Annual herb, taprooted, without tubers or stolons, 30-70 cm tall, pubescent, densely armed with prickles 2-9 mm long; hairs 0.2-0.8 mm long, simple over most of the plant, frequently gland-tipped. LEAVES: alternate, deeply dissected, bipinnatifid to compound, broadly ovate to obovate or deltoid, 4-11 cm long, with irregular-stellate trichomes on the lower surface, and simple, gland-tipped trichomes on the upper and lower surface, the stellate trichomes with 2-5 rays, the main veins armed with scattered prickles; petioles 2-7 cm long, armed with prickles; base of leaf or leaflets variable, acute to cordate; apices of ultimate lobes rounded to obtuse. INFLORESCENCE: 4-10 cm long, raceme-like monochasial cymes, 5-9-flowered; peduncles 1-5 cm long. FLOWERS: somewhat zygomorphic, perfect or having nonfunctional stigmas and abortive ovules in the terminal portions of the inflorescence; pedicel 0.5-2 cm long; calyx campanulate, the tube 1.5-2.2 mm long, the lobes deltoid, 2-5 mm long; corollas stellate-pentagonal, 1-1.7 cm wide, loosely folded between the lobes, violet to blue; stamens unequal; anthers oblong, of two sizes, the lowermost extended anther 3.5-5 mm long, purple-tinged, incurved at the tip, the upper shorter anthers 2-4 mm long and yellow; filaments ca. 1/5 as long as the anthers; styles slender, extending out beyond the anthers; stigmas 0.3-0.6 mm across. FRUITS: spherical, 0.9-1.2 cm in diam., tightly invested by the densely armed, burr-like accresent tube of the calyx; seeds dark brown, 2.5-2.9 mm long, lenticular to broadly ovate, minutely pitted. NOTES: Silty, sandy, or gravelly soils of dunes, streambeds, washes, and open hillsides: Cochise, Graham, Maricopa, Santa Cruz, Yavapai cos. (Fig. 1F); 590-1650 m (1950-5400 ft).; Jun-Oct; NM; n and c Mex. Whalen (1979) cites three varieties of S. heterodoxum as occurring in the United States and Mexico. S. h. var. setigeroides, with stems densely covered in needle-like prickles and pentagonal corollas with broadly deltoid lobes, is probably the only variety in AZ.