4 M. scabratum Mx. Lvs. pinnatifid in whorls of 4s and 5s; fls. verticillate, axillary, upper fls. , with 4 stam., lower ones ; floral lvs. linear, pectinately toothed; fr. 8-angled, the ridges tuberculate. - Plymouth, Mass. (Oak(s), Block Island (Robbins), S. and W. States. St. 6 to 12' high. Segm. of the lvs. linear-capillary.
5 M. tenellum Bw. Erect and almost leafless; floral lvs. or bracts alternate, minute, entire, obtuse; fls. ; petals linear; stam. 4; carp. smooth, not ridged-- About the edges of ponds and rivers, Providence, R. I. (Olney), northern part of
N. Y. to Newfoundland. Rhizome prostrate, creeping, sending up several stems or scapes which are simple and 4 to 12' high. Fls. small, purplish white, sessile, alternate, a little shorter than the bracts, the upper ones . Jl. 6 M. ambiguum Nutt. Lvs. many, submersed ones pinnate, with capillary segments, middle ones pectinate, upper linear, petiolate, toothed or entire; fls. mostly ; petals oblong, somewhat persistent; stam. 4; carpels smooth, not ridged on the back. - In ponds and ditches, Penn. to Mass. Sts. floating, upper end emerged, with ininute fls. and linear floral lvs. (M. natans DC.) In other situations it varies as follows.
β. limosum Nutt. St. procumbent and rooting; lvs. all linear, rigid, often en tire. - Muddy places, where it is a small, creeping and branching plant. (M. procumbens Bw.) gapillaceum Torr. Lvs. all immersed and capillary. - Ponds.
11. HIPPU'RIS, L. Mare's Tail. (Gr. a horse, a tail.)
Calyx with a minute, entire limb crowning the ovary; corolla none; stamen 1, inserted on the margin of the calyx; anther 2-lobed, compressed; style 1, longer than the stamen, stigmatic the whole length in a groove of the anther; seed 1.- Aquatic herbs. St. simple. Lvs. verticillate, entire. Fls. axillary, minute.
H. vulgaris L. Lvs. in verticils of 8 to 12, linear, acute, smooth, entire; fls. solitary, often . - In the borders of ponds and lakes, Penn. to Arc. Am., very rare. Rhizome with long, verticillate fibers. St. erect, jointed, 1 to 2f high. The flowers are the simplest in structure of all that are called perfect, consisting merely of 1 stamen, 1 pistil, 1 seed in a 1-celled ovary, with neither calyx lobes nor corolla. May, Jn.