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..
Annual or perennial herbs, with alternate simple entire lobed or divided or dissected leaves, and yellow white or red flowers. Sepals mostly 5, deciduous. Petals equal in number or more, conspicuous or minute, provided with a nectariferous pit and a scale at the base of the blade. Carpels 00, 1-ovuled. Achenes capitate or spicate, generally flattened, smooth, papillose or echinate, tipped with a minute or an elongated style. [Latin for a small frog, in allusion to the marsh habitat of many species.]
Some 275 species, widely distributed in the temperate and cool regions of both hemispheres and on mountain tops in the tropics. In addition to those here described, many others inhabit the western and northwestern parts of the continent. The names Crowfoot or Buttercup are popularly applied to most of the species with large flowers and divided leaves. Type species: Ranunculus auricomus L.
Aquatics; leaves orbicular, palmately divided.
Glabrous; leaves 3-lobed or 3-cleft, cuneate at the base; arctic.
Plant scapose from filiform rootstocks.
†† Plants of swamps or muddy shores; leaves entire or denticulate.
Annuals; achenes beakless.
Petals 1"-2" long; stamens few.
Petals 2"-3" long; stamens numerous.
Perennials, rooting from the nodes; achenes beaked.
Stems trailing; achenes minutely beaked.
R. rep tans.
Stems ascending or erect; achenes subulate-beaked.
††† Terrestrial or marsh species with some or all the leaves toothed, lobed or divided.
Calyx glabrous or pubescent; flowers yellow.
1. Achenes smooth, neither papillose, muricate nor spiny.
Plant low, arctic-alpine; leaves small, palmately lobed.
Plants neither arctic nor alpine.
Basal leaves, some or all of them, merely crenate (deeply cleft in no. 11).
Head of fruit oblong, 2-3 times as long as thick.
Flowers 6" broad or less; sepals slightly hairy.
Flowers 8"-10" broad, sepals densely tomentose.
Head of fruit globose or subglobose.
Petals longer than the sepals.
Petals not twice as long as the loosely villous sepals.
Petals several times longer than the glabrous calyx.
Basal leaves oval or ovate, not cordate.
Basal leaves reniform or orbicular, cordate.
Petals small, shorter than or equalling the sepals.
Styles very short.
Basal leaves cordate; plant glabrous or nearly so.
Basal leaves not cordate; plant villous at least below.
Styles subulate, hooked, nearly one-half as long as the achene.
Leaves all lobed or divided.
Plant glabrous; stem hollow; flowers very small.
Plants more or less pubescent.
Beak of the achene strongly hooked; flowers 4"-5" wide.
Beak of the achene short.
Erect plants, naturalized in fields; flowers 1' broad.
Calyx spreading; roots fibrous.
Calyx reflexed; stem bulbous-thickened at base.
Erect or ascending plants of moist soil; flowers 3"-6" broad.
Petals not longer than the reflexed sepals.
Petals longer than the sepals.
Ascending and creeping by stolons; flowers 1' broad.
Beak of the achene long, stout or slender; flowers 6"-18" broad.
Roots slender; beak stout; leaflets cuneate at base.
Roots thickened; beak of achene slender, subulate.
Leaf-segments broad, oblong or obovate.
Leaf-segments narrow, linear-oblong.
2. Achenes with scattered papillae, at least near the margins; perennial.
3. Achenes rough-papillose all over; annual.
4. Achenes muricate or spiny.
Leaf-lobes broad, obtuse.
Leaf-lobes narrow, subacute.