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FNA | Family List | FNA Vol. 3 | Ranunculaceae

22. Thalictrum Linnaeus, Sp. Pl. 1: 545. 1753; Gen. Pl. ed. 5, 242, 1754.

Meadow-rue, pigamon [ Thaliktron, an ancient name used by Dioscorides]

Marilyn M. Park & Dennis Festerling Jr.

Herbs , perennial, from woody rhizomes, caudices, or tuberous roots. Leaves basal and cauline, proximal leaves petiolate, distal leaves sessile; cauline leaves alternate. Leaf blade 1-4×-ternately or -pinnately compound; leaflets cordate-reniform, obovate, lanceolate, or linear, sometimes 3-lobed or more, margins entire or crenate. Inflorescences terminal, sometimes also axillary, (1-)2-200-flowered panicles, racemes, corymbs, umbels, or flowers solitary, to 41 cm; involucres absent or present, involucral bracts 2-3 (these compound, often resembling whorl of 6-9 simple bracts), leaflike, not closely subtending flowers. Flowers all bisexual, bisexual and unisexual on same plant, or all unisexual with sexes on same or different plants, radially symmetric; sepals not persistent in fruit, 4-10, whitish to greenish yellow or purplish, plane, lanceolate to reniform or spatulate, 1-18 mm; petals absent; stamens 7-30; filaments filiform to clavate or distally dilated; staminodes absent between stamens and pistils; pistils 1-16, simple; ovule 1 per pistil; style present or absent. Fruits achenes, usually aggregate, sessile or stipitate, ovoid to obovoid, falcate, or discoid, sides prominently veined or ribbed; beak present or absent, terminal, straight to coiled, 0-4 mm. x = 7.

Species 120-200 (22 in the flora): nearly worldwide, mostly temperate.

Thalictrum is a taxonomically difficult genus that should be carefully researched through additional population-based field studies. Past treatments of Thalictrum have often emphasized leaf characters that are highly variable in most species; they are therefore of poor diagnostic value and not indicative of true relationships. Because of the paucity of field studies and a continuing emphasis on highly variable characters, the literature is replete with names that do not represent distinct entities. Often mixes of character states can be found within a single population; many of the character states used in past studies were neither ecologically nor geographically distinct.

Some species of Thalictrum have been divided into varieties by previous authors. In the absence of carefully collected, supporting evidence from field studies, we are unwilling to perpetuate the use of any infraspecific names.

Characters useful in identifying species of Thalictrum include leaflet shape, degree of dilation of filaments, anther length, shape of anther apex, achene shape and venation patterns, and vestiture (glands and/or hairs) of leaves and achenes. Leaflets described in this treatment are the central, distalmost of a midstem leaf; proximal and distal leaves are more variable and often not representative of the species. Stigma and filament colors refer to fresh material in the following descriptions.

In Thalictrum species, the stigma extends down the side of the style, so length of style in fruit (beak) includes the stigma.

For many species no reliable characteristics for the identification of staminate material are known. Extensive field work and careful analysis are required to determine if such characteristics exist.

In a narrow strip from southeastern Ontario to Ohio to Louisiana, some individuals of some species in Thalictrum section Leucocoma may lack their normal vestiture. In the absence of glands or pubescence, the differences among species are difficult to describe. The remaining characteristics overlap considerably. The species involved may be identified in the final couplets of the key as follows: if the plant in hand falls into the area of overlap for the first character of the couplet, go on to the next character, and so forth, until a distinguishing character is found. One or more of the characters offered should distinguish the infrequent, problematic individual.

Several species of Thalictrum are used as ornamentals. At least one species, T . aquilegiifolium Linnaeus, occasionally escapes cultivation in Ontario and Quebec and possibly elsewhere. The plant is tall (40-100 cm); flowers bisexual, mauve to pink; and achenes few, filiform, 3-winged, stipitate, very small, and hidden at anthesis among the bases of long, rigid stamens.

Numerous alkaloids have been identified from plants of the genus, some with pharmacologic potential. Some exhibit antimicrobial activity; others inhibit growth of tumors or lower blood pressure in mammals.

