Micranthes

Herbs, sometimes rhizomatous, sometimes stoloniferous; herbage usually hairy, sometimes stipitate-glandular, sometimes glabrous; caudex or rhizome scaly, sometimes with bulbils. Flowering stems ± erect, leafless, 2–125 cm. Leaves basal, (proximally cauline and crowded in M. tolmiei); stipules absent; petiole present (absent in M. bryophora, M. tolmiei; ± absent in M. foliolosa, M. stellaris; appearing absent in M. tempestiva); blade obtrullate, spatulate, fan-shaped, or obovate to oblanceolate, linear to oblong, elliptic or rhombic, to lanceolate, ovate, triangular, deltate, round, or reniform, lobed or unlobed, base attenuate or cuneate to truncate or cordate, ultimate margins entire, crenate, serrate, or dentate, (without lime-secreting hydathodes), apex acute to obtuse or rounded; venation pinnate or palmate. Inflorescences cymes or thyrses, sometimes solitary flowers, from terminal buds in rosettes, 2–70+[–1000]-flowered, (flowers sometimes replaced by bulbils in M. bryophora, M. ferruginea, M. foliolosa, M. petiolaris, M. stellaris), (scapose), bracteate; (bracts reduced or sometimes leaflike, only proximal in M. tolmiei). Flowers radially symmetric (± bilaterally symmetric in M. ferruginea, M. petiolaris); hypanthium free from or 1/4–3/4 adnate to ovary, free to 0.5 mm, usually green, rarely purplish; sepals 5, green, sometimes reddish at tips; petals 5 (absent or 1–5 in M. apetala, M. subapetala), usually white or cream, sometimes greenish, yellowish, pinkish, pink, purplish, purple, or reddish purple, sometimes purple-margined, sometimes orange-, yellowish green-, or yellow-spotted; nectary disc present or not; stamens 10, (distinct); filaments linear and ± flattened or club-shaped; pistils 2(–3+), 2(–3)-carpellate, carpels distinct or connate to ca. 1/2 their lengths (rarely more); ovary superior or ± inferior, sometimes appearing more superior in fruit, 2(–3)-locular; placentation axile (when connate); styles 2(–3); stigmas 2–3. Capsules folliclelike or 2(–3)-beaked. Seeds brown, oblong, ellipsoid, or ovoid, longitudinally ribbed (ribs ribbonlike or pectinate). x = 8.

Distribution

North America, Mexico, South America, Eurasia, mostly north-temperate, arctic, and alpine regions.

Discussion

Species 68–93 (45 in the flora).

The ovules of Micranthes species are unitegminate, except those of M. merkii and M. tolmiei (sect. Merkianae), which are bitegminate. Pollen exine is reticulate. These characters help distinguish Micranthes from Saxifraga (see discussion for differences among the genera).

Some circumboreal species (e.g., Micranthes nelsoniana), which show only slight variation in Europe (D. A. Webb and R. J. Gornall 1989), are more variable in North America.

Molecular phylogenies (D. E. Soltis et al. 2001, and references therein) have shown that Saxifraga in the broad sense is not monophyletic and comprises two main lineages, Saxifraga in the narrow sense and Micranthes, which are not sister to each other (for morphologic differences, see discussion of 23. Saxifraga). We recognize these two as distinct genera, as well as the monospecific Cascadia.

Supernumerary chromosomes have been reported in some species (D. E. Soltis 1983), leading to doubts about the accuracy of some reported chromosome numbers, especially those published before 1950. There is also well-documented and extensive occurrence of polyploidy and dysploidy (sometimes reported as aneuploidy) throughout the genus (P. E. Elvander 1984; D. A. Webb and R. J. Gornall 1989).

The stigmas and styles are typically two (except for three in Micranthes texana); occasional flowers on plants in some species may have three or, rarely, four stigmas and styles. In addition, ovary position in some taxa can change from mostly inferior at flowering to superior in fruit; ovary position is described at the flowering stage.

