6. Lactuca serriola L. (prickly lettuce)

Pl. 259 c, d; Map 1082

Plants annual or biennial. Latex white. Stems 30–150(–200) cm long, hollow between the nodes, glabrous but often short-prickly toward the base, whitish tan. Leaves well developed along the stems, extremely variable; the basal and lower stem leaves mostly (5–)15–20(–30) cm long, sessile or with a winged petiole, narrowly oblong, ovate, or oblong-ovate in outline, unlobed to more commonly deeply pinnately lobed, the margins sharply toothed and prickly but otherwise glabrous, usually with a pair of narrowly to broadly triangular basal lobes clasping the stem, the surfaces glabrous but the undersurface short-prickly along the midvein; the middle and upper stem leaves mostly linear to lanceolate, unlobed, the margins sometimes lacking teeth but with short prickles, the base more or less rounded, with a pair of narrowly triangular basal lobes clasping the stem, the undersurface usually short-prickly along the midvein. Inflorescences mostly well-branched panicles with 50–100 or more heads. Involucre narrowly cup-shaped to cylindrical or urn-shaped, 7–8 mm long at flowering, elongating to 10–15 mm at fruiting, the bracts 17(–19). Florets (12–)18–24(–30). Corollas light yellow to lemon yellow, occasionally bluish- or purplish-tinged on the outer surface, sometimes turning blue with age or upon drying. Pappus 4–5 mm long. Fruits with the body 3–4 mm long, 1.5–2.0 mm wide, yellowish brown to grayish brown, somewhat flattened, hairy toward the tip, angled along the margins and with 5–7 roughened to minutely awned nerves or ridges on each face, tapered to a slender beak 1–2 times as long as the body. 2n=18. July–October.

Introduced, scattered to common nearly throughout the state (native of Europe, Asia; introduced widely in the U.S., Canada). Banks of streams and rivers, margins and openings of mesic upland forests, and disturbed portions of upland prairies; also crop fields, fallow fields, roadsides, railroads, and open, disturbed areas.

The species epithet has sometimes been spelled L. scariola in the older literature (Steyermark, 1963) based on the inconsistent spelling of the name by Linnaeus in his various publications (de Vries and Jarvis, 1987). As in other Lactuca species, variants have been named to account for phases with different degrees of leaf division. The nominate form has deeply lobed leaves, and plants with narrow, more or less unlobed leaves currently are known as f. integrifolia (Gray) S.D. Prince & R.N. Carter. Prince and Carter (1977) speculated that this trait may be under the control of a single major gene. Interestingly, the stem leaves often become twisted at the base during development and assume a vertical position at maturity.

This weedy species is genetically very close to the cultivated lettuce, L. sativa, and it has been used as a genetic resource in lettuce breeding. Recent DNA fingerprinting studies (Koopman et al., 2001) have suggested that the closely related L. sativa L., L. serriola, L. dregeana DC., and L. altaica Fisch. & C.A. Mey. are all so closely related genetically that they should be considered part of the same evolutionary gene pool and thus combined into a single species under the name L. sativa. However, as the garden lettuce is a cultigen whose origins may have included other ancestral taxa in addition to the preceding list, it seems preferable to maintain it as a separate species for the present time until further, more intensive studies document the genetic limits of this crop plant. Lactuca sativa was reported for Missouri from the St. Louis railyards by Mühlenbach (1983), based on a single plant that was uprooted before it could become fully mature. Thus, because it has not been documented to reproduce itself in the wild and has not been seen in the state since the original report, the garden lettuce is not treated fully in the present work. The species has become established outside of cultivation sporadically in the United States and Canada, but it does not appear to persist in the wild anywhere for very long. Lactuca sativa is similar to L. serriola but differs in its usually leafier stems with broadly ovate to nearly circular leaves that tend to have a corrugated or ruffled appearance along the margins (or to be entire and unlobed with a rounded tip; Pl. 259 e–g). The leaves and stems usually lack prickles.