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.