Milk thistle is an erect, stout, annual or biennial plant, that grows to 1.5—3 m in height. It has large prickly leaves, large purple flowering heads, and strongly spinescent stems (Figure 90.1). When broken, the leaves and stems exude a milky sap. The glabrous leaves are dark green, oblong, sinuate-lobed or pinnatified, with spiny margins. The leaves have milk-white veins that give the leaves, which initially form a flat rosette, a diffusely mottled appearance. During the flowering season, from June to September, each stem bears a terminal head containing a single large, purple, slightly fragrant flower ridged with sharp spines. The achenes, 6—7 mm in length and transversely wrinkled, are dark in color, gray-flecked with a yellow ring at the apex. Attached to the achene is a long white pappus. The fruits are glossy brown or gray, with spots (Evans, 2002).
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