Order 6. =Hydrocharitaceæ.= This order differs chiefly from the preceding in its _epigynous_ flowers. These are in general unisexual (_diœcious_), and surrounded by a 2-leaved or bipartite _spathe_; they are 3-merous in all whorls, but the number of whorls is generally greater than 5, sometimes even indefinite. The perianth is divided into _calyx_ and _corolla_. The ovary is _unilocular_ with parietal placentation, or more or less incompletely plurilocular. The fruit is berry-like, but usually ruptures irregularly when ripe. Embryo straight.--Most often submerged water-plants, leaves seldom floating on the surface. Axillary scales (_squamulæ intravaginales_). _Hydrocharis._ Floating water-plants with round cordate leaves; S3, P3 (folded in the bud); ♂-flowers: 3 (-more) flowers inside each spathe; stamens 9–15, the most internal sterile. ♀-flowers solitary; three staminodes; ovary 6-locular, with many ovules attached to the septa; styles 6, short, bifid. [The petals of the ♀-flowers bear nectaries at the base. In this and the following genus the pollination is without doubt effected by insects.] ~_H. morsus ranæ_ (Frog-bit) has runners; it hibernates by means of special winter-buds.~--_Stratiotes_; floating plants with a rosette of linear, thick, stiff leaves with spiny margin, springing from a short stem, from which numerous roots descend into the mud. Inflorescence, perianth, and ovary nearly the same as in _Hydrocharis_, but the ♂-flower has 12 stamens in 3 whorls, of which the outer 6 are in 1 whorl (dédoublement), and inside the perianth in both flowers there are numerous (15–30) nectaries (staminodes?). _S. aloides_ (Water-soldier); in N. Eur. only ♀-plants.--~_Vallisneria spiralis_ is a tropical or sub-tropical plant, growing gregariously on the mud in fresh water. The leaves are grass-like, and the plants diœcious; the ♂-flowers are detached from the plant, and rise to the surface of the water, where they pollinate the ♀-flowers. These are borne on long, spirally-twisted peduncles which contract after pollination, so that the ♀-flower is again drawn under the water, and the fruits ripen deeply submerged.--_Elodea canadensis_ is also an entirely submerged plant. The leaves are arranged in whorls on a well-developed stem. Only ♀-plants in Europe (introduced about 1836 from N. Am). This plant spreads with great rapidity throughout the country, the reproduction being entirely vegetative. _Hydrilla_, _Halophila_, _Thalassia_, _Enhalus_.--In many of these genera the number of whorls in the flower is remarkably reduced; for example, in _Vallisneria_, in the ♂-flowers to 2: Pr 3, A (1-) 3, in the ♀ to 3: Pr 3, Staminodes 3, G 3.--About 40 species; Temp. and Trop.~ Family 2. =Glumifloræ.=