Herbs, annual. Leaves: basal rosette present, persistent or not, proximalmost internodes to 5 mm; blade without broad basal lobes, margins entire, toothed, pinnate, or pinnatisect, flat. Flowers: stamens all fertile, filaments monomorphic, filiform, or heteromorphic, 5 outermost dorsiventrally flattened, linear or elliptic, not petaloid, inner filiform, usually unlobed, rarely distally 2-lobed; ovules (and seeds) oriented parallel to long axis of ovary. Capsules cylindric or clavate, axillary straight to strongly curved, sometimes S-shaped, terminal straight to slightly curved. Seeds triangular prisms with grooves along longitudinal edges or irregularly polygonal (angular or rounded), not winged (occasionally winged in M. lindleyi); seed-coat cells polygonal, ± isodiametric, anticlinal walls straight.

Distribution

w North America, nw Mexico, South America (Argentina), South America (Chile)

Discussion

Species ca. 22 (21 in the flora).

Both homoploid hybridization and allopolyploidy have played important roles in the diversification of sect. Trachyphytum (J. M. Brokaw 2009; Brokaw and L. Hufford 2010, 2010b). H. J. Thompson and H. Lewis (1953) and J. E. Zavortink (1966) reported diploid, tetraploid, hexaploid, and octoploid species in the section. The small desert and grassland annuals of this section are often difficult to distinguish due to the simplicity of their shoot systems, inter-population variability, and morphological intermediacy of allopolyploids. In contrast, homoploid hybridization appears to be rare among existing populations. Evidence of homoploid hybridization has been found primarily in molecular studies (Brokaw and Hufford 2010) and has been observed in natural populations only among hexaploid species (Zavortink).

Seed characteristics have been some of the most reliable characters for discrimination of the common, widespread species but often require an advanced developmental stage for observation. However, the two major clades in sect. Trachyphytum, “Affines” [containing Mentzelia affinis, M. dispersa, and M. micrantha (J. M. Brokaw and L. Hufford 2010)] and “Trachyphyta” [containing all other diploids and most polyploids (Brokaw and Hufford 2010, 2010b)], can be distinguished relatively early in seed development. Even when immature, plants of the “Affines” clade have a single triangular ovule in the cross-sectional view of the top of the ovary. With the exception of the narrowly distributed M. packardiae, immature plants of the “Trachyphyta” clade have two or more ovules at the top of the ovary. Further, these two groups differ in the size of their seed coat cells, which can be distinguished early in seed development: cells of the “Affines” clade are small and barely discernible at 10× magnification, whereas those of the “Trachyphyta” clade are large and clearly visible at 10× magnification, resulting in a tessellate to tuberculate appearance of the seed coat.

The only species in sect. Trachyphytum not found in the flora area is the South American Mentzelia bartonioides (C. Presl) Urban & Gilg.

Selected References

Brokaw, J. M. 2009. Phylogeny of Mentzelia Section Trachyphytum: Origins and Evolutionary Ecology of Polyploidy. Ph.D. dissertation. Washington State University.

Brokaw, J. M. and L. Hufford. 2010. Phylogeny, introgression, and character evolution of diploid species in Mentzelia section Trachyphytum (Loasaceae). Syst. Bot. 35: 601–617.

Brokaw, J. M. and L. Hufford. 2010b. Origins and introgression of polyploid species in Mentzelia section Trachyphytum (Loasaceae). Amer. J. Bot. 97: 1457–1473.

