Herbs, winter-annual, biennial, or short-lived perennial. Leaves: basal rosette present, not persistent, proximalmost internodes to 10+ mm; blade without broad basal lobes, margins entire, toothed, pinnate, or pinnatisect, flat. Flowers: stamens all fertile or 5+ outer sterile; filaments heteromorphic, outer dorsiventrally flattened, linear, elliptic, spatulate, or oblanceolate, usually petaloid, sometimes not, inner filiform, unlobed; ovules (and seeds) oriented perpendicular to long axis of ovary. Capsules cupshaped to lingulate or cylindric, straight. Seeds ovoid, flattened dorsiventrally, winged; seed-coat cells polygonal, ± isodiametric, anticlinal walls straight, wavy, or sinuous.

Distribution

w, c North America, n Mexico, s South America, arid to mesic regions

Discussion

Species 51 (51 in the flora).

In the following keys and descriptions, three trichome types are defined as follows. Simple grappling-hook trichomes have a cylindric stalk that is unbarbed except at the apex, which terminates with four (or sometimes fewer) retrorse barbs. Complex grappling-hook trichomes have a tapered stalk that is retrorsely barbed along its length and capped usually by four retrorse barbs. Needlelike trichomes have swollen bases resting on a pedestal formed by a ring of epidermal cells, are relatively long and pointed, and bear antrorse barbs or knobs along their length.

Leaf blade intersinus distance is measured across the leaf blade between opposite sinuses; for leaves with entire margins, it is the width of the leaf blade. Anther papillae and seed coat anticlinal wall shape can be observed at 10/x magnification; greater magnification (30–40/x) is needed to see seed coat cell papillae in species with numerous small papillae.

Selected References

Schenk, J. J. and L. Hufford. 2011. Phylogeny and taxonomy of Mentzelia section Bartonia (Loasaceae). Syst. Bot. 36: 711–720.

Lower Taxa

Mentzelia albescens, Mentzelia argillicola, Mentzelia argillosa, Mentzelia candelariae, Mentzelia canyonensis, Mentzelia chrysantha, Mentzelia collomiae, Mentzelia conspicua, Mentzelia cronquistii, Mentzelia decapetala, Mentzelia densa, Mentzelia filifolia, Mentzelia flumensevera, Mentzelia goodrichii, Mentzelia holmgreniorum, Mentzelia hualapaiensis, Mentzelia humilis, Mentzelia integra, Mentzelia inyoensis, Mentzelia laciniata, Mentzelia laevicaulis, Mentzelia lagarosa, Mentzelia leucophylla, Mentzelia librina, Mentzelia longiloba, Mentzelia marginata, Mentzelia memorabilis, Mentzelia mexicana, Mentzelia multicaulis, Mentzelia multiflora, Mentzelia nuda, Mentzelia oreophila, Mentzelia paradoxensis, Mentzelia perennis, Mentzelia polita, Mentzelia procera, Mentzelia pterosperma, Mentzelia puberula, Mentzelia pumila, Mentzelia reverchonii, Mentzelia rhizomata, Mentzelia rusbyi, Mentzelia saxicola, Mentzelia shultziorum, Mentzelia sivinskii, Mentzelia speciosa, Mentzelia springeri, Mentzelia strictissima, Mentzelia tiehmii, Mentzelia todiltoensis, Mentzelia uintahensis

Key

1	Petals white.	> 2
2	Anther epidermis papillate.	> 3
3	Leaf blade widest intersinus distances 9.5–29 mm.	Mentzelia hualapaiensis
3	Leaf blade widest intersinus distances 0.8–9.1 mm.	> 4
4	5 outermost stamens without anthers; New Mexico, Texas.	Mentzelia humilis
4	5 outermost stamens with anthers; Arizona.	Mentzelia canyonensis
2	Anther epidermis smooth.	> 5
5	Petals 13–22.7 mm wide; stamens white to yellow.	Mentzelia decapetala
5	Petals 1.9–10.3 mm wide; stamens white.	> 6
6	Petals 22.6–49 × 3.6–10.3 mm; bracts pinnate.	Mentzelia nuda
6	Petals 14.7–22(–24.4) × 1.9–4.4 mm; bracts entire or dentate.	Mentzelia strictissima
1	Petals light to golden yellow.	> 7
7	Petals hairy abaxially.	> 8
8	Branches perpendicular to stem, especially proximal and mid-stem, or slightly antrorse, especially distal, ± equal; Montrose County, Colorado.	Mentzelia paradoxensis
8	Branches antrorse, proximal longer than distal, all usually extending to near the distal end of plant; Arizona, Colorado (including Montrose County), New Mexico, Utah.	> 9
9	5 outermost stamens with anthers; margins of proximal leaf blades with teeth 0.5–4 mm; Colorado.	Mentzelia marginata
9	5 outermost stamens usually without anthers; margins of proximal leaf blades with teeth or lobes 1.2–6(–9) mm, always some more than 4 mm; Arizona, Colorado, New Mexico, Utah.	Mentzelia cronquistii
7	Petals glabrous abaxially.	> 10
10	Anther epidermis papillate.	> 11
11	5 outermost stamens with anthers.	> 12
12	Leaf blades 4–12.2 mm wide, always some more than 10 mm wide, margins usually entire, rarely dentate, teeth 0(–4), 0.1–1.4 mm; seed coat cell papillae 3–5 per cell; Utah.	Mentzelia argillosa
12	Leaf blades 1.5–8.8 mm wide, margins entire, dentate, or pinnate, teeth or lobes 0–8 (always some leaves with 5+ teeth or lobes), 0.1–2 mm (always some more than 1.4 mm); seed coat cell papillae 5–8 per cell; Nevada.	Mentzelia argillicola
11	5 outermost stamens without anthers.	> 13
13	Leaf blade widest intersinus distances 9.5–29 mm.	Mentzelia hualapaiensis
13	Leaf blade widest intersinus distances 1–4.2 mm.	> 14
14	Second whorl of stamens without anthers.	Mentzelia perennis
14	Second whorl of stamens with anthers.	> 15
15	Seed coat anticlinal cell walls straight, papillae 38–45 per cell; Texas.	Mentzelia saxicola
15	Seed coat anticlinal cell walls sinuous, papillae 6–12 per cell; New Mexico.	Mentzelia todiltoensis
10	Anther epidermis smooth.	> 16
16	Capsules longitudinally ridged.	Mentzelia rusbyi
16	Capsules not longitudinally ridged.	> 17
17	Stems multiple, from subterranean branching caudices.	> 18
18	5 outermost stamens without anthers.	Mentzelia springeri
18	5 outermost stamens with anthers.	> 19
19	Bases of distal leaf blades clasping.	> 20
20	Leaf blade surfaces densely hairy, whitish, abaxial surfaces with complex grappling-hook and needlelike trichomes; Ash Meadows, Nye County, Nevada.	Mentzelia leucophylla
20	Leaf blade surfaces moderately hairy, green, abaxial surfaces with complex grappling-hook trichomes only; se California and s Nevada (including Nye County).	> 21
21	Leaf blades 5.8–9.1 mm wide, widest intersinus distances 4.6–7.1 mm; margins of distal leaf blades with 0–6 teeth; seed coat cell papillae 15–46 per cell.	Mentzelia tiehmii
21	Leaf blades 7.6–41.2 mm wide, widest intersinus distances 5.1–35.3 mm; margins of distal leaves with 6–16 teeth; seed coat cell papillae 6–17 per cell.	Mentzelia oreophila