D. A. Webb and R. J. Gornall (1989) provided much ecological and horticultural information on the few European species of the genus.

Selected References

Elvander, P. E. 1984. The taxonomy of Saxifraga (Saxifragaceae) section Boraphila subsection Integrifoliae in western North America. Syst. Bot. Monogr. 3: 1–44.

Webb, D. A. and R. J. Gornall. 1989. A Manual of Saxifrages and Their Cultivation. Portland.

Lower Taxa

Micranthes apetala, Micranthes aprica, Micranthes bryophora, Micranthes californica, Micranthes calycina, Micranthes careyana, Micranthes caroliniana, Micranthes eriophora, Micranthes ferruginea, Micranthes foliolosa, Micranthes fragosa, Micranthes gaspensis, Micranthes gormanii, Micranthes hieraciifolia, Micranthes hitchcockiana, Micranthes howellii, Micranthes idahoensis, Micranthes integrifolia, Micranthes lyallii, Micranthes marshallii, Micranthes micranthidifolia, Micranthes nelsoniana, Micranthes nidifica, Micranthes nivalis, Micranthes nudicaulis, Micranthes occidentalis, Micranthes odontoloma, Micranthes oregana, Micranthes palmeri, Micranthes pensylvanica, Micranthes petiolaris, Micranthes razshivinii, Micranthes reflexa, Micranthes rhomboidea, Micranthes rufidula, Micranthes spicata, Micranthes stellaris, Micranthes subapetala, Micranthes tempestiva, Micranthes tenuis, Micranthes texana, Micranthes tischii, Micranthes tolmiei, Micranthes unalaschcensis, Micranthes virginiensis

