Asclepias syriaca, common milkweed, is a big native perennial herbaceous plant that blooms in the summer and is frequently associated with Monarch butterflies. Milkweeds have an unusual flower anatomy which results in unusual pollination techniques. Wildlife is rewarded with its rich supply of nectar.
Flowers
Inflorescence/Flower cluster:
The big flower clusters emerge from near the top of the stem from the axils of the upper leaves. The clusters are about 2 to 4 inches across and can have up to 100 flowers each. All the flowers in the cluster originate from a single point and is called an umbel. Each stem can have 1 to 3 clusters. The flowering lasts about 1-1½ months from early to mid-summer.
Individual Flower:
The Asclepias syriaca flower has both male and female parts. It has a corolla with 5 petals and a 5 parted calyx (sepals). The flowers are a pale light pink to pale reddish purple. Each flower is about a half inch across. When the flowers are mature, the petals are reflexed. The outer surface of the petals and the flower stalks are hairy.
If you look at the individual flowers, you will easily notice it does not look like a traditional flower. Actually, all the milkweeds have similar flower structure.
The most conspicuous parts are the hoods and horns. The hoods hold nectar which attracts insects for pollination. The hoods and horns together are called the corona. They are slick and waxy structures that pollinators land on.
Diagram of a general milkweed flower:
- pt: petal, reflexed
- hd: hood
- hn: horn
- cp: corpusculum
- ss: stigmatic slit
The green comet milkweed (Asclepias viridiflora) has hoods but no horns.
Internet Archive Book Images, No restrictions, via Wikimedia Commons
Common milkweed (Asclepias syriaca) flowers. Sometimes the tip of the hood has lobes. The petals are maroonish and the hoods and horns are whitish.
Where are the reproductive structures of the common milkweed?
Pollen: Most plants have powdery pollen that separates on release. However, the milkweed has sticky pollen that is packed into a sac.
Instead of individual pollen grains, the yellow grains are together in a sac called the pollinium. The pollinia (pollen sacs) are connected to the corpusculum, the black looking object, by connectors called translator arms. These three parts together are called pollinarium .
Diagram of pollen apparatus (pollinarium):
- ps: pollen sac- pollinium
- ta: pollen sacs are paired and connected by translator arms
- cp: corpusculum - black looking object - sticky and clasping
Internet Archive Book Images, No restrictions, via Wikimedia Commons
Micrographs: pollinarium and pollen grains of common milkweed (Asclepias syriaca, )
Gynostegium: This is the central flat topped columnar structure. It is partially fused pistil and stamen parts. There are 2 carpels (pistils) and 5 stamens.
- Anthers: On the outer vertical surface of the gynostegium are 5 anthers that appear as 10 anther pouches. They are not obvious from the top view. See photos below. However, easily seen are the 5 corpusculum of the pollinaria (pollen apparatus).
- Stigmatic slits: There are 5 stigmatic slits. Each slit has a protruding structure around it. At the top of each slit is a corpusculum (dark) and on each side is an anther pouch. See photos below. These slits open into stigmatic chambers
- Stigmatic disc: It is the female stigma part of the gynostegium. Each stigmatic slit opens into the gynostegium that contains stigmatic chambers. The 5 chambers 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.
- Ovaries: There are 2 ovaries/carpels. One ovary style goes to 3 stigmatic chambers and the other ovary style goes to the other 2 chambers.
Thanks to Belinda Beetham for the detailed photo to which I was able to add labels.
Below are some micrographs of the dissected flower of the common milkweed with the hoods and horns removed exposing the sides of the gynostegium
In the first photo below, clearly seen is the structure of the gynostegium, the dark corpusculum (blue arrows) of the pollen apparatus and the structure of the stigmatic slits (red arrows).
In the second photo, on both sides of the slit is the yellow pollen sacs in anther pouches (blue-green arrows). The anther pouch walls are almost transparent so the pollen sacs can be seen inside the anther pouch.
In third photo, the pollen sacs attached to the dark corpusculum is partially lifted out of the anther pouches. The semi-transparent anther pouch walls are clearly seen.
Photo below: This is a cross-section through the gynostegium. The left pointed object is the stigmatic slit (red arrow). Just behind it to the right is the stigmatic chamber with stigmatic walls (orange arrow). The chamber is empty. Just above and below the stigmatic chamber are empty anther pouches (blue-green arrows).
Photos below: They are longitudinal sections. The first shows the gynostegium with a pollen sac on the left and a stigmatic slit on the right. Below the gynostegium is one of 2 styles going down to the ovary that is show in the second photo. The second photo also shows the ovules in the ovary (red arrow). The third photo shows the 2 ovaries with some ovules.
Pollination
Asclepias syriaca, common milkweed, has a copious supply of high-quality nectar that is readily accessible at the base of the hoods. The nectar is produced in the stigmatic chