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237. Tetradymia de Candolle in A. P. de Candolle and A. L. P. P. de Candolle, Prodr. 6: 440. 1838.

[Greek tetradymos, fourfold, evidently alluding to numbers of phyllaries and florets in heads of original species]

John L. Strother

Shrubs, mostly 30–200 cm. Stems 1–5+, erect to spreading (tomentose to pannose or glabrous, often spiny). Leaves (usually with fascicles of secondary leaves in axils of the primary; primaries sometimes becoming spines) cauline; alternate; sessile (or obscurely petiolate); blades obscurely palmately nerved (usually 1-nerved), linear to filiform, narrowly lanceolate, or oblanceolate, margins entire, faces glabrous or lanate to tomentose, sometimes glabrescent. Heads discoid, (1–3) in distil axils or (3–8) in corymbiform clusters. Calyculi 0. Involucres turbinate to cylindric or hemispheric, 3–10+ mm diam. Phyllaries persistent, 4–6 in 1–2 series, erect, distinct, mostly oblong to ovate or lanceolate, equal or subequal, margins obscurely scarious. Receptacles flat, foveolate, epaleate. Ray florets 0. Disc florets 4–9, bisexual, fertile; corollas cream to bright yellow, tubes longer than funnelform or abruptly dilated throats, lobes 5, recurved, linear to lanceolate; style branches stigmatic in 2 lines, apices usually truncate to rounded-truncate, sometimes truncate-penicillate (appendages essentially 0). Cypselae prismatic to obconic or fusiform, obscurely 5-ribbed, glabrous or hirsute to densely pilose (hairs often obscuring pappi); pappi 0 or of 70–150 bristles or of 20–30, white to stramineous, subulate to setiform scales. x = 30.

Species 10 (10 in the flora): w North America, nw Mexico.


Strother, J. L. 1974. Taxonomy of Tetradymia (Compositae: Senecioneae). Brittonia 26: 177–202.

1 Indumentum of stems evenly pannose; cypselae with white hairs that obscure or replace pappi; pappi 0 or of 20–30, subulate to setiform scales   (2)
+ Indumentum of stems interrupted by linear, tomentose to nearly glabrous streaks; cypselae glabrous or with induments shorter than cypselae, not obscuring or replacing pappi; pappi of 70–150 bristles   (5)
2 (1) Primary leaves forming rigid spines; heads in clusters of 1–3 (in axils of spines)   (3)
+ Primary leaves flaccid or stiff and pungent, not forming spines; heads in corymbiform clusters of 3–7   (4)
3 (2) Spines straight, 10–50 mm, glabrescent; involucres mostly turbinate, 7–9 mm; cypselae 4–5 mm   10 Tetradymia axillaris
+ Spines recurved, 5–25 mm, tomentose; involucres mostly campanulate, 8–12 mm; cypselae 6–8 mm   9 Tetradymia spinosa
4 (2) Blades of primary leaves lance-linear to spatulate, becoming stiff, pungent; florets 5–8; cypselae 6–8 mm, epappose   8 Tetradymia comosa
+ Blades of primary leaves filiform, flaccid; florets 4(–5); cypselae 4–6 mm, pappose   7 Tetradymia tetrameres
5 (1) Primary leaves not forming spines   (6)
+ Primary leaves becoming rigid, ± persistent spines   (8)
6 (5) Blades of primary leaves subulate and ± appressed to stems, contrasting with subtended clavate, glabrous or glabrescent blades of secondary leaves   3 Tetradymia glabrata
+ Blades of primary leaves filiform to spatulate, spreading, similar to subtended tomentose to sericeous blades of secondary leaves   (7)
7 (6) Blades of primary leaves spatulate to lanceolate; pappus bristles weakly, if at all, distally dilated   1 Tetradymia canescens
+ Blades of primary leaves linear to filiform; pappus bristles distally dilated   2 Tetradymia filifolia
8 (5) Blades of secondary leaves filiform to clavate; cypselae glabrous   4 Tetradymia argyraea
+ Blades of secondary leaves spatulate; cypselae densely hirsute or villous to densely pilose   (9)
9 (8) Blades of secondary leaves greenish, usually villous, sometimes glabrescent; florets mostly 4; cypselae 4–6 mm, densely hirsute   5 Tetradymia nuttallii
+ Blades of secondary leaves silvery sericeous; florets mostly 5; cypselae 6–8 mm, villous to densely pilose   6 Tetradymia stenolepis

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