Key

1	Inflorescences with all or some flowers replaced with bulbils	> 2
1	Inflorescences without bulbils	> 6
2	Plants tufted to mat-forming, with ± branched caudices or stolonlike rhizomes.	Micranthes stellaris
2	Plants usually solitary or tufted, with caudices	> 3
3	Leaf blade margins entire, subentire, or toothed distally; inflorescences narrow, cylindric or open; petals each with 1-2 yellow spots	> 4
3	Leaf blade margins coarsely toothed; inflorescences open; petals: 3 with 2 yellow spots and 2 without spots, or all not spotted	> 5
4	Leaf blades oblanceolate; pedicels erect to ascending.	Micranthes foliolosa
4	Leaf blades linear to elliptic; pedicels reflexed.	Micranthes bryophora
5	Leaf blades usually fleshy, margins usually with fewer than 12 coarse teeth; petioles indistinct; inflorescence bracts reduced; w North America	Micranthes ferruginea
5	Leaf blades thin, not fleshy, margins with 15+ lobelike teeth; petioles distinct; inflorescence bracts leaflike, gradually reduced distally; e North America.	Micranthes petiolaris
6	Ovaries ca. 1/2 inferior to inferior (sometimes appearing more superior in fruit)	> 7
6	Ovaries ± superior to at most 1/2 inferior (hypanthium to 1/3 adnate to ovary)	> 29
7	Leaf blade margins serrate or toothed distally; pistils connate 1/2+ their lengths	> 8
7	Leaf blade margins entire to subentire or apically denticulate, or crenulate, denticulate, serrate, or dentate on distal 2/3 to throughout; pistils connate to 1/2 their lengths	> 10
8	Leaf blades broadly obovate to round; petals ± clawed, 3.5-5 mm.	Micranthes unalaschcensis
8	Leaf blades rhombic to fan-shaped, sometimes nearly round, or oblanceolate to spatulate; petals not clawed, 2-4 mm	> 9
9	Leaf blades oblanceolate to narrowly oblanceolate or narrowly spatulate, sometimes ± narrowly obovate, margins (3-)5-7(-9)-toothed (teeth 0.5 mm); inflorescence axes and branches glabrous.	Micranthes razshivinii
9	Leaf blades rhombic to fan-shaped, sometimes nearly round, margins 7-11-toothed (teeth 1-3 mm); inflorescence axes glabrous or sparsely long-hairy, branches ± densely long-hairy.	Micranthes calycina
10	Petals (1-)5 and pink to purple, or 1-5 and white to greenish white or cream, or absent	> 11
10	Petals 5, white to greenish white, cream, or, rarely, purple, sometimes pink or purplish tinged	> 14
11	Inflorescences glomerate, congested; petals absent or white to greenish white.	Micranthes apetala
11	Inflorescences spikelike, constricted or open; petals white, cream, or pink to purple	> 12
12	Leaf blades ovate to elliptic, 2-6 cm.	Micranthes hieraciifolia
12	Leaf blades linear to elliptic, ovate to obovate, or oblanceolate, 3-25 cm	> 13
13	Inflorescences ± constricted to ± open thyrses; petals usually absent, sometimes 1-5, 1-2 mm; Rocky Mountains.	Micranthes subapetala
13	Inflorescences open, often lax thyrses; petals 5, 2-3 mm; e North America.	Micranthes pensylvanica
14	Leaf blades linear to oblanceolate or obovate, or elliptic to ovate; petioles indistinct	> 15
14	Leaf blades ± round to ovate or triangular, elliptic, or oblong, or, rarely, obovate, obtrullate, or deltate; petioles distinct	> 18
15	Inflorescences 3-10(-15) cm; petals shorter than sepals.	Micranthes tempestiva
15	Inflorescences usually 15-125 cm; petals equaling or longer than sepals, sometimes shorter	> 16
16	Leaf blade margins serrate to dentate, surfaces densely tangled-hairy; inflorescences flat-topped thyrses.	Micranthes hitchcockiana
16	Leaf blade margins entire or nearly so or serrulate to denticulate, surfaces glabrous, sparsely hairy, or densely short-hairy; inflorescences conic to cylindric thyrses	> 17
17	Leaf blade surfaces glabrous or sparsely hairy (margins ciliate); petals broadly elliptic to obovate and to 2 times as long as sepals, sometimes oblanceolate and equaling or shorter than sepals; w North America.	Micranthes oregana
17	Leaf blade surfaces short-hairy; petals linear to narrowly elliptic, longer than sepals; e North America.	Micranthes pensylvanica
18	Leaf blade margins serrate to crenate or dentate, rarely subentire	> 19
