Leaves 1-4+ cm, lobes 0.5-1(-3) mm wide. Ray laminae 1-2+ mm. Disc corollas ca. 2.5 mm. Cypselae 5-18 mm; pappi 0.5-1(-3) mm. 2n = 48, 50. Flowering Jun-Oct. Igneous and calcareous soils; 900-2500 m; Ariz., N. Mex., Tex.; Mexico; Central America. Heterosperma pinnatum occurs throughout much of the southwestern United States. No specimens have been seen to substantiate claims of occurrences in Maryland and Massassachusetts, where it may have been ephemeral. Calyculi of H. pinnatum have been interpreted as leaves, the heads being interpreted as terminal and sessile.
FNA 2006, Martin and Hutchins 1980, MacDougall 1973, Allred and Ivey 2012
Duration: Annual Nativity: Native Lifeform: Forb/Herb General: Slender annuals, to 30 cm tall; stems erect, branching above, striate (having longitudinal stripes), usually nearly hairless. Leaves: Opposite, at least below the inflorescence; blades 15-50 mm long, once or twice pinnately lobed, the lobes linear to filiform, or rarely lanceolate, upper and lower surface glabrous. Flowers: Flower heads small, yellow, and radiate, though the rays are small and inconspicuous; heads borne singly or in loose, cymose arrays; involucre narrowly campanulate to subcylindric, 5-6 mm high and 3-4 mm wide, the bracts (phyllaries) in 2 distinct series; outer phyllaries 3-5 per flower head, narrow (linear to lanceolate) and green, sometimes with a few gland-tipped teeth on the margins; inner phyllaries wider (ovate), straw-colored, and striate with 10-15 dark-colored nerves; paleae (bracts on the receptacle that subtend each floret) appearing similar to the inner phyllaries, with dark colored striations and apiculate tips; ray florets 1-3 per flower head, the laminae (ray petals) 1-2 mm long, pale yellow to orange; disc florets 3-10 perflower head, yellow. Fruits: Achenes from ray and disc florets have different morphology; ray achenes are oval-shaped and winged, 3-5 mm long and reddish brown; disc achenes are narrower, up to 14 mm long, and topped with a pair of awns. Ecology: Found on open rocky slopes, often on igneous and calcareous soils, from 3,000-8,500 ft (914-2591 m); flowers June-October. Distribution: AZ to TX; south to MEX Notes: With its 2-barbed achenes, this species could easily be mistaken for Bidens. Get your hand lens and look at the phyllaries; H. pinnatum has 2 obviously different types. The outer layer consists of only a few narrow green bracts. The inner phyllaries look the same as the recepticular bracts; they are oval shaped, light-colored, and have numerous dark nerves. The other characteristic that distinguishes this species is the dimorphic achenes. Ray achenes are oval-shaped, while the disc achenes are narrow and 2-barbed. Ethnobotany: Unknown Etymology: Heterosperma comes from Greek heteros, different and sperma, seed, referring to the dimorphic achenes; pinnatum means pinnate, referring to the leaf morphology. Synonyms: None Editor: SBuckley 2010, AHazelton 2017