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Published In: Species Plantarum 2: 755. 1753. (1 May 1753) (Sp. Pl.) Name publication detailView in BotanicusView in Biodiversity Heritage Library

Project Name Data (Last Modified On 8/29/2017)
Acceptance : Accepted

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Astragalus L. (milk vetch, loco weed) (Barneby, 1964; Isely, 1983a, 1984, 1985, 1986a; Welsh, 2007)

Plants perennial herbs (annuals or shrubs elsewhere) sometimes woody at the base. Stems erect to prostrate, unarmed, glabrous or pubescent with unbranched or branched, hairs (these positioned flat along the stem and/or leaves and with 2 opposite branches, thus appearing as a straight line attached near the midpoint). Leaves alternate, odd-pinnately compound with 9 to numerous leaflets (sometimes trifoliate or simple elsewhere), short-petiolate. Stipules well-developed, free or fused to the petiole toward the base; stipels absent. Leaflets of various sizes and shapes, pinnately veined, but the lateral veins sometimes inconspicuous. Inflorescences axillary racemes, elongate and spikelike to short and dense (appearing as clusters), stalked, the bracts small and shed early; bractlets short and inconspicuous or absent. Flowers stalked, in some species appearing pendant or drooping. Calyces 5-lobed, the tube cylindric to bell-shaped, actinomorphic or zygomorphic, sometimes somewhat pouched basally on 1 side, the lobes usually shorter than tube, triangular. Corollas papilionaceous, purple, yellow, or white, the banner oblanceolate, longer than the wings, often curved or bent backward, rounded but often shallowly notched at the tip, the wings oblong, the keel shorter than wings, boat-shaped, fused, usually rounded or blunt at the tip, usually somewhat curved upward. Stamens 10, in an alternating set of 5 slightly shorter and 5 slightly longer stamens, 9 of the filaments fused and 1 free, the anthers attached near the base, mostly yellow to orange. Ovary sometimes short-stalked, the style straight or curved, glabrous, persistent at fruiting, the stigma minute, terminal. Fruits legumes, variable in shape from globose to elongate, often inflated, sometimes flattened, straight to arched or curved, with 1 or 2 locules (separated by a membranous partition), variously dehiscent or not. Seeds 1 to more commonly several to numerous, variously shaped, but most commonly shallowly notched at the attachment point. Perhaps 2300–2500 species (or more), nearly worldwide, except Australia; most diverse in arid and seasonally dry regions of the Northern hemisphere.

Astragalus is the largest genus of Fabaceae and one of the largest genera of flowering plants Sanderson and Wojciechowski, 1996). The genus is very diverse in habit, pubescence, leaf morphology, and fruit characters. The majority of the approximately 375 species in the United States occurs as narrow endemics in arid, physiographically and geologically diverse regions in the western states, primarily in the Great Basin and Colorado Plateau. Astragalus canadensis is widespread across North America and closely related to some Old World species. Species such as A. lotiflorus and A. crassicarpus are found mostly in the central Great Plains, their ranges barely extending into western Missouri.

The large number of species in Astragalus makes it difficult to work with. It has been subdivided into smaller genera in the past (Rydberg, 1929), but most botanists currently recognize a single, very large genus (Barneby, 1964; Isely, 1998; Welsh, 2007). Most of the New World species are aneuploids with chromosome numbers of x=11–15. The Old World species are mostly euploids with a base chromosome number of Sequence data indicate that most of the New World species comprise a natural lineage (Wojciechowski et al., 1999). Oxytropis is very similar to Astragalus, but differs mainly in having the keel pointed at the tip and a different base chromosome number.

Many species of Astragalus and Oxytropis are toxic to humans and livestock. Toxic species are particularly problematic in western states, where ranching is prevalent and Astragalus is common. Astragalus canadensis, A. crassicarpus, and A. lotiflorus in Missouri are considered toxic. Consuming large amounts of plant material causes neurological damage and pronounced behavioral changes. Sick animals commonly become dazed, uncoordinated, frantic, or violent, and are referred to as loco, hence the common name locoweed. Animals often lose weight and may starve, or die of accidents, predation, or heart and respiratory failure. Fetuses may be aborted or born with deformities. There is no cure and damage is irreversible. The toxicity is caused primarily by several indolizidine alkaloids, especially swainsonine (Burrows and Tyrl, 2001). This alkaloid occurs throughout the plant, but levels are highest in the seeds and fruits. Even the pollen is toxic. Additional toxicity problems are associated with the production of 3-mitropropionic acid by some species, causing a condition known as cracker-heels for the sound made by the pelvic bones of afflicted animals. Some western species accumulate high levels of selenium from the soil, which is also toxic. Not all species of Astragalus are toxic, and some Asian species are considered valuable as forage plants. The Middle Eastern species A. gummifer Labill. and a few other Old World species are the source of gum tragacanth, a collection of water soluble polysaccharides that originates from the dried sap (Gentry, 1957) and that has a wide variety of uses, including as a burnishing compound in leatherworking, a textile stiffener, a binder in incense sticks and artists’ pastels, and in the tobacco industry (as an adhesive for the paper or other outer layer in rolled cigars and cigarettes), as well as an emulsifier and thickener in foods and medicinally for coughs and burns.

Steyermark (1963) mentioned several additional species of Astragalus that had been reported from Missouri in the older botanical literature, but which he excluded because they could not be confirmed as growing in the state. The list included A. gracilis Nutt., A. missouriensis Nutt., A. racemosus Pursh, and A. tennesseensis A. Gray ex Chapm. As no evidence has been uncovered since Steyermark’s treatment to support the addition of these species to the Missouri flora, they remain excluded for the present.


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1 Stems mostly 60–150 cm long; stipules united at the base and sheathing the stems; inflorescences 12–30 cm long, elongate, spikelike racemes, with 30–70 flowers Astragalus canadensis
+ Stems 5–60 cm long; stipules free, not sheathing the stem; inflorescences 1–8(–12) cm long, subcapitate to somewhat elongated, appearing as clusters or short, spikelike racemes with 3–25 flowers (2)
2 (1) Stems 8–40(–60) cm long; calyces with the tube 3–9 mm long; fruits oblong-obovoid to nearly globose, fleshy when immature, becoming leathery with age, 2-locular Astragalus crassicarpus
+ Stems 5–25 cm long; calyces with the tube 2–3 mm long; fruits elongate, tapered to sharply pointed ends, not fleshy, leathery or hardened, 1-locular (3)
3 (2) Stems 8–25 cm long; leaflets with the undersurface sparsely pubescent with unbranched hairs, these mostly appressed, straight, attached at their bases (best observed carefully with magnification), occasionally glabrous; flowers all of one type, open, with well-developed petals, the corollas purple (rarely white); ovary glabrous; fruits arched or curved Astragalus distortus
+ Stems 1–8(–12) cm long; leaflets pubescent with branched hairs, these appressed and with 2 opposite branches, thus appearing as a straight line attached toward the midpoint (best observed carefully with magnification); flowers of 2 types, the open ones with well-developed, greenish yellow petals, the cleistogamous ones (produced in short racemes from the stem base, either on separate plants or late in the growing season) with poorly developed corollas; ovary hairy; fruits more or less straight Astragalus lotiflorus
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