SELECTED REFERENCES

Boivin, B. 1944. American Thalictra and their Old World allies. Rhodora 46: 337-377, 391-445, 453-487. Boivin, B. 1948. Key to Canadian species of Thalictra. Canad. Field-Naturalist 62: 169-170. Lecoyer, J. C. 1885. Monographie du genre Thalictrum. Bull. Soc. Roy. Bot. Belgique 24: 78-324. Park, M. M. 1992. A Biosystematic Study of Thalictrum Section Leucocoma (Ranunculaceae). Ph.D. dissertation. Pennsylvania State University. Tamura, M. 1968. Morphology, ecology, and phylogeny of the Ranunculaceae. VIII. Sci. Rep. Coll. Gen. Educ. Osaka Univ. 17: 41-56. Tamura, M. 1992. A new classification of the family Ranunculaceae. Acta Phytotax. Geobot. 43: 53-58.


1 Inflorescences umbels or flowers solitary (sect. Anemonella).   1 Thalictrum thalictroides
+ Inflorescences panicles, racemes, or corymbs.   (2)
       
2 (1) Flowers bisexual; sepals 5 (often 4 in T. alpinum).   (3)
+ Flowers unisexual, or unisexual and bisexual, rarely only bisexual; sepals 4(–6).   (7)
       
3 (2) Achenes sessile to nearly sessile; filaments filiform (sect. Thalictrum).   (4)
+ Achenes stipitate; filaments ± dilated distally.   (5)
       
4 (3) Stems 15–150 cm; sepals 3–4mm; achenes 3–15.   2 Thalictrum minus
+ Stems (3–)5–20(–30) cm; sepals 1–2.3(–2.7) mm; achenes 2–6.   3 Thalictrum alpinum
       
5 (3) Filaments weakly dilated; achenes short-stipitate, stipe less than 1.5 mm, body (4–)5–6 m; Canada, w United States (sect. Omalophysa).   6 Thalictrum sparsiflorum
+ Filaments strongly clavate; achenes long-stipitate, stipe 1–3.5(–4) mm, body 2.5–5 mm; se United States (sect. Physocarpum).   (6)
       
6 (5) Adaxial margin of achene concave, ca. 2 times length of stipe; filaments 2.5–4 mm.   4 Thalictrum clavatum
+ Adaxial margin of achene straight, ± equaling length of stipe; filaments 2–3 mm   5 Thalictrum mirabile
       
7 (2) Leaflets apically 3–12-lobed, lobe margins crenate (rarely entire in T. debile); filaments variously colored, rarely white, filiform (sect. Heterogamia).   (8)
+ Leaflets undivided or 3-lobed apically, lobe margins entire (some leaflet margins on some individuals rarely crenate); filaments usually white, rarely lavender, filiform to clavate (sect. Leucocoma).   (18)
       
8 (7) Lateral veins of achene anastomosing-reticulate.   17 Thalictrum polycarpum
+ Lateral veins of achene not reticulate, veins parallel, converging, or rarely branched.   (9)
       
9 (8) Achenes laterally compressed.   (10)
+ Achenes not laterally compressed, or very slightly so.   (11)
       
10 (9) Leaf blade membranous, green; leaflets (5–)10–20 × (6–)8– 12(–18) mm; stems (20–)30–60(–150) cm; achenes 7–11(–14) per flower.   15 Thalictrum fendleri
+ Leaf blade leathery, glaucous; leaflets 5–8 × 4–5 mm; stems 14–50 cm; achenes 4–5(–6) per flower.   16 Thalictrum heliophilum
       
11 (9) Achenes stipitate; stipe 0.7–2.5 mm.   (12)
+ Achenes nearly sessile; stipe 0–0.3 mm.   (13)
       
12 (11) Achenes erect; beak 1.5–3 mm.   8 Thalictrum coriaceum
+ Achenes spreading to reflexed; beak 3–4.5(–6) mm.   14 Thalictrum occidentale
       
13 (11) Achenes incurved.   (14)
+ Achenes straight.   (15)
       
14 (13) Beak (2–)2.5–4(–5) mm; adaxial surface of achene 4–6 mm.   12 Thalictrum confine
+ Beak 1.5–2.5(–3) mm; adaxial surface of achene 3– 4(–6) mm.   13 Thalictrum venulosum
       
