Trans. Amer. Philos. Soc., n. s.7: 297. 1840
sw United States, nw Mexico.
Species 10 (9 in the flora).
Xylorhiza species were considered by A. Cronquist and D. D. Keck (1957) to be primitive members of the genus Machaeranthera, where they were recently maintained (Cronquist 1994). T. J. Watson (1977) and other contemporary botanists, however, have recognized Xylorhiza as a distinct genus characterized by the following features: perennial herbs, subshrubs, and shrubs from large, woody taproots with woody, branched, persistent caudices; large, single heads; long phyllaries with erect or spreading apices; white or bluish rays; large cypselae; and vernal flowering. The base chromosome number of Xylorhiza (x = 6) also contrasts with that of Machaeranthera (x = 4 or 5). Two sections were recognized by Watson: sect. Xylorhiza (subshrubs or perennial herbs) and sect. Californica T. J. Watson (shrubs, including X. cognata, X. orcuttii, and the Mexican endemic X. frutescens).
|1||Leaf margins (many of them) toothed||> 2|
|1||Leaf margins all or nearly all entire||> 5|
|2||Leaf blades mostly 1–2.5 mm wide, margins involute||Xylorhiza confertifolia|
|2||Leaf blades mostly (2–)2.5–15 mm wide, margins flat||> 3|
|3||Leaf blades linear-oblong to linear-lanceolate, bases truncate or rounded-auriculate||Xylorhiza linearifolia|
|3||Leaf blades oblanceolate to oblong-spatulate, bases tapering||> 4|
|4||Stems leafy for 2/3 + their lengths; peduncles 2–6 cm; involucres 7–14 × 12–25 mm||Xylorhiza glabriuscula|
|4||Stems leafy in proximal 1/2 – 2/3; peduncles 5–20 cm; involucres 12–20 × 20–35 mm||Xylorhiza venusta|
|5||Shrubs; stems branched to near apices; style-branch appendages shorter than stigmatic lines; s California||> 6|
|5||Subshrubs or perennials; stems branched mostly in proximal 3/4; style-branch appendages longer or shorter than stigmatic lines; various regions||> 7|
|6||Younger stems and branchlets stipitate-glandular; phyllaries loosely appressed, out-ermost stipitate-glandular (at least margins), innermost shorter than mid||Xylorhiza cognata|
|6||Younger stems and branchlets eglandular; phyllaries tightly appressed; outermost glabrous, innermost equaling or longer than mid||Xylorhiza orcuttii|
|7||Leaf blades spatulate to obovate or oblong, rarely elliptic; Texas||Xylorhiza wrightii|
|7||Leaf blades linear-lanceolate to lanceolate, oblanceolate, or elliptic-oblong; n to w of Texas||> 8|
|8||Leaf blades 2.5–6 mm wide, usually shallowly spinulose-toothed, sometimes entire; rays 13–17, white||Xylorhiza cronquistii|
|8||Leaf blades 3–20(–25) mm wide, sharply spinulose-toothed; rays (15–)18–60(–85), usually lavender, seldom white||Xylorhiza tortifolia|