Ruppia L.

Monoecious. Annual or perennial submerged aquatic herbs. Rhizomes creeping, monopodial, but often also laterally branched; in annual species often considerably reduced. Central cylinder with a vascular strand, with in the centre a xylem canal; cortex consisting of parenchyma with a circle of air channels. From each node 1 or 2 unbranched roots with numerous very fine root hairs arise, as well as an erect shoot. Shoots very short to up to more than 2.5 m high, in the latter case profusely branched. Internodes elongate, variable in length. Leaves linear, distichous, with very many tannin cells; leaf sheath amplexicaulous, with on either side a slightly auriculate membranous flap; flaps overlapping; no ligula; leaf blade with only a midrib; margins smooth, but near the leaf tip irregularly serrulate; on either side of the midrib a wide air lacune. Uppermost leaves of generative branches opposite. Inflorescence terminal, consisting of a peduncle which has at its top a two-flowered spike. Peduncle arising from between the inflated sheathing bases of the 2 apical leaves; short, erect and sometimes thickened after flowering, or thin and varying in length from a few cm to more than a metre (often still lengthening itself by cell stretching during the flowering process), and in most of the species after flowering coiled or spirally contracted, pulling the ripening fruits down to the bottom. Flowering takes place at the water surface, or submerged. Flowers placed at opposite sides of the axis, but very closely together, bisexual, without a perianth, consisting of 2 opposite stamens and 4-~carpels. Stamens consisting of one (sub)sessile, bilocular anther; connective broad with at each side a theca; thecae circular to broad-elliptic, extrorsely dehiscent, shed after emission of pollen. Pollen boomerang-shaped with reticulate exine. Pollination aerial, on the water surface, or under water in an air bubble. Carpels free, sessile or subsessile; ovary ovoid; no style, but a small peltate disc-like stigma. In most species a podogynium develops at the base of each carpel after fertilisation, giving the infructescence an umbellate appearance. Ovule solitary, pendulous, campylotropous. Fruit an achene, sessile or stalked (podogyne and fruit form a morphological entity without abscission zone), symmetric to very asymmetric; exocarp spongy, soon decomposing; endocarp hard, persistent with beak and usually a podogyne; at the apical part of the endocarp a small foramen occurs, the shape of which has diagnostic value at the species level.