15 (13) Roots fibrous; stems erect, 30–80 cm; largest leaflets more than 15 mm wide.   7 Thalictrum dioicum
+ Roots tuberous; stems reclining to erect, usually less than 30(–45) cm; largest leaflets less than 15 mm wide.   (16)
       
16 (15) Beak 0.5–1 mm; achenes ovoid; stems erect; roots black when dry.   11 Thalictrum texanum
+ Beak 1.3–2mm; achenes oblong to elliptic-lanceolate; stems reclining or decumbent; roots brown.   (17)
       
17 (16) Achenes 0.7–1.2 mm wide, veins 6–8, prominent; beak 1.3–3 mm.   9 Thalictrum debile
+ Achenes 1.5–2 mm wide, veins 10–12; beak (1.3–)2.3–3 mm.   10 Thalictrum arkansanum
       
18 (7) Achenes, peduncles, abaxial surfaces of leaflets, and/or petioles and rachises with stipitate glands.   19 Thalictrum revolutum
+ Achenes, peduncles, abaxial surfaces of leaflets, and/or petioles and rachises without stipitate glands.   (19)
       
19 (18) Achenes, peduncles, abaxial surfaces of leaflets, and/or petioles and rachises with minute papillae (i.e., sessile glands), may also be pubescent.   (20)
+ Achenes, peduncles, abaxial surfaces of leaflets, petioles, and rachises without papillae, may be pubescent or glabrous.   (21)
       
20 (19) Leaflet length 0.9–5.25 times width; nonglandular trichomes absent; filaments 2.5–7.8 mm; anthers (0.7–)1.2–2.7(–3) mm; stipe 0.2–1.7 mm; e North America, rare w of Missouri.   19 Thalictrum revolutum
+ Leaflet length 0.9–2.6 times width; nonglandular trichomes present or absent; filaments 2–6.5 mm; anthers 1–3.6(–4) mm; stipe 0–1.1 mm; c North America, very rare e of Ohio. .   20 Thalictrum dasycarpum
       
21 (19) Achenes, peduncles, abaxial surfaces of leaflets, and/or petioles and rachises pubescent.   (22)
+ Achenes, peduncles, abaxial surfaces of leaflets, and/or petioles and rachises glabrous.   (23)
       
22 (21) Anthers less than 1.5 mm, apex blunt or slightly apiculate; filaments rigid, ascending, prominently clavate; beak straight or coiled distally, ca. 1/2 as long as achene body.   18 Thalictrum pubescens
+ Anthers usually 1–3.6(–4) mm, apex usually strongly apiculate; filaments flexible, drooping, filiform, scarcely dilated distally; beak ± straight, filiform, about as long as achene body.   20 Thalictrum dasycarpum
       
23 (21) Leaflets linear to narrowly lanceolate or oblanceolate, (2.6–)4–26 times longer than wide.   22 Thalictrum cooleyi
+ Leaflets nearly orbiculate to ovate, or lanceolate to obovate, usually less than 4 times longer than wide.   (24)
       
24 (23) Leaflets undivided or apically 2–3-lobed, largest usually less than 22 mm wide; filaments 1.8–4 mm; se United States.   21 Thalictrum macrostylum
+ Leaflets apically 3-lobed, seldom undivided, largest usually 15–60 mm or more wide; filaments 1.5– 7.8 mm; Ontario to Ohio to Louisiana.   (25)
       
25 (24) Anthers 0.5–1.5(–2.1) mm; stigma straight or distally coiled; flowers often bisexual.   18 Thalictrum pubescens
+ Anthers (0.7–)1–3.6 mm; stigma straight, ± filiform; flowers rarely bisexual (included here are very infrequent forms of T. dasycarpum and T. revolutum).   (26)
       
26 (25) Leaflet length 0.9–5.25 times width; filaments 2.5–7.8 mm; anthers (0.7–)1.2– 2.7(–3) mm; stipe 0.2–1.7 mm; e North America, infrequent w of Missouri.   19 Thalictrum revolutum
+ Leaflet length 0.9–2.6 times width; filaments 2–6.5 mm; anthers 1–3.6(–4) mm; stipe 0–1.1 mm; w, c North America, very infrequent e of Ohio. .   20 Thalictrum dasycarpum

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