24. Rosa L. (rose)

Plants shrubs, sometimes lianas or the stems leaning or climbing over other vegetation, sometimes colonial from rhizomes or root suckers. Stems prostrate to erect or ascending, sometimes climbing or loosely twining, all those found in Missouri armed with prickles, at least toward the stem base, these variously slender and straight or broad-based, flattened, and curved. Leaves petiolate, the petioles sometimes armed with prickles. Stipules conspicuous, usually persistent, leaflike, fused to the petiole laterally, each with an earlike basal auricle, the margins entire, toothed, or less commonly dissected into slender lobes. Leaf blades trifoliate or pinnately compound with 5–11 leaflets, the rachis with the underside sometimes armed with prickles, the leaflets sessile or short-stalked (the terminal leaflet almost always short-stalked), variously shaped, the margins bluntly to sharply toothed, the surfaces glabrous or hairy, sometimes with gland-tipped hairs. Inflorescences terminal on long branches or on short, lateral branches, loose clusters or panicles, sometimes reduced to a solitary flower, the stalks with bracts, these of various size, shape, and number. Flowers perfect (but occasionally functionally imperfect), deeply perigynous (but appearing epigynous unless dissected), the hypanthium globose to urn-shaped with a circular opening at the tip, the enclosed ovaries not fused to the inner wall above their bases. Sepals 5, sometimes persistent at fruiting, erect, horizontally spreading, or reflexed, mostly lanceolate to narrowly ovate, angled to a long-tapered tip, the margins entire or toothed (in a few species dilated and with lateral appendages). Petals usually 5 (except in doubled flowers), white to pink or red (yellow or purple in some cultivated forms), obovate to broadly obovate, broadly rounded and usually shallowly and broadly notched at the tip. Stamens numerous (fewer in doubled flowers) in several dense whorls, the filaments relatively short, free, attached inside the hypanthium rim, the anthers mostly yellow to orangish yellow. Pistils mostly numerous, attached along the inner surface of the hollow hypanthium, each with 1 carpel. Ovaries free, each with 1 ovule. Style 1 per ovary, slender, the styles within a flower either free or more or less fused into a column toward the tip, protruding slightly from the mouth of the hypanthium, the stigma disc-shaped, the stigmas in a flower often tightly clustered into a flattened mass. Fruits hips, the hypanthium becoming fleshy and usually reddish-colored, enclosing the several to numerous, small, hard-walled achenes. Achenes asymmetrically obovoid to ellipsoid, usually more or less rounded but sometimes longitudinally grooved dorsally and angled to wedge-shaped on the other side, the surface appearing smooth, light yellow to more commonly tan or light brown, densely pubescent with long hairs (glabrous in R. rugosa), often mostly along 1 side and/or only toward the tip, sometimes becoming glabrous or nearly so with age. About 150 species, North America, Europe, Asia, Africa.

Rosa is a taxonomically difficult genus, with abundant hybridization, polyploidy, and apomixis. A complex classification exists and numerous cultivars have been developed. Interested readers should consult the literature and electronic resources of the American Rose Society or the Royal Horticultural Society as a gateway to the massive amounts of information available on all aspects of the genus. Numerous species are cultivated as ornamentals and the genus also provides an extremely important source of cut flowers. The fleshy hips can be consumed raw or made into jellies, jams, or preserves. They are an important food source for wildlife and also are an important commercial source of vitamin C. Some species are cultivated for their fragrances and used in rose water and perfumes.

The present account follows closely the forthcoming treatment of the genus Rosa in the Flora of North America series by Walter Lewis, Barbara Ertter, and Anne Bruneau.