Lactuca serriola, prickly lettuce

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Intermountain Flora
Annual or winter-annual, 3-15 dm tall, glabrous; leaves prckkly on the midrib beneath, more finely on the margins, pinnately lobed or lobeless, twisted the base to stand erect in two rows on opposite sides of the stem in a single vertical plane, sagittate-clasping, mostly 5-30 cm long and 1-10 cm wide, the midrib typically pale like the stem; heads numerous, 13-27-flowered, the flowers yellow, often drying blue; involucre 10-15 mm high in fruit; achenes olive-gray or yellowish, the body 3-4 mm long and a third as wide, compressed, prominently several-nerved on each side, spinulose above, at least marginally, the slender beak about equaling or up to twice as long as the body; 2n = 18.

Cronquist, A., Holmgren, A.H., Holmgren, N.H., Reveal, J.L., and Holmgren, P.K. (1994) "Intermountain Flora" pg. 455, vol. 5, The New York Botanical Garden New York.