Order 4. =Najadaceæ.= Only one genus _Najas_ (about 10 species); annual fresh water plants with leaves in pairs and solitary, unisexual flowers. The ♂ flower is remarkable in having a terminal stamen, which has either 4 longitudinal loculi or 1 central one; on this account the stamen of _Najas_ is considered by some authorities to be a stem and not a leaf-structure. The unilocular gynœceum and the single, erect, anatropous ovule are also terminal. Pollination takes place under the water. Order 5. =Alismaceæ.= The regular, _hypogynous_ flowers are in some species unisexual by the suppression of either andrœcium or gynœceum; they have a 6-merous perianth, _generally_ differentiated into 3 sepals and 3 petals; generally 6 _stamens in the outer whorl_ (by the division of the 3; Fig. 282) and often several 3-merous whorls inside these, and 6–∞ _free_ carpels arranged cyclically or spirally. Fruit a syncarp.--Marsh- or water-plants with radical leaves and long-stalked inflorescences. =A.= _Butomeæ. Follicles with many seeds, which are borne on nearly the whole of the inner surface of the cyclic carpels_ (as in Nymphæaceæ). Embryo _straight_.--_Butomus_ (Flowering Rush, Fig. 282), has an umbel (generally composed of 3 helicoid cymes). _S_ 3, _P_ 3, stamens 9 (6 + 3, _i.e._ the outer whorl doubled), _G_ 3 + 3. ~_B. umbellatus_; creeping rhizome with triangular Iris-like leaves.--_Hydrocleis. Limnocharis._~ [Illustration: FIG. 282.--Diagram of _Butomus_: _f_ bracteole.] =B.= _Alismeæ._ Fruit achenes. Latex common (in the intercellular spaces). The flowers are arranged most frequently in single or compound whorls. Embryo _curved_, horse-shoe shaped.--_Alisma_ has _S_ 3, _P_ 3, _A_ 6 (in 1 whorl, grouped in pairs, _i.e._ doubled in front of the sepals), and 1 _whorl_ of 1-seeded achenes on a flat receptacle. The leaves are most frequently radicle, long-stalked; the lamina have curved longitudinal veins, and a richly branched venation. _A. plantago._--_Elisma_ (_E. natans_) has epitropous (turned inwards) ovules, whilst the ovules of _Alisma_, _Sagittaria_ and others are apotropous (turned outwards).--_Echinodorus_ (_E. ranunculoides_) has a convex receptacle, carpels many, united and capitate. _Damasonium_.--_Sagittaria_ (Arrow-head) has _monœcious_ flowers, several whorls of stamens and _spirally-arranged achenes_ on a very convex receptacle. ~_S. sagittifolia_ reproduces by tuberous buds formed at the end of long, submerged branches. The leaves, in deep and rapidly running water, are long and strap-shaped, but in the air arrow-shaped.~ Honey is secreted in the flower and pollination effected by insects. _Alisma plantago_ has 12 nectaries. The submerged flowers of _Elisma natans_ remain closed and are self-pollinated. _Butomus_ has protandrous flowers. There are about 50 species, which mostly grow outside the Tropics.--Uses insignificant. The rhizome of some is farinaceous.