19	Bases of distal leaf blades not clasping.	> 22
22	Margins of proximal leaf blades 3-fid or pinnate to pinnatisect (rarely entire in M. librina).	> 23
23	Filaments of 5 outermost stamens broadly spatulate, strongly clawed.	> 24
24	Margins of proximal leaf blades pinnate to pinnatisect, lobes 4–12.	Mentzelia uintahensis
24	Margins of proximal leaf blades usually 3-fid, rarely entire or pinnate, teeth or lobes usually 2, rarely 0 or 4.	Mentzelia librina
23	Filaments of 5 outermost stamens narrowly spatulate or oblanceolate, slightly clawed.	> 25
25	Petals 2.9–3.8 mm wide; filaments of 5 outermost stamens less than 1.3–1.9 mm wide; Utah.	Mentzelia flumensevera
25	Petals 5.1–9 mm wide; filaments of 5 outermost stamens 2–4.5 mm wide; Colorado.	Mentzelia multicaulis
22	Margins of proximal leaf blades entire or dentate to serrate.	> 26
26	Stems glabrescent, smooth to the touch.	Mentzelia polita
26	Stems hairy.	> 27
27	Margins of proximal leaves entire.	> 28
28	Petals 7.4–11 × 2.6–3.4 mm; margins of distal leaf blades entire; filaments of 5 outermost stamens slightly clawed; Arizona.	Mentzelia memorabilis
28	Petals 12.5–17.2 × 6.1–11.2 mm; margins of distal leaf blades entire or dentate; filaments of 5 outermost stamens strongly clawed; Colorado.	Mentzelia rhizomata
27	Margins of proximal leaf blades dentate or serrate, with 4–12 teeth.	> 29
29	Petals 6.7–9.2 mm wide; seed coat cell papillae 11–15 per cell.	Mentzelia goodrichii
29	Petals 2.3–5.9 mm wide; seed coat cell papillae 3–6 per cell.	> 30
30	Stems zigzag; margins of proximal leaf blades with 4–6 teeth, each 0.5–0.6 × 1.2–1.5 mm.	Mentzelia shultziorum
30	Stems straight; margins of proximal leaf blades with 8–12 teeth, each 1–4 × 1.7–6 mm.	Mentzelia puberula
17	Stems solitary or, if multiple, from ground-level caudices.	> 31
31	5 outermost stamens not petaloid, with anthers, filaments linear to narrowly elliptic or narrowly spatulate, 0.5–2(–2.5) mm wide, if narrowly spatulate, to 1.5 mm wide.	> 32
32	Petals 23.5–70 mm; styles 20.4–57 mm; 5 outermost stamens 17.7–55 mm.	Mentzelia laevicaulis
32	Petals (6.1–)7.3–15.8(–18) mm; styles 3.7–13 mm; 5 outermost stamens 4–13.2 mm.	> 33
33	Petals 11.7–15.8(–18) mm; 5 outermost stamens 10.2–13.2 mm.	Mentzelia inyoensis
33	Petals (6.1–)7.3–10 mm; 5 outermost stamens 4–8 mm.	Mentzelia candelariae
31	5 outermost stamens petaloid, with or without anthers, filaments narrowly spatulate to elliptic or oblanceolate, 1–9.7 mm wide, if narrowly spatulate and less than 1.5 mm wide, then without anthers.	> 34
34	Margins of distal leaf blades pinnatisect (sometimes pinnate in M. laciniata).	> 35
35	Petals 30–42.2 mm; 5 outermost stamens 27–37.4 mm; styles 24–32.4 mm.	Mentzelia conspicua
35	Petals 8.3–23.8(–26) mm; 5 outermost stamens 6.5–20.4 mm; styles 6.1–12.5(–14) mm.	> 36
36	Petals 8.3–13 mm; 5 outermost stamens 6.5–10.7 mm; Colorado, Nevada, Utah.	Mentzelia lagarosa
36	Petals 13–23.8(–26) mm; 5 outermost stamens 10.2–20.4 mm; Arizona, Colorado, New Mexico.	> 37
37	Lobes of distal leaf blades strongly antrorse; branches upcurved; Arizona, New Mexico.	Mentzelia holmgreniorum
37	Lobes of distal leaf blades slightly antrorse or perpendicular to leaf axis; branches straight; Arizona, Colorado, New Mexico.	> 38
38	Seed coat cell papillae 42–48 per cell; Chuska Mountains of Arizona and New Mexico.	Mentzelia filifolia
