Flora of North America species comparison

Of conservation concern.

Rosa minutifolia was first collected in the United States in 1985 (San Diego County; G. A. Levin 1986). Before being extirpated by a development project, the single population was re-established in a nearby protected site, where the transplants are reportedly doing well (C. Burrascano, pers. comm.). Existence of R. minutifolia in the United States remains of conservation concern. Although common where found in coastal scrub of Baja California, Mexico, the habitat there is considered threatened by development.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Species ca. 140 (33 in the flora).

Most species of Rosa occur in the cooler parts of the northern hemisphere. Only three or four species extend south of the Tropic of Cancer in the Old World, none in the New World.

In an infrafamilial classification of the Rosaceae by D. Potter et al. (2007), Rosa is included in subfam. Rosoideae, supertribe Rosodae (no tribe). Distinctive features of Rosa include shrubby habit, concave or urn-shaped hypanthia, relatively numerous pistils, and fruits aggregated achenes (achenetum) inside fleshy hypanthia (hips).

Character states often are part of a continuum instead of being discrete. When addressed by differing taxonomic philosophies, radically different treatments of Rosa in North America have resulted. P. A. Rydberg (1918) recognized 129 native species and dozens of hybrids among them. E. W. Erlanson (1934) accepted 15 species, plus seven ecotype species and four putative hybrids based on cytogenetics, experimental hybridizations, common garden experiments, and field observations. Although no consensus treatment has been adopted by subsequent regional (for example, B. Ertter 1993e; A. Cronquist and N. H. Holmgren 1997) and national floras (for example, H. J. Scoggan 1978–1979, vol. 1) or monographic treatments (W. H. Lewis 1958, 1959, 1959b, 1962, 1965), a unifying feature has been a conservative approach to Rosa taxonomy, with relatively few species recognized (Ertter and Lewis 2008). Caution is advised in relying on names in synonymy for updating the identification of old collections; synonymy given here often differs from previous applications.

The approach of R. Elven et al. (1999) is followed here in using subspecies as the main infraspecific rank. The rank of variety also is used, either within subspecies or as subunits of species.

The following summary describes the methods of obtaining data. Information on introduced species was obtained chiefly from North American plants with only supplemental use of specimens from Eurasia.

Prickles, including aciculi, of all Rosa are epidermal outgrowths of stems and not thorns, which are modified stems. Acicular prickles are shaped like aciculi, needle-shaped and relatively short. Declining prickles are retrorse, being turned inward toward the axis. Infrastipular prickles are proximal to stipules, single or paired in contrast to internodal. Setaceous prickles are shaped like bristles or stiff hairs (setae). Pricklets are miniature prickles on petioles and rachises closely resembling infrastipular/internodal prickles found on stems except that they are smaller.

Herbarium specimens often do not have information on plant height, habit, or bark; field research and observations in gardens have helped provide information for these and other characters. First-year rhizomatous shoots (suckers, turions) are densely covered with mixed internodal prickles and aciculi; leaves of these often develop an extra pair of lateral leaflets when compared to older stems. Identifications using these shoots are difficult because specific differentiation at an early stage of development is limited; no attempt was made to incorporate such data in this treatment. Whenever possible, first-year rhizomatous shoots should be collected, particularly if they flower the first year, as Rosa arkansana, or when mowing or otherwise broken shoots stimulate early flowering. Data for prickles and aciculi were obtained using distal stems and distal, usually fertile branches at roughly the third or fourth internode from branch tips; measurements of infrastipular prickles (or, if none, internodal) consisted of prickle lengths to base × base lengths; relatively few aciculi or setae were measured. Leaves: stipules were measured for length of adnation to petioles, width across both sides of petioles near distal regions, and length of distally free auricles on one side. Petioles and rachises often have pricklets resembling main prickles of stems. Terminal leaflet measurements of serrations per side exclude terminal serration, and for double- or multi-serrations only each main serration is counted. Inflorescences include measurements of pedicel lengths; pedicels are slender to stout, usually varying between 0.5–1 mm diam. Flowers: sepal base width measurements at junctures with hypanthia were made of the three outer sepals; sepal margins are characteristically tomentose or pubescent and are not described (varying glandularity is), nor are induments of sepal adaxial surfaces (also tomentose or pubescent); petal numbers are either single and five or double and ten or more; available stamen number averages are from E. W. Erlanson (1934). Achenes are bony walled, round or angular (not in descriptions unless different); when unusually small, shriveled, or darkened, they are considered abortive and, if counted, are provided separately from fertile achenes. Cytology: ploidy levels obtained from flow cytometry calculations are based on nuclear DNA amounts using fresh leaflets; these are provided with the ploidy level followed by DNA only in species discussions (all research by Anne Bruneau).