18	Leaf blade margins entire or subentire to crenulate or denticulate	> 24
19	Inflorescences lax, open (with distant lateral branches)	> 20
19	Inflorescences ± congested, glomerate or capitate to spicate	> 21
20	Sepals reflexed, sometimes spreading.	Micranthes californica
20	Sepals erect.	Micranthes eriophora
21	Inflorescences yellow- to cream-tipped stipitate-glandular; petals white, 1.5+ times as long as sepals.	Micranthes rhomboidea
21	Inflorescences purple- to red-tipped stipitate-glandular, sometimes glabrous proximally or glabrate distally; petals white, sometimes pink- or purple-tipped, margined, or tinged, equaling or slightly longer than sepals	> 22
22	Abaxial leaf surfaces glabrate (margins sometimes ciliate); petals white with purple margins, usually pink or purple tinged.	Micranthes tenuis
22	Abaxial leaf surfaces ± densely, tangled, reddish brown-hairy; petals white (sometimes pink tinged)	> 23
23	Inflorescences (5-)10-40-flowered, congested, almost capitate, (3-)7-20 cm; leaf blades ovate to ± round, sometimes elliptic or oblong; arctic and alpine North America	Micranthes nivalis
23	Inflorescences 3-10-flowered, subcapitate to spicate, 2-10 (-15) cm; leaf blades obtrullate or obovate to ovate; Gaspé Peninsula.	Micranthes gaspensis
24	Inflorescences at least somewhat congested, conic or capitate, 3.5-15(-35) cm	> 25
24	Inflorescences lax, cylindric to conic thyrses, (10-)15-50 cm	> 27
25	Carpels 3+.	Micranthes texana
25	Carpels 2(-3+)	> 26
26	Inflorescences densely stipitate-glandular; leaf blades narrowly to broadly ovate, surfaces hairy.	Micranthes integrifolia
26	Inflorescences sparsely stipitate-glandular; leaf blades obovate to elliptic, surfaces glabrous or sparsely hairy	Micranthes aprica
27	Inflorescences cylindric, with ± compact lateral cymules; petals 1-1.9 mm, equaling or shorter than sepals.	Micranthes nidifica
27	Inflorescences conic, with ± open cymules on lateral branches; petals 2-4 mm, longer than sepals	> 28
28	Plants solitary or in clumps, caudices often with bulbils; leaf blades usually longer than petioles.	Micranthes integrifolia
28	Plants often mat-forming, long, thin, rhizomatous; leaf blades usually equaling or shorter than petioles.	Micranthes fragosa
29	Leaves cauline, proximally crowded; sepals erect to spreading.	Micranthes tolmiei
29	Leaves basal; sepals reflexed, sometimes spreading or ascending to erect	> 30
30	Pistils connate 1/2+ their lengths	> 31
30	Pistils distinct almost to base	> 44
31	Capsules dark purple or purple-black	> 32
31	Capsules green to yellow and purple-tinged, purple, or distal 1/2 purple	> 36
32	Filaments club-shaped.	Micranthes nelsoniana
32	Filaments linear, flattened	> 33
33	Leaf blades somewhat reniform, ± evenly lobed, base cordate to truncate, margins entire; inflorescences sparsely purple-tipped stipitate-glandular.	Micranthes nudicaulis
33	Leaf blades round or obovate to oblanceolate or spatulate, or rhombic to fan-shaped, unlobed, base cuneate to attenuate, margins toothed distally; inflorescences densely tangled-hairy or glabrous	> 34
34	Leaf blades broadly obovate to round, base cuneate; petals 3.5-5 mm	Micranthes unalaschcensis
34	Leaf blades oblanceolate to narrowly oblanceolate or narrowly spatulate, or rhombic to fan-shaped, sometimes nearly round, base attenuate to cuneate; petals 2-4 mm	> 35
35	Leaf blades oblanceolate to narrowly oblanceolate or narrowly spatulate, sometimes ± narrowly obovate, margins (3-)5-7(-9)-toothed (teeth 0.5 mm); inflorescence axes glabrous, branches glabrous.	Micranthes razshivinii
35	Leaf blades rhombic or fan-shaped, margins 7-11-toothed (teeth 1-3 mm); inflorescence axes glabrous or sparsely long- hairy, branches ± densely long tangled hairy	Micranthes calycina
36	Leaf blades round, reniform, or spatulate to obovate; filaments ± club-shaped	> 37
36	Leaf blades linear or elliptic to oblanceolate, obovate, or spatulate; filaments linear, flattened	> 40
37	Leaf blades spatulate to obovate, base cuneate to slightly attenuate.	Micranthes lyallii
37	Leaf blades round to ovate or reniform, base cordate to truncate	> 38
38	Inflorescences open, lax thyrses.	Micranthes odontoloma
38	Inflorescences ± congested, spikelike to glomerulate, sometimes conic thyrses	> 39
39	Inflorescences ± congested, conic to glomerate thyrses; petals white to pinkish, rarely orange-spotted; filaments distinctly club-shaped.	Micranthes nelsoniana
39	Inflorescences spikelike thyrses; petals cream to yellowish; filaments narrowly club-shaped.	Micranthes spicata
40	Plants tufted or mat-forming, with ± branched caudices or stolonlike rhizomes; inflorescences open, lax cymes.	Micranthes stellaris
40	Plants solitary or tufted, with caudices; inflorescences cylindric or open, lax thyrses	> 41
41	Leaf blades linear to elliptic or oblanceolate, margins entire or subentire to 1-3-toothed distally; petals each with 1-2 yellow spots	> 42
41	Leaf blades spatulate to obovate or oblanceolate, margins coarsely toothed in distal 1/3+; petals: 3 with 2 yellow spots, 2 without spots	> 43
42	Leaf blades oblanceolate; pedicels erect to ascending.	Micranthes foliolosa
42	Leaf blades linear to narrowly elliptic; pedicels reflexed.	Micranthes bryophora
43	Leaves ± fleshy; petioles indistinct; blade margins usually with fewer than 12 coarse teeth; inflorescence bracts usually reduced; w North America.	Micranthes ferruginea
43	Leaves thin; petioles distinct; blade margins with 15+ lobelike teeth; inflorescence bracts leaflike; e North America.	Micranthes petiolaris
44	Petals white, each with 1-2 greenish to yellow spots (sometimes faded in dried material); filaments usually strongly club-shaped or petaloid	> 45
44	Petals white to greenish, sometimes purplish tipped or margined, usually not spotted; filaments linear, flattened or slightly club-shaped	> 49
45	Leaf blades lanceolate to oblanceolate, base attenuate; petioles ± indistinct (broad).	Micranthes micranthidifolia
45	Leaf blades ovate or elliptic to oblong, sometimes almost orbiculate, base ± attenuate to ± truncate; petioles distinct	> 46
46	Leaf blades densely white- or reddish brown-hairy; petal spots usually distinct even in dried material; filaments longer than petals	> 47
46	Leaf blades glabrate adaxially; petal spots often faded in dried material; filaments ± equaling petals	> 48
47	Leaf blade margins deeply dentate (teeth 2+ mm); inflorescences open, lax; e North America.	Micranthes caroliniana
47	Leaf blade margins shallowly serrate (teeth to 1 mm); inflorescences with flowers crowded at branch tips; nw North America.	Micranthes reflexa
48	Leaf blade margins crenate-serrate (teeth ca. 2 mm); inflorescences open, lax; filaments strongly club-shaped, sometimes petaloid.	Micranthes marshallii
48	Leaf blade margins shallowly serrate to dentate (teeth usually to 1 mm); inflorescences with flowers ± crowded at tips; filaments club-shaped, not petaloid.	Micranthes idahoensis
49	Sepals erect to ascending	> 50
49	Sepals ascending or spreading to strongly reflexed	> 52
50	Petal lengths ca. 2 times sepals; ovaries 1/3-1/2 inferior.	Micranthes eriophora
50	Petal lengths 2+ times sepals; ovaries ± superior	> 51
51	Leaf blade margins irregularly crenate to serrate.	Micranthes virginiensis
51	Leaf blade margins entire or subentire.	Micranthes palmeri
52	Leaf blade bases truncate; petals often with 2 faint yellow spots.	Micranthes careyana
52	Leaf blade bases attenuate to ± truncate; petals usually not spotted (rarely present)	> 53
53	Petals greenish, often purple-margined.	Micranthes tischii
53	Petals white	> 54
54	Bracts and sepals often tangled brown-hairy; inflorescences flat-topped.	Micranthes rufidula
54	Bracts and sepals glabrous, rarely sparsely tangled brown-hairy; inflorescences open and conic or ± flat-topped, or glomerulate	> 55
55	Leaf blades oblong to narrowly ovate; inflorescences 5-10(-20)-flowered; sepals strongly reflexed.	Micranthes howellii
55	Leaf blades ovate to elliptic; inflorescences usually (10-)30+-flowered; sepals ascending to spreading	> 56
56	Inflorescences with flowers crowded in 1+ glomerules, branches ascending; ovaries ± superior.	Micranthes occidentalis
56	Inflorescences open, lax, branches strongly divaricate; ovaries nearly 1/3 inferior.	Micranthes gormanii