38	Seed coat cell papillae 5–14 per cell; Colorado, New Mexico (not Chuska Mountains).	Mentzelia laciniata
34	Margins of distal leaf blades entire, dentate, serrate, or pinnate.	> 39
39	Bases of distal leaf blades clasping (rarely a few not clasping in M. pterosperma).	> 40
40	Capsules cup-shaped; Arizona, California, Colorado, Nevada, Utah.	Mentzelia pterosperma
40	Capsules cylindric; Kansas, Missouri, Oklahoma, Texas.	Mentzelia albescens
39	Bases of distal leaf blades not clasping (sometimes a few clasping in M. mexicana).	> 41
41	Filaments of 5 outermost stamens strongly clawed; on black (to sometimes reddish) volcanic cinder cone soils in San Francisco Volcanic Field, Coconino County, Arizona.	Mentzelia collomiae
41	Filaments of 5 outermost stamens slightly clawed; not on black (to sometimes reddish) volcanic cinder cone soils in San Francisco Volcanic Field, Coconino County, Arizona.	> 42
42	Seed coat cell anticlinal walls straight.	> 43
43	Capsule length more than 2 times diam. (occasionally a few slightly less in M. reverchonii).	> 44
44	Styles 8.7–11.4(–13) mm; Colorado, New Mexico, Oklahoma, Texas.	Mentzelia reverchonii
44	Styles 5.6–7.8 mm; Montana, Utah, Wyoming.	Mentzelia pumila
43	Capsule length to 2 times diam. (sometimes a few more in M. longiloba).	> 45
45	Capsule bases tapering; seed coat cell papillae 4–6 per cell.	Mentzelia longiloba
45	Capsule bases rounded; seed coat cell papillae 8–45 per cell.	> 46
46	Seed coat cell papillae 8–12 per cell; leaf blade widest intersinus distances 3.4–19 mm.	Mentzelia mexicana
46	Seed coat cell papillae 38–45 per cell; leaf blade widest intersinus distances 1.6–3.4 mm.	Mentzelia saxicola
42	Seed coat cell anticlinal walls wavy to sinuous.	> 47
47	Margins of proximal leaves with 4–12 teeth or lobes.	> 48
48	Plants candelabra-form; proximal branches not decumbent; largest leaf trichomes with pearly white bases; leaf blade margin teeth with proximal side antrorse, distal side perpendicular to leaf axis; Arizona, Nevada, Utah.	Mentzelia integra
48	Plants bushlike; proximal branches decumbent; largest leaf trichomes without pearly white bases; leaf blade margin teeth or lobes perpendicular to leaf axis; c Colorado.	Mentzelia densa
47	Margins of proximal leaves with 14–38 teeth or lobes (occasionally 10–38 in M. speciosa).	> 49
49	Seed coat cell anticlinal walls wavy; petals golden yellow; leaf blades 43.6–233 mm, always some 146+ mm.	Mentzelia speciosa
49	Seed coat cell anticlinal walls sinuous; petals light to golden yellow; leaf blades 32.8–125(–146) mm.	> 50
50	5 outermost stamens with anthers.	> 51
51	Leaf blade widest intersinus distances 2.3–7.1 mm, always some leaves 4+ mm; branches upcurved.	Mentzelia longiloba
51	Leaf blade widest intersinus distances 1–2.9 mm; branches straight.	Mentzelia sivinskii
50	5 outermost stamens without anthers.	> 52
52	Leaf blade widest intersinus distances 1–3.9 mm; petals light yellow.	Mentzelia procera
52	Leaf blade widest intersinus distances 2.1–13.7(–14) mm, always some leaves 4+ mm; petals light to golden yellow.	> 53
53	Seed coat cell papillae 10–21 or 67–106 per cell; Arizona, California, Colorado, New Mexico, Utah.	Mentzelia longiloba
53	Seed coat cell papillae 29–48 per cell; Colorado, Nebraska, New Mexico, Wyoming.	> 54
54	Branches distal or along entire stem, straight.	Mentzelia chrysantha
54	Branches distal, upcurved.	Mentzelia multiflora

"not elongating" is not a number.