When using the keys, be aware that most species known to possess prickles also have some stems and branches lacking armature.

Phenotypic variability in Rosa may be due to seasonal or annual environmental factors, varying gene expressions, hybridization, and polyploidy; in sect. Caninae, hemisexual reproduction and such features represent confounding factors in rose morphology and taxonomy.

All species of the gall-inducing cynipid wasps Diplolepis are restricted to native and introduced roses and induce structurally distinct galls. For example, in Ontario, D. polita induces a single-chambered gall on leaves of Rosa acicularis subsp. sayi; D. spinosa induces a multichambered gall on stems of R. blanda (J. D. Shorthouse 1991).

Most Rosa species have general usages, such as fleshy hips long and widely used for eating raw and as prepared foods, such as jams, jellies, and preserves, and for food supplements due to high vitamin C content. Ethnobotanical uses provided for species are based on D. E. Moerman (1998) and W. H. Lewis and M. P. F. Elvin-Lewis (2003), together with original articles and personal communications.

North American rose species have contributed modestly to the development of hybrids for the horticulture industry. Such contributions are reported in publications associated with the American Rose Society and Royal Horticultural Society (J. H. McFarland 1938; P. Harkness 2003; C. Quest-Ritson and B. Quest-Ritson 2003).

The infrageneric structure used here is relatively well established in floral and taxonomic works (with some ambivalent results from molecular analyses). The main difference is that sect. Rosa replaces the better-known sect. Cinnamomeae. This results from conservation of the type of Rosa on R. cinnamomea (J. McNeill et al. 2006, App. IIIA) over the hybrid R. ×centifolia Linnaeus in sect. Gallicae.

Two species of sect. Chinenses de Candolle ex Seringe, both cultivated in warmer areas, are reported as occasionally escaping in North America. Key distinguishing features of the section include shrubs erect or climbing, stems with curved prickles, leaflets usually 3–5 with relatively narrow, adnate stipules, sepals reflexed and deciduous after anthesis, and styles free, exsert, nearly equaling stamens. Rosa chinensis Jacquin, China rose, has been confirmed as occurring at some time in Arkansas, Florida, Georgia, Louisiana, Mississippi, South Carolina, and Virginia. The closely allied R. ×odorata (Andrews) Sweet, tea rose, was also found at some time in Florida, Georgia, Louisiana, and South Carolina, and unconfirmed in Prince Edward Island, Pennsylvania, and Utah. Rosa chinensis is distinguished from R. ×odorata by its shrubby habit (versus climbing or scrambling lianas to 20 m), leaves with 3–5(–7) leaflets (versus 5–9), stipules and auricles marginally stipitate-glandular (versus eglandular or glandular only on auricle margins), flowers usually 4 or 5, rarely solitary (versus 1–3), slightly or not fragrant (versus fragrant), 3–5 cm diam. (versus 5–10 cm diam.), with often pinnately lobed sepals (versus entire or slightly incised), and hips ovoid or pyriform (versus depressed-globose). Both species are of major importance in breeding new garden roses, especially the hybrid teas, by incorporating continuous flowering into roses.

(Discussion copyrighted by Flora of North America; reprinted with permission.)