Trans. Amer. Philos. Soc., n. s. 7: 320. 1840.
Perennials or subshrubs, (4–)20–120(–150) cm (bases often woody). Stems usually strictly erect, few-branched, glabrous or hispidulous, villous, or tomentose, usually gland-dotted (sessile or slightly sunken in pits), sometimes stipitate-glandular, rarely eglandular, often resinous. Leaves mostly cauline; alternate; sessile; blades 1-nerved, linear to oblanceolate or obovate, margins entire or toothed to pinnatifid (teeth or lobes often spinulose-tipped), faces glabrous or hispidulous, villous, or tomentose, usually gland-dotted (in pits). Heads discoid, (sessile or subsessile) in compact clusters borne in terminal corymbiform arrays, rarely borne singly. Involucres obconic to turbinate or campanulate, (3–9.5 ×) 2–8 mm. Phyllaries 15–30 in (3–)4–6 series, 1-nerved (midnerves usually barely evident; flat to convex), oblong- to elliptic-lanceolate, unequal, margins narrowly scarious, apices sometimes green, faces glabrous or tomentose, sometimes gland-dotted, sometimes with resin pockets. Receptacles flat, pitted, epaleate. Disc florets 8–34, bisexual, fertile; corollas yellow with dark orange-resinous veins, tubes (with at least a few glandular hairs) longer than abruptly ampliate, goblet-shaped throats (tube elevating corollas above involucres at flowering), lobes 5, erect, deltate (elongating at maturity, unequal, outer prominently bent or leaning outward, abruptly enlarged); style-branch appendages narrowly triangular. Cypselae (brownish) obpyramidal, terete or subterete, 5–11-ribbed (sometimes thick and resinous), faces sericeous; pappi persistent, of 40–50 unevenly thick, unequal, barbellate, apically attenuate bristles in 2(–3) series. x = 6.
sw United States, Mexico.
Species 16 (10 in the flora).
Isocoma is recognized by its subshrubby habit, gland-dotted (sometimes stipitate-glandular), often resinous leaves, compactly clustered discoid heads in terminal, corymbiform arrays, goblet-shaped disc corollas, and base chromosome number of x = 6. H. M. Hall (1928) regarded the group as a section of Haplopappus, as did A. Cronquist (1994). Naturally occurring hybrids in south-central Mexico have been observed between Isocoma veneta (Kunth) Greene and Xanthocephalum humile (Kunth) Bentham (R. L. Hartman and M. A. Lane 1991).
|1||Phyllary apices with strongly developed resin pockets||> 2|
|1||Phyllary apices gland-dotted (rarely so in I. coronopifolia), lacking resin pockets or pockets small, weakly developed||> 3|
|2||Teeth or lobes of leaf margins apically rounded-obtuse; florets 8–27; phyllary apices not spinulose-aristate, or if spinulose-aristate, then florets 20–27||Isocoma acradenia|
|2||Teeth or lobes of leaf margins apically spinulose; florets 10–13; phyllary apices spinulose-aristate||Isocoma arguta|
|3||Leaves pinnatifid||> 4|
|3||Leaves entire or shallowly toothed to lobed, not pinnatifid||> 6|
|4||Herbage minutely hispidulous or sparsely puberulous (at least distal stems)||Isocoma tenuisecta|
|4||Herbage glabrous||> 5|
|5||Involucres 5–7.5 mm wide; florets 18–25||Isocoma azteca|
|5||Involucres 2.5–4 mm wide; florets 12–15||Isocoma coronopifolia|
|6||Phyllary apices distinctly aristate-spinulose; florets 10–13; California||Isocoma arguta|
|6||Phyllary apices not aristate-spinulose; florets 8–34; California and eastward||> 7|
|7||Involucres 3.2–7 × 2.5–4 mm; florets 8–17(–21)||> 8|
|7||Involucres 5–9 × 4.5–8 mm; florets (15–)18–34||> 9|
|8||Leaves never stipitate-glandular but almost always resinous, glabrous; involucres 5–7 mm||Isocoma coronopifolia|
|8||Leaves sometimes stipitate-glandular, never resinous, margins hispidulous, sometimes also on faces; involucres 3.2–5.5 mm||Isocoma pluriflora|
|9||Leaves 5–10(–18) mm; corollas 4–5 mm||Isocoma humilis|
|9||Leaves (in I. menziesii 5–)13–50 mm; corollas 5–7.5 mm||> 10|
|10||Leaf margins entire or toothed to lobed; California||Isocoma menziesii|
|10||Leaf margins entire; e of California||> 11|
|11||Involucres (5.5–)6–9.5 × 5–7.5 mm; phyllary apices yellowish to greenish yellow, sparsely or not at all gland-dotted; florets 19–25, corollas 5–6.5 mm; cypsela ribs forming hornlike extensions; Arizona, Colorado, New Mexico, Utah||Isocoma rusbyi|
|11||Involucres (5.5–)6.5–8 × 4.5–5.5(–7) mm; phyllary apices usually green, usually densely gland-dotted; florets (18–)22–34, corollas 5.8–7.5 mm; cypsela ribs not forming hornlike extensions; Texas||Isocoma drummondii|