This section is from the book "Harper's Guide To Wild Flowers", by Caroline A. Creevey. Also available from Amazon: Harper's Guide To Wild Flowers.
E. densum - Color, pink, or sometimes white (see p. 98). Calyx-tube, 5-lobed. Petals, 4, notched. Stamens, 8. Stigma, club-shaped. Leaves, narrow, 1 or 2 inches long, opposite or alternate, with smaller ones clustered in the axils, their edges turned back. Flowers, in the upper axils, on slender pedicels, small. Stem, much branched near the top, whitish with soft down, from a perennial root. July to September.
In swamps or open, low fields north of New Jersey and Delaware. A plant slightly hairy, with pods on long stalks and seeds crowned with fine hairs by which they are blown about.