Lower Taxa

Mentzelia affinis, Mentzelia albicaulis, Mentzelia congesta, Mentzelia crocea, Mentzelia desertorum, Mentzelia dispersa, Mentzelia eremophila, Mentzelia gracilenta, Mentzelia jonesii, Mentzelia lindleyi, Mentzelia micrantha, Mentzelia mollis, Mentzelia monoensis, Mentzelia montana, Mentzelia nitens, Mentzelia obscura, Mentzelia packardiae, Mentzelia pectinata, Mentzelia ravenii, Mentzelia thompsonii, Mentzelia veatchiana

Key

1	Seeds in 1 row distal to mid fruit, triangular prisms.	> 2
2	Stamen filaments heteromorphic, 5 outermost elliptic, distally 2-lobed, inner filiform, unlobed.	Mentzelia micrantha
2	Stamen filaments monomorphic, filiform, unlobed.	> 3
3	Petals 10–20 mm; styles 6–14 mm; Malheur County, Oregon.	Mentzelia packardiae
3	Petals 2–12 mm; styles 2–6.5 mm; w North America, including Malheur County, Oregon.	> 4
4	Blades of basal leaves usually deeply to moderately lobed, sinuses extending more than 1/4 to midvein, rarely entire; styles 3–6.5 mm; capsules often prominently longitudinally ribbed; Arizona, s California, below 1200 m.	Mentzelia affinis
4	Blades of basal leaves usually dentate, sinuses extending less than 1/4 to midvein, or entire, rarely deeply lobed; styles 2–3.5(–5); capsules usually inconspicuously longitudinally ribbed; w North America, including Arizona and s California, where above 1200 m.	Mentzelia dispersa
1	Seeds in 2+ rows distal to mid fruit, irregularly polygonal or occasionally triangular prisms proximal to mid fruit.	> 5
5	Basal leaves not persisting; blade margins of proximalmost remaining leaves (proximal cauline) dentate or entire; usually on barren, alkaline or saline soils.	> 6
6	Petals 8–12 mm; sepals 3–5.5 mm; Idaho, Nevada, Oregon.	Mentzelia mollis
6	Petals 2–4 mm; sepals 1–3 mm; Colorado, New Mexico, Utah.	Mentzelia thompsonii
5	Basal leaves persisting; blade margins of proximalmost leaves (basal) usually deeply to shallowly lobed, rarely entire; on wide variety of substrates.	> 7
7	Bract margins entire, bracts green.	> 8
8	Petals (6–)8–25 mm.	> 9
9	Sepals (7–)9–16 mm; styles 7–15 mm; e Kern County and nw San Bernardino County, California.	Mentzelia eremophila
9	Sepals (2–)3–8(–10) mm; styles 4–10 mm; Arizona, e California, Nevada.	> 10
10	Seeds without recurved flap over hilum; seed coat cell outer periclinal wall domes on seed edges more than 1/2 as tall as wide at maturity.	Mentzelia jonesii
10	Seeds usually with recurved flap over hilum; seed coat cell outer periclinal wall domes on seed edges less than 1/2 as tall as wide at maturity.	Mentzelia nitens
8	Petals 2–8 mm.	> 11
11	Seeds dark brown or tan and moderately to densely dark-mottled; seed coat cell outer periclinal walls domed, domes on seed edges more than 1/2 as tall as wide at maturity.	Mentzelia albicaulis
11	Seeds tan, not or occasionally sparsely dark-mottled; seed coat cell outer periclinal walls flat to slightly convex, or if domed, domes on seed edges less than 1/2 as tall as wide.	> 12
12	Capsules 6–15 mm, axillary curved to 20°; 2000–2500 m; Mono County, California.	Mentzelia monoensis
12	Capsules 11–31 mm, axillary curved to 250°; 30–1700 m; sw United States, but not Mono County, California.	> 13
13	Blade margins of basal leaves usually shallowly lobed, lobes rounded; seed coat cell outer periclinal walls flat or slightly convex.	Mentzelia desertorum
13	Blade margins of basal leaves usually deeply lobed, lobes pointed; seed coat cell outer periclinal walls domed.	Mentzelia obscura
7	Bract margins toothed or lobed, or if entire, bracts green with white base.	> 14
