M. aquaticum: stems weak, often a foot or more in length; lower leaves small, cordate-ovate, acuminate, stalked, the upper ones sessile or stem-clasping, 1-2 inches long; flowers white, in the forks of leafy cymes, the pedicels turned down after flowering; styles usually five; petals narrow, deeply cleft, about one-half longer than the calyx. - Wet places, along ditches and streams. Fl. June.