Herbs aquatic with the habit of seaweeds, with alternate, dissected leaves, with flowers minute, perfect, naked or with 3 sepals, stamens 1 or many, hypogynous.

Ovary compound, 2 to 3-celled, with as many stigmas, and numerous ovules. Fruit a many-seeded capsule, ribbed and somewhat pedicelled. Albumen none.

Genera 20, species 100. frequent in S. America and E, India. 1 only in N. America. They all prow in running water, attached to stones like the following species.

PODOSTE'MUM, L. C. Rich. Threadfoot. River Weed. (Gr. a foot, the stamens being apparently on a common foot-stalk,) Stamens 2, with the filament3 united below; ovary oblong-ovoid; stigmas 2, sessile recurved; capsule 2-celled; seeds minute. - Small, submersed herbs, adhering to stones and pebbles.

P. ceratophyllum Mx Lvs. dichotomously dissected; fls. solitary, axillary. - Mid. W. and S. States, in shallow streams. St. a few inches long, usually destitute of roots and attached to stones by lateral, fleshy processes. Lvs. numerous, olive-green, alternate, coriaceous, divided into many long, linear-setaceous segments. Fls. on short, thick peduncles, the 2 stamens and styles at length bursting through the lacerated calyx. Jl. (Lacis ceratophylla Bougard.)