14	Bracts green.	> 15
15	Styles 15–35 mm; stamens 11–40 mm, filaments heteromorphic, 5 outermost linear, inner filiform.	> 16
16	Petals usually elliptic to ovate, rarely obovate, 8–17(–21) mm wide; west slope of Sierra Nevada, California.	Mentzelia crocea
16	Petals obovate, (12–)16–33 mm wide; Coast Ranges, California.	Mentzelia lindleyi
15	Styles 2–15 mm; stamens 3–11 mm, filaments monomorphic, filiform.	> 17
17	Petals 3–7(–10) mm; styles 2–6 mm.	> 18
18	Petals red to orange proximally, orange to orange-yellow distally; styles (3–)3.5–6 mm.	Mentzelia veatchiana
18	Petals orange proximally, yellow distally; styles usually less than 3.5 mm.	> 19
19	Bract margins 3-lobed or entire, lateral lobes never prominent; capsules 8–28(–35) mm (longest capsules usually more than 15 mm), axillary curved to 180°; 0–2300 m.	Mentzelia albicaulis
19	Bract margins usually 3–7-lobed, rarely entire, lateral lobes usually prominent; capsules 6–17(–20) mm, axillary curved to 45°; 600–3400 m.	Mentzelia montana
17	Petals 8–25 mm; styles 5–15 mm.	> 20
20	Petals yellow; bracts usually entire, rarely 2-lobed; w Mojave Desert, e Kern County, nw San Bernardino County, California.	Mentzelia eremophila
20	Petals red to orange proximally, orange to yellow distally; bracts 3–7-lobed; s San Joaquin Valley, Inner Coast Ranges, s Sierra Nevada foothills, California.	> 21
21	Petals orange proximally, yellow distally; Fresno, Monterey, and San Benito counties, California, 200–1400 m, and s San Luis Obispo, Santa Barbara, and Ventura counties, California, 1500–1700 m.	Mentzelia gracilenta
21	Petals red to orange proximally, orange to yellow distally; Kern, San Luis Obispo, Santa Barbara, and Tulare counties, California, 200–1400 m.	Mentzelia pectinata
14	Bracts green with white base or mostly white with green margins.	> 22
22	Bracts concealing capsules, mostly white with green margins.	Mentzelia congesta
22	Bracts not concealing capsules, green with white base.	> 23
23	Petals usually 8+ mm, usually orange, rarely yellow, proximally, yellow distally; seed coat cell outer periclinal wall domes on seed edges 1/2 as tall as wide at maturity.	> 24
24	Plants of grasslands, pine-oak woodlands; Coast and Transverse ranges, California.	Mentzelia gracilenta
24	Plants of desert scrub, Joshua-tree woodlands; ne Los Angeles and w Riverside counties, California.	Mentzelia ravenii
23	Petals usually less than 5 mm, red to orange proximally, orange to yellow distally; if petals 4+ mm, then seed coat cell outer periclinal wall domes on seed edges more than 1/2 as tall as wide at maturity.	> 25
25	Styles usually less than 3.5 mm.	> 26
26	Bracts 3–4.1 × 1.1–1.7 mm, margins entire; seeds not mottled, seed coat cell outer periclinal wall domes on seed edges less than 1/2 as tall as wide at maturity.	Mentzelia monoensis
26	Bracts 5.9–9.2 × 1.7–5 margins usually 3–7-lobed, rarely entire,; seeds moderately to densely dark-mottled, seed coat cell outer periclinal wall domes on seed edges more than 1/2 as tall as wide.	Mentzelia montana
25	Styles usually 3.5+ mm.	> 27
27	Petals red to orange proximally, orange to orange-yellow distally; bracts usually 3–7-lobed, rarely entire.	Mentzelia veatchiana
27	Petals orange proximally, yellow distally; bracts 3–7-lobed or entire.	> 28
28	Plants of desert scrub, Joshua-tree woodlands; ne Los Angeles and w Riverside counties, California.	Mentzelia ravenii
28	Plants of oak-pine woodlands, grasslands; Arizona, California, Nevada, Oregon.	Mentzelia veatchiana

"not elongating" is not a number.