The common garden lettuce, L. sativa L., is a familiar salad plant in all its forms, probably originating in or near the Middle East area and derived from L. serriola. It has apparently been collected only once as possibly out of cultivation in Michigan: on Belle Isle, Wayne Co., in 1892 (O. A. Farwell 1315, BLH); however, the evidence is too sparse to include this species in the state’s flora. It can be recognized by its very broad leaves with smooth midrib; otherwise, it is much like L. serriola.
1. Beak (if any) of achene stout and distinctly less than half as long as the body; heads usually with blue to white ligules.
2. Pappus brownish or gray; inflorescence various, but often elongate-cylindrical with crowded heads.
2. Pappus white; inflorescence open, the branches widely spreading.
1. Beak of achene distinct, filiform, when fully mature nearly half or more as long as the body; heads usually with yellow ligules (drying blue in L. saligna and L. serriola).
3. Mature achenes with several strong nerves on each face; leaves either prickly on midrib beneath or linear-lanceolate and entire.
4. Leaves smooth on the midrib (rarely a few prickles toward base), linear-lanceolate (ca. 12–30 times as long as broad), entire and unlobed.
4. Leaves prickly along the midrib beneath, ± oblong or oblanceolate in outline (less than 4 times as long as broad), prickly-margined and often ± lobed.
3. Mature achenes with at most 1 strong nerve on each side (sometimes a weak pair besides); leaves neither prickly nor all linear-lanceolate and unlobed.
5. Involucres (8–) 9–12.5 mm long at maturity; mature achenes 4.5–6 mm long, with pappus ca. 5–7 mm.
5. Involucres 15–21 mm long at maturity; mature achenes (7.5–) 8–9 mm long (including beak), with pappus 8–10 (–14) mm long.