 Milkweed flowers are so bizarre and
interesting that they represent one of the "extreme" ways that blossoms can
behave. In eastern North America, probably the most famous milkweed is the one whose
flowers are shown at the right -- the Butterfly Weed, Asclepias tuberosa. You
really need the Latin name here because several very different plants go by the English
name of Butterfly Weed. Of course it's true that butterflies just flock to this plant.
Like our Standard Blossom, milkweed flowers
possess five sepals and five corolla lobes. However, at
that point milkweed flowers depart drastically from what's normal among flowers, as the
next picture clearly shows.
The greenish thing in the picture's center is yet another structure not found on our
Standard Blossom. Composed of the flower's grown-together stigma and anthers,
it's part of a larger structure known as the gynoecium, the bottom of
which, the ovary, is hidden by the corona bases in the picture. Notice the thing on the
grown-together stigma and anthers labled
So, imagine this: An insect pollinator is attracted to the milkweed's bright flower and begins figuring out how to get to the blossom's nectar. As the insect crawls all over the flower one of its legs slips between two of the flower's corona limbs. When the leg is jerked upward, it snags between two pollinia and catches at the pollinarium gland. As the leg is withdrawn upward, the entire, upside-down-V-shaped pollinarium is removed from the flower and remains stuck to the insects leg! By now you can guess that when the insect visits the next flower, the pollinarium dislodges from the leg, the pollinia come into contact with the blossom's stigma, and then pollination takes place. Sneaky, huh? If you ever find a wild, flowering milkweed, examine its blossoms with your hand lens, looking especially for the extremely small pollinarium glands. If you find one, place a pin beneath it and, lifting upward, try to remove the entire upside-down-V-shaped pollinarium.
The picture at the right shows the inside of the flower of a Green Milkweed, Asclepias viridis, a common wildflower in a field near my home. I include it here to show that milkweed hoods, coronas and other parts come in many, many interesting and pretty configurations. |
In the top picture, the downward-spreading orange items
are corolla lobes. To make the image at the left I removed part of the flower's top
appendages to show what's inside. In that picture, note that arising from atop the
corolla-lobe bases are five scoop-shaped corona limbs or hoods.
The five limbs considered together are termed the corona or crown.
The corona is completely absent from our 
pollinarium gland. At the right you
can see that the pollinarium gland is barely visible when intact flowers are viewed
from their sides. Now let's try to understand what that pollinarium gland is all
about
At the left you see a much magnified image of a pollinarium gland at the tip
of a silvery pin's head. Below the dark pollinarium gland are two honey-colored pollinia.
The whole upside-down-V-shaped structure with its pollinarium gland and two pollinia is
called a pollinarium. We begin thinking that all this might start making
sense when we learn this: Among milkweeds, pollen sticks together into granular masses,
and those masses are called pollinia. The pollinia at the left are yellowish
because they are made of stuck-together pollen grains.
And while you have a flower, be sure to look for
broken-off insect legs stuck in the flower, because sometimes an insect's leg breaks off
before the pollinarium comes free!