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, or rarely climbing shrubs, with acrid sap. Leaves alternate (except in Clematis), simple or compound. Stipules none, but the base of the petiole often clasping or sheathing. Pubescence, when present, composed of simple hairs. Sepals 3-15, generally caducous, often petal-like, imbricate, except in Clematis. Petals about the same number (occasionally more), or wanting. Flowers regular or irregular. Stamens 00, hypogynous, their anthers innate. Carpels 00 or rarely solitary, I-celled, I-many-ovuled. Ovules anatropous. Fruit achenes, follicles or berries. Seeds with endosperm.
About 35 genera and 1100 species, distributed throughout the world; not abundant in the tropics.
*Carpels several-ovuled (1-2-ovuled in nos. 1 and 8); fruit a follicle or berry; sepals imbricated in the bud. (Helleboreae.)
Flowers regular; leaves palmately nerved or palmately compound.
Carpels ripening into a head of red berries.
Carpels ripening into a head of dry follicles.
Petals present, narrow or small, linear, flat.
Petals present, narrow or small, tubular, at least at the base.
Sepals persistent; stem tall, leafy.
Sepals deciduous; stem scape-like, bearing one leaf.
Flowers regular; leaves ternately or pinnately compound or decompound.
Petals not spurred.
Low herbs with solitary or panicled flowers.
Carpels and follicles stalked.
Carpels and follicles sessile.
Low shrub with racemose flowers, the fruits follicles.
Tall erect herbs with racemose flowers.
Petals prolonged backward into hollow spurs.
Posterior sepal spurred.
Posterior sepal hooked helmet-like.
** Carpels 1-ovuled; fruit an achene.
Sepals imbricated in the bud. (Anemoneae.)
Flowers subtended by involucres remote from the calyx or close under it.
Styles short, glabrous or pubescent, or none.
Involucre remote from the calyx; styles short, subulate.
Involucre of 3 simple sessile leaves close under the flower.
Involucre of 3 compound sessile leaves; leaflets stalked; stigma sessile.
Styles elongated, densely plumose.
Flowers not subtended by involucres.
Small annual herbs; leaves basal, linear; sepals spurred.
Low or tall herbs, mostly with both basal and stem leaves; sepals spurless.
Petals none; leaves palmately lobed.
Petals present, bearing a nectariferous pit at the base of the blade.
Achenes transversely wrinkled; flowers white.
Achenes swollen, smooth; sepals 3; petals mostly 8.
Achenes compressed or terete, longitudinally nerved.
Petals none; leaves ternately decompound.
Petals present, with no nectar-bearing pit; leaves dissected.
Sepals valvate in the bud; leaves opposite. (Clematideae.)
Sepals and stamens spreading; flowers panicled.
Sepals and stamens erect or ascending; flowers mostly solitary.
Petals present, small, spatulate.