Asclepias syriaca is an herbaceous perennial with
broad, simple, entire, opposite leaves and a white, sticky
sap that is the source of the common name, "milkweed".
The flowers are very unusual and easily recognized. Sepals
and petals are reflexed and the upper portion of the flower
is derived from fusion and modification of the stamens and
carpels. The filaments of the anthers are adnate (fused
to one another) to form the showy "corona", including
5 "hoods" each with a "horn". The lower
portion of the filaments are adnate to form a tube around
the 2 carpels, one of which is mostly hidden behind the
other in this view. The obvious carpel has been dissected
to expose the interior and a close look at flower photo
2 will reveal numerous ovules within. The two carpels are
separate at the base, but are joined at the stigma.
The anthers are also fused together and to the 2 stigmas
to form a structure known as the "gynostegium"
(see flower photo 2 above). Note the "stigmatic slit"
on the first flower photo. The pollinating insect must pass
through this opening (at least in part) to get to the inner
area of the gynostegium to reach the nectar and to pollinate
the flowers. Usually only one of the carpels will mature
to produce the familiar "follicle" or milkweed
"pod" that contains numerous seeds, each with
a tuft of hairs to aid in wind dispersal.
Although the scientific name suggests origination in Syria,
Asclepias syriaca is a native species of the Eastern USA, ranging west to Texas in the south and
Montana and Oregon in the north. It grows throughout Wisconsin
on sunny, disturbed sites.