Artemisia subg. Tridentatae

(Rydberg) McArthur

Amer. J. Bot. 68: 590. 1981

Basionyms: Tridentatae Rydberg in N. L. Britton et al., N. Amer. Fl. 34: 282. 1916
Treatment appears in FNA Volume 19. Treatment on page 509. Mentioned on page 504, 508, 510, 511, 512, 513, 515, 524.
Shrubs; fibrous rooted, caudices woody, rhizomes absent. Stems not wandlike (relatively numerous; new stems may sprout from caudices). Leaves (pungently aromatic) deciduous or persistent, cauline (in lateral fascicles on vegetative shoots). Heads discoid (except A. bigelovii with, rarely, 1–2 raylike florets). Receptacles epaleate, glabrous. Pappi 0. Florets: 3–20, bisexual, fertile; corollas (pale yellow) funnelform.

Distribution

North America, nw Mexico.

Discussion

Species 10 (10 in the flora).

Difficulty in classification of Artemisia subg. Tridentatae has been complicated by transfer of North American species to Seriphidium (Y. R. Ling 1995b; W. A. Weber 1984b), a disposition not followed here. Species circumscription varies among authors, but most modern treatments recognize the species as defined here. The most useful field characteristics in sagebrush taxonomy are size of the plant, shape and lobing of the vegetative leaves, and size and shape of the flowering heads (A. A. Beetle 1960; A. H. Winward 1970). Differences in chromosome number are more useful in defining subspecies than species (E. D. McArthur et al. 1981; G. H. Ward 1953), and introgression among subspecies is common (McArthur et al. 1988; McArthur and S. C. Sanderson 1999). The following key relies on vegetative characteristics, and unless noted, descriptions of leaf size and lobing refer to the leaves found in the vegetative shoots proximal to arrays of heads. These ‘vegetative leaves’ occur in bundles, or fascicles that are part of the lateral shoots. They are subtended by an elongate leaf (termed ‘ephemeral’), which is attached to the primary stem and falls off early in the season. With the exception of Artemisia spiciformis, which retains its ephemeral leaves through most of the growing season, ephemeral leaves normally drop from the plant before the onset of flowering.

References

Key

1 Leaves deciduous, blades usually entire, sometimes irregularly lobed; moist habitats Artemisia cana
1 Leaves deciduous or persistent, blades usually lobed, sometimes entire; dry habitats > 2
2 Leaves bright green, pinnately lobed, lobes 3–7 (gypsum or shale) Artemisia pygmaea
2 Leaves gray-green, usually palmately lobed, lobes 0 or 3 or 3–6 > 3
3 Leaf lobe lengths 1/3+ blade lengths, widths 1–1.5 mm > 4
3 Leaf lobe lengths to 1/3 blade lengths, widths (1–)1.5–5 mm > 5
4 Leaves rigid (lava scablands, Oregon and Washington) Artemisia rigida
4 Leaves not rigid Artemisia tripartita
5 Shrubs, 50–200(–250) cm > 6
5 Shrubs, 10–50 cm > 8
6 Leaves mostly deciduous (variable in size and shape, entire or irregularly 3–6-lobed, lobes rounded or acute); involucres broadly campanulate. Artemisia spiciformis
6 Leaves persistent (lobes 3, uniform, lengths to 1/3 blade lengths); involucres lanceoloid or ovoid > 7
7 Leaves light or dark gray-green, sticky-resinous; involucres ovoid, 3–5 × 4–6 mm; florets 12–20 (California) Artemisia rothrockii
7 Leaves gray-green, not sticky-resinous (widespread, including California); involucres lanceoloid, (1–)1.5–4 × 1–3 mm; florets 3–8. Artemisia tridentata
8 Leaves silver-green, blades narrowly cuneate, lobes acute; heads mostly nodding; involucres globose Artemisia bigelovii
8 Leaves dark green to gray-green, blades broadly cuneate, lobes obtuse or rounded; heads mostly erect; involucres campanulate, globose-ovoid, or turbinate. > 9
9 Leaves on flowering stems entire (heads mostly pedunculate); involucres narrowly turbinate; phyllaries sparsely hairy or glabrous Artemisia nova
9 Leaves on flowering stems entire or lobed (heads mostly sessile); involucres campanulate or globose-ovoid; phyllaries densely pubescent or tomentose > 10
10 Leaves on flowering stems 3-lobed, not sticky-resinous Artemisia arbuscula
10 Leaves on flowering stems entire, sticky-resinous or densely hairy and not sticky Artemisia rothrockii