This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Perennial herbs, prostrate and commonly rooting at the joints, with palmately lobed or veined, often peltate leaves, the bases of the petioles with 2 scale-like stipules, and small white flowers in peduncled or sessile simple or proliferous umbels or heads. Bracts of the involucre few and small, or none. Calyx-teeth minute. Petals entire. Disk flat. Fruit laterally compressed, orbicular or broader than high. Carpels with 5 primary ribs, the lateral ones usually curved; no large oil-tubes, but an oil-bearing layer of tissue beneath the epidermis. [Greek, water-cup.]
About 75 species of wide distribution. Besides the following another occurs in the Southwest and on the Pacific Coast. Type species: Hydrocotyle vulgaris L. The species are known as Marsh-, or Water-pennywort, or Water-cup.
Leaves nearly orbicular, peltate.
Umbels simple, rarely slightly proliferous; pedicels slender.
Umbels, at least some of them, proliferous; pedicels, or some of them, short
Fruit notched at each end.
Fruit not notched at either end.
Leaves nearly orbicular, cordate, or reniform, not peltate.
Leaves 5-9-lobed; umbels nearly sessile.
Leaves 3-7-cleft; umbels long-peduncled.
Umbels capitate, the beads peduncled.
Hydrocotyle umbellata L. Sp. Pl. 234. 1753.
Glabrous, stem creeping, several inches long, the subterranean branches tuberiferous. Petioles slender, erect, or ascending, 1'-6' long; leaves peltate, orbicular, or broader than long, sometimes cordate at the base, 1/2'-2' wide, crenately 7-11-lobed, the lobes broad, not deep, mostly crenulate; peduncles elongated; umbels simple or rarely with a proliferous extension; pedicels slender, 2"-6" long; mature fruit notched at both ends, 1"-1 1/2" broad, not quite as long; intermediate ribs corky-thickened; dorsal rib obtuse.