4. M. scabràtum, Michx. Stem rather slender; lower leaves pinnately parted with few capillary divisions; floral leaves linear (rarely scattered), pectinate-toothed or cut-serrate; carpels strongly 2-ridged and roughened on the back.—Shallow ponds, S. New Eng. to S. C., west to Mo. and La.
[*][*][*] Stamens 4; petals rather persistent; carpels even on the back, leaves chiefly scattered, or wanting on the flowering stems.
5. M. ambíguum, Nutt. Immersed leaves pinnately parted into about 10 very delicate capillary divisions; the emerging ones pectinate, or the upper floral linear and sparingly toothed or entire; flowers mostly perfect; fruit (minute) smooth.—Ponds and ditches, Mass. to N. J. and Penn.; also in Ind.—Var. capillàceum, Torr. & Gray, has stems floating, long and very slender, and leaves all immersed and capillary. Var. limòsum, Torr., is small, rooting in the mud, with leaves all linear, incised, toothed, or entire.
6. M. tenéllum, Bigelow. Flowering stems nearly leafless and scape-like (3–10´ high), erect, simple; the sterile shoots creeping and tufted, bracts small, entire; flowers alternate, monœcious; fruit smooth.—Borders of ponds, Newf. to N. Eng., west to Mich.
2. PROSERPINÀCA, L. Mermaid-weed.
Flowers perfect. Calyx-tube 3-sided, the limb 3-parted. Petals none. Stamens 3. Stigmas 3, cylindrical. Fruit bony, 3-angled, 3-celled, 3-seeded, nut-like.—Low, perennial herbs, with the stems creeping at base, alternate leaves, and small flowers sessile in the axils, solitary or 3–4 together, in summer. (Name applied by Pliny to a Polygonum, meaning pertaining to Proserpine.)
1. P. palústris, L. Leaves lanceolate, sharply serrate, the lower pectinate when under water; fruit sharply angled.—Wet swamps, N. Eng. to Fla., west to Minn. and Tex.
2. P. pectinàcea, Lam. Leaves all pectinate, the divisions linear-awl-shaped; fruit rather obtusely angled.—Sandy swamps, near the coast, Mass. to Fla. and La.
3. HIPPÙRIS, L. Mare's Tail.
Flowers perfect or polygamous. Calyx entire. Petals none. Stamen one, inserted on the edge of the calyx. Style single, thread-shaped, stigmatic down one side, received in the groove between the lobes of the large anther. Fruit nut-like, 1-celled, 1-seeded.—Perennial aquatics, with simple entire leaves in whorls, and minute flowers sessile in the axils in summer. (Name from ἵππος a horse, and οὐρά, a tail.)