Isocoma

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.

Distribution

sw United States, Mexico.

Discussion

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).

Selected References

Nesom, G. L. 1991c. Taxonomy of Isocoma (Asteraceae: Astereae). Phytologia 70: 69–114.

Lower Taxa

Isocoma acradenia, Isocoma arguta, Isocoma azteca, Isocoma coronopifolia, Isocoma drummondii, Isocoma humilis, Isocoma menziesii, Isocoma pluriflora, Isocoma rusbyi, Isocoma tenuisecta

Key

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