Flowers monœcious or polygamous. Calyx of the sterile flowers 4-parted, of the fertile 4-toothed. Petals 4, or none. Stamens 4--8. Fruit nut-like, 4-celled, deeply 4-lobed; stigmas 4, recurved.--Perennial aquatics. Leaves crowded, often whorled; those under water pinnately parted into capillary divisions. Flowers sessile in the axils of the upper leaves, usually above water in summer; the uppermost staminate. (Name from μυρίος, _a thousand_, and φύλλον, _a leaf_, i.e., Milfoil.) [*] _Stamens 8; petals deciduous; carpels even; leaves whorled in threes or fours._ 1. M. spicàtum, L. Leaves all pinnately parted and capillary, except the _floral ones or bracts_; these _ovate, entire or toothed, and chiefly shorter than the flowers_, which thus form an interrupted spike.--Deep water, Newf. to N. Eng. and N. Y., west to Minn., Ark., and the Pacific. (Eu.) 2. M. verticillàtum, L. _Floral leaves much longer than the flowers, pectinate-pinnatifid_; otherwise nearly as n. 1.--Ponds, etc., common. (Eu.) [*][*] _Stamens 4; petals rather persistent; carpels 1--2-ridged and roughened on the back; leaves whorled in fours and fives, the lower with capillary divisions._ 3. M. heterophýllum, Michx. Stem stout; _floral leaves ovate and lanceolate_, thick, crowded, sharply serrate, the lowest pinnatifid; _fruit obscurely roughened._--Lakes and rivers, Ont. and N. Y. to Fla., west to Minn. and Tex. 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.