• 5 hoods(hd): It is the reservoir for the nectar.
• 5 horns(hn): At the base of the hoods and horns is a plentiful supply of nectar for pollinators and other insects.
• Central gynostegium (gy)- reproductive parts: In the center is the fused parts of the pistil and the 5 stamens. It is columnar and flat topped.
  • The center is the stigmatic chamber/s that is internally divided into 5 chambers that each contain active stigmatic surfaces. The stigmatic chambers also generate nectar that is stored in the hoods. The nectar also serves as the germination fluid for pollen grains. There are 2 ovaries, 3 of the chambers go to one ovary, and the other 2 chambers go to the second ovary.
  • 5 stigmatic slits(ss). They are openings into the stigmatic chambers from the outside.
  • 5 anthers which form 10 anther pouches(an) surround the outer surface of the gynostegium(gy).
• For more close up photos see - common milkweed and poke milkweed and references below.

Diagram of a general milkweed flower:

• hn: horn
• cp: corpusculum
• ss: stigmatic slit
• hd: hood
• pt: petal, reflexed
• cp: corpusculum
• ta: translator arm
• ps: pollinia (pollen sac)

Internet Archive Book Images, No restrictions, via Wikimedia Commons

Pollen:

• Instead of individual pollen grains, the grains are together in a pollen sac(ps) called the pollinium (pollinia - plural). See diagram above.
• The pollen sacs are paired and connected by translator arms(ta) to a dark sticky/clamp-like corpusculum(cp). These three components make up the pollen apparatus (pollinarium)
• Each anther is separated into two anther pouches(an), one on each side of a stigmatic slit(ss).
• Each anther pouch(an) contains a pollen sac/pollinium(ps).

• The way it works: A bee/pollinator comes along and while it is enjoying the nectar at the base of the hoods and horns, a leg slips into a stigmatic slit.
• When the bee tries to remove its leg out of the stigmatic slit, it hits the dark corpusculum which is sticky and sticks on to the bee's leg. While removing its leg with the corpusculum stuck to it, the two pollen sacs that are connected to corpusculum by the translator arms, get lifted out of the top of the anther pouches!
• The bee continues with the whole pollen apparatus dangling from its leg and goes to another milkweed plant
• Amazingly, when the bee lands on another milkweed plant, the pollen sac can be angled so that it goes into the stigmatic slit. Then the stigmatic surface inside the gynostegium chamber get pollinated!
• It is a complicated and amazing way to get a flower pollinated but it works well enough to keep the species alive. It is not always so straight forward. Lots can and do go wrong. Read more about this amazing process in the references below.