner whorls are often barren; the two to fifteen carpels form an inferior ovary containing generally numerous ovules on often large, produced, parietal placentas . The fruit is leathery or fleshy, opening irregularly . The seeds contain a large embryo and no endosperm .

In Hydrocharis (fig . I), which is dioecious, the flowers are

borne above the surface of the water, have conspicuous white petals, contain honey and are pollinated by in-sects . Stralioles has similar flowers which come above the surface only for pollination, becoming sub-merged again during ripening of the fruit . In Vallisneria (fig . 2), which is also dioecious, the small male flowers are borne in large numbers in short-stalked spathes; the petals are minute and scale-like, and only two of the three stamens are !fer- FIG. z.—Vallisneria spiralis—Eel grass--tile; the flowers about i natural size . A, Female plant; B, become detached Male plant . before opening and rise to the surface, where the sepals expand and form a float bearing the two projecting semi-erect stamens . The female flowers are solitary and are raised to the surface on. a long, spiral stall:; the ovary bears three broad styles, on, which some of the large, sticl.•y pollen-grains from the Iloatmg male floc: ers get de-posited (fig . 3) . After pollination the female flower becomes drawn below the surface by the spiral con-traction of the long stalk, and the fruit ripens near the bottom . Elodea has poly- FIG . 3 .

gamo.us flowers (that is, male, female and hermaphrodite), solitary, in slender, tubular spathes; the male flowers become detached and rise to the surface; the

females are raised to the surface when mature, and receive the floating pollen from the male . The flowers of Halophila are submerged and apetalous . The order is a widely distributed one; the marine forms are tropical or subtropical, but the fresh-water genera occur also in the temperate zones .