Rhynchosia Lour.
(Grear, 1978)
Plants perennial
herbs (shrubs elsewhere), with thick, woody rootstalks. Stems prostrate or
trailing to ascending, climbing, or rarely erect, usually twining, ridged or
angled, unarmed, densely pubescent with mostly downward-angled or -curved hairs
(variously hairy elsewhere). Leaves pinnately
trifoliate or the lowermost leaves sometimes lacking lateral leaflets and thus
appearing simple, the petioles of the lowermost leaves long, progressively
shorter toward the stem tip, the uppermost leaves short-petiolate,
the petioles all densely hairy, the terminal leaflet stalk 5–20 mm long.
Stipules narrowly lanceolate to
ovate, sharply pointed at the tip, mostly shed early; stipels
absent (present elsewhere). Leaflets broadly ovate to elliptic, somewhat
rhombic, or nearly circular, the lateral leaflets sometimes somewhat asymmetric
with an oblique base, otherwise rounded to broadly angled at the base, rounded
or more commonly angled to a bluntly or sharply (but broadly) pointed tip, the
margins entire, short-hairy, the upper surface moderately to densely pubescent
with short, fine, spreading to curved hairs, the undersurface spreading-hairy,
mostly along the veins, also dotted with minute, yellow to orange, more or less
globose resin glands, the venation pinnate but with 3
main veins from the leaflet base, raised on the undersurface. Inflorescences
axillary, short to elongate racemes or small clusters, the stalk mostly short,
the bracts 3–7 mm long, linear to narrowly lanceolate,
mostly shed early; bractlets absent. Calyces
moderately to densely pubescent with fine, ascending hairs, sometimes mostly
along the nerves and margins, also dotted with minute, yellow to orange, more
or less globose resin glands, often becoming slightly
enlarged at fruiting, the tube bell-shaped (cylindric
elsewhere), shorter than the lobes, more or less 2-lipped, the upper 2 lobes
fused to at or above the midpoint, the free portions narrowly triangular-ovate,
the lower 3 lobes as long as or slightly shorter than the other lip, narrowly
oblong-elliptic to lanceolate, all of the lobes
angled or tapered to sharply pointed tips. Corollas papilionaceous,
not much longer than the calyx, the petals lemon yellow to orangish
yellow, the banner occasionally streaked with red or reddish-tinged on the
outer surface, short-stalked, the expanded portion with a pair of small,
incurved auricles at the base, obovate to nearly
circular, rounded to minutely notched at the tip, shallowly keeled, strongly
curved or bent backward from toward the base (but the flowers sometimes not
fully opening), glabrous or short-hairy on the outer surface, the wings oblong-obovate with a minute auricle at the base, straight or
nearly so, rounded at the tips, the keel oblong, somewhat curved upward,
bluntly pointed at the tip. Stamens 10, all of similar lengths, 9 of the
filaments fused and 1 free to about the midpoint, the fused portion longer than
the free portion, curved upward toward the tip, the anthers small, attached
below the midpoint, yellow, often darker-colored around the attachment. Ovary
sessile, densely hairy and usually also glandular, the style curved
upward, thickened toward the tip, glabrous, the stigma terminal, minute. Fruits
legumes, oblong to elliptic-oblong or asymmetrically ovate in outline,
flattened not or only slightly narrowed or indented between the seeds, tapered
asymmetrically to a short beak at the tip, the 2 valves dark brown at maturity,
densely hairy and dotted with minute, more or less globose
glands, dehiscent, the valves becoming contorted or somewhat spirally twisted
during dehiscence, (1)2-seeded. Seeds in our species 3–4 mm long, broadly
oblong to more or less circular in outline, flattened, the surface brown,
reddish brown, gray, or black, sometimes with darker mottling, smooth, shiny. About 230 species, nearly worldwide, most diverse in tropical
regions.
Rhynchosia is recognized by the gland-dotted,
trifoliate leaves, yellow corollas, and small, mostly 2-seeded legumes. Grear (1978) noted that in most of the species the flowers
frequently do not open fully and that they do not produce significant
quantities of nectar. During his extensive field work he never observed insects
actually visiting Rhynchosia
flowers. He concluded that the species are most commonly self-pollinated, which
would limit opportunities for interspecific hybridization. Earlier bagging
studies by Walraven (1967) indicated that all
American species of Rhynchosia are self-fertile, with pollination
occurring before the flowers open. There were no obvious meiotic irregularities
noted in that study that might indicate that any hybridization had occurred.
In his revision of the genus, Grear (1978) cited a collection of R. minima (L.) DC. (least snoutbean) made by Reverchon in
1903 at Sheldon, which he mistakenly mapped from Vernon County, Missouri. Julien Reverchon (1834–1905) was
an inveterate collector of the Texas flora, but although his personal herbarium
was acquired by the Missouri Botanical Garden after his death there is no
evidence that Reverchon ever botanized in the state
of Missouri. The specimen cited by Grear (1978)
actually originated from a Sheldon located near Houston, Texas, rather than the
one in Missouri. Rhynchosia minima is an Old World species that is
widespread as a presumed introduction in Latin America, the New World range
extending northward sporadically into the southern United States, from Texas
and southeastern Arkansas eastward to Georgia. Within the genus, it does not
appear to be closely related to the two Missouri taxa, differing
morphologically in a syndrome of floral features, especially its relatively
short calyx lobes and smaller corollas, as well as its relatively long, curved
fruits.