3. Rosa canina L. (dog rose, dog briar)

Pl. 538 c, d; Map 2488

Stems 50–250(–500) cm long, arching or sprawling, less commonly ascending, slender, few- to several-branched, green, with prickles mostly confined to and paired at the nodes, these 5–7 mm long, 4–9 mm wide, the bases broadened, flattened, curved or hooked, not gland-tipped. Leaves 6–11 cm long, the petiole and rachis glabrous, occasionally with a few small prickles. Stipules 10–22 mm long, the margins entire or shallowly glandular-toothed, the auricle flared, 3–5 mm long. Leaflets 5 or 7, 15–40 mm long, 12–20 mm wide, the terminal leaflet with a stalk 5–11 mm long, the blades elliptic to obovate, mostly rounded to broadly angled at the base, the margins simply or doubly toothed with 20–30 teeth per side, these sharply or bluntly pointed, usually gland-tipped, the upper surface dull or shiny, dark green, sometimes glaucous, the undersurface green to light green, glabrous or minutely hairy along the veins. Inflorescences at the tips of current year’s stems, sometimes also on lateral branches from second year’s stems, of solitary flowers or more commonly small clusters of 2–6 flowers, the flower stalks 8–20 mm long, glabrous or with stalked glands, with 1 or more bracts. Flowers not doubled, the hypanthium 7–9 mm long, glabrous, the mouth about 1.0–1.5 mm in diameter. Sepals 10–17 mm long, 3–5 mm wide, strongly unequal, some of them with prolonged dilated, somewhat leaflike tips and the margins with 2 or 3 pairs of deep, lanceolate lobes, the undersurface glabrous, spreading to recurved after flowering, usually shed by fruit maturity. Petals 18–25 mm long, light to bright pink or white. Pistils 25–36, the styles free, hairy, extending 1–2 mm beyond the mouth of the hypanthium, the mass of stigmas thus slightly elevated above the hypanthium mouth. Hips 10–16(–24) mm long, 6–16 mm wide, globose or ellipsoid-ovoid, fleshy, the surface red, lacking stalked glands. Achenes 14–23, 5–6 mm long. 2n=35. May–July.

Introduced, uncommon and sporadic in the state (native of Europe, Asia, Africa; introduced widely but sporadically in the eastern and western U.S.; Canada, Mexico). Edges of mesic upland forests; also fencerows and roadsides.

Steyermark (1963) and some other authors recognized two varieties within R. canina, with the name var. canina applied to plants with the leaflet undersurface hairy with glandular and often also nonglandular hairs, whereas var. dumetorum (Thuill.) Poir. (R. corymbifera Borkh.; R. ×dumetorum Thuill.) was applied to plants with glabrous leaflets. In their molecular study of then dog roses, Ritz et al. (2005) found no significant differences between the ITS sequences of plants assignable to the two types. The exact disposition of these varieties will have to await more intensive research, but for now there is no evidence to support their distinctness.

The dog roses (sect. Caninae DC. ex Ser.), with about 20 species native to Europe and adjacent portions of Asia and Africa) are all stable pentaploids and have a unique mechanism of meiosis that results in the formation of tetraploid eggs and haploid pollen, which fertilize to result in a pentaploid zygote. The evolution of the group appears to have involved multiple hybridization events between some extinct or as-yet undiscovered diploid dog rose with members of one or more other sections of Rosa (Ritz et al., 2005).