2. Cynanchum L.
clear latex and thus watery sap (milky or yellow latex elsewhere). Stems
twining, usually climbing, branched or unbranched. Leaves opposite, short- to
long-petiolate. Leaf blades narrowly to broadly ovate or triangular-ovate, the
base rounded, truncate, or cordate, the tip tapered to a sharp point, the
margins flat or somewhat wavy, sometimes slightly curled under. Inflorescences solitary
in the leaf axils, mostly relatively short-stalked, consisting of sometimes
branched or slightly racemose, umbellate clusters. Calyces with the lobes
spreading or loosely ascending at maturity, narrowly triangular or lanceolate
to ovate, sparsely short-hairy on the outer surface. Corollas erect or
ascending at flowering, white to cream-colored or dark purple to nearly black.
Gynostegium appearing sessile or nearly so, the corona as long as or usually
longer than the anther/stigma head, modified into 5 erect, lateral, petaloid
segments or a shallow, cuplike, fleshy disk. Fruits pendant, narrowly
elliptic-lanceolate to ovate in outline, circular or slightly flattened in
cross-section (not angled), the surface smooth, glabrous or short-hairy. Seeds
strongly flattened and narrowly winged, brown to dark brown, with a tuft of
long, white, silky hairs at the tip. Two hundred to 400 or more species, nearly
worldwide, mostly in tropical and warm-temperate regions.
and the circumscriptions and relationships among subgroups in the Cynanchum
alliance are among the most poorly understood in the Asclepiadaceae. Rosatti
(1989) noted that because previous authors included so many species with
exceptions to the morphological character states used to define most of the
subgroups, comparisons of different classifications have been nearly
impossible. Liede (1997) and her colleagues have been studying the infrageneric
classification of the genus and its segregates for a number of years, but this
classification should still be viewed as preliminary and not fully resolved.
The two species present in Missouri have very different coronal structures and
probably are not closely related. The North American Ampelamus group
contains only the species treated here as Cynanchum laeve, a relatively
isolated taxon whose relationships are not well understood (Sundell, 1981;
Liede, 1996, 1997). Cynanchum louiseae is part of the Vincetoxicum
group, a relatively cohesive segregate that comprises up to 100 or more Old
World species (Liede, 1996), of which 3 have become introduced in temperate
North America as escapes from cultivation (Sheeley and Raynal, 1996). It is
likely that ongoing molecular studies will result in the breakup of Cynanchum
in the broad sense into a number of smaller genera, but until further data
become available in the literature it seems premature at the present time to
recognize more than a single polymorphic genus.