Tropicos | Name - !Lactuca L.

!Lactuca L.

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39. Lactuca L. (lettuce)

Contributed by Kuo-Fang Chung

Plants annual, biennial, or perennial herbs, usually taprooted, but often also with fibrous roots at maturity, occasionally with rhizomes. Latex white or light tan to pale orange. Stems erect or ascending, finely to less commonly coarsely ridged, glabrous or hairy. Leaves alternate and often also basal, mostly 4–20 times as long as wide or, if 2–3 times as long as wide (in L. sativa), then the blade strongly and irregularly crisped or curled and sessile or nearly so, glabrous or hairy, sessile or short-petiolate, the basal leaves mostly withered before flowering, the stem leaves slightly expanded or with a small pair of lobes and somewhat clasping the stem. Leaf blades unlobed or pinnately lobed, linear to lanceolate or ovate in outline, the margins entire or finely to coarsely toothed, sometimes irregularly so. Inflorescences spikes, racemes, or panicles. Involucre becoming elongated as the fruits mature, narrowly cylindrical to narrowly cup-shaped or urn-shaped, the bracts in 3–5 overlapping series, ovate to lanceolate or narrowly oblong-lanceolate, often purplish- or reddish-tinged, or purple-tipped, glabrous, the margins often thin and pale, the tip appressed-ascending or somewhat outward-curved. Receptacle naked, usually minutely pitted at the base of each floret. Ligulate florets 9–55 per head. Corollas yellow, blue, purple, or rarely white. Pappus of numerous apparently smooth (microscopically barbed) bristles, these white (grayish white elsewhere). Fruits with the body ovate to elliptic, lanceolate, or oblanceolate in outline, flattened, sometimes tapered to a short or long beak at the tip, the pappus attached to an expanded disc at the very tip, with 1 to several longitudinal nerves or ridges on each face, often also finely cross-wrinkled. Fifty to 75 species, North America, Central America, Europe, Asia, Africa.

The generic circumscription of Lactuca has been somewhat controversial, with some authors (Bremer, 1994) adopting a narrow generic concept in which (among other segregates) L. tatarica and its Old World relatives are classified in Mulgedium Cass. Based on earlier biosystematic studies (Dille, 1976), this species and its relatives had been suggested as possible ancestors of the North American allopolyploid species complex that includes L. canadensis, L. floridana, L. hirsuta, and L. ludoviciana. A study utilizing molecular data (Koopman et al., 1998) lent support to a relatively broad generic concept of Lactuca to include the L. tatarica group, which is adopted here.

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1	1. Heads relatively large, the involucre 12–20 mm long at flowering, elongating to 15–23 mm at fruiting

2	2. Corollas light purplish blue to blue; fruits with 3–5 nerves or ridges on each face ... 7. L. TATARICA	Lactuca tatarica

3	2. Corollas yellow to orangish yellow, sometimes turning blue with age or upon drying; fruits with 1 nerve or ridge on each face

4	3. Leaves best-developed toward the stem base, much smaller above the stem midpoint, the margins of both lobed and unlobed leaves sharply toothed and sparsely short-hairy but not prickly, also hairy but not prickly on the undersurface; florets (12–)15–20(–22) per head ... 3. L. HIRSUTA	Lactuca hirsuta

5	3. Leaves well developed along the stem, the margins of both lobed and unlobed leaves sharply toothed and glabrous but somewhat prickly, also glabrous but prickly on the undersurface midvein; florets 20–30(–55) per head ... 4. L. LUDOVICIANA	Lactuca ludoviciana

6	1. Heads relatively small, the involucre 7–10 mm long at flowering, elongating to 10–15(–18) mm at fruiting

7	4. Corollas lavender to purplish blue or blue, rarely white; fruits beakless or with a short, stout beak much less than 1/2 as long as the body ... 2. L. FLORIDANA	Lactuca floridana

8	4. Corollas light yellow to orangish yellow or orange, sometimes turning blue with age or upon drying; fruits with a noticeable, slender beak somewhat shorter than to twice as long as the body

9	5. Corollas yellowish orange to orange; fruits with 1 nerve or ridge on each face, tapered to a beak somewhat shorter than to about as long as the body; sap light tan to pale orange ... 1. L. CANADENSIS	Lactuca canadensis

10	5. Corollas light yellow to bright yellow; fruits with 5–7 nerves or ridges on each face, tapered to a beak 1–2 times as long as the body; sap white

11	6. Leaves with the margins and undersurface not prickly, the margins of both lobed and unlobed leaves lacking teeth; inflorescences spikelike panicles ... 5. L. SALIGNA	Lactuca saligna

12	6. Leaves with prickly margins and sharp teeth, also prickly on the undersurface midvein; inflorescences usually well-branched, open panicles ... 6. L. SERRIOLA	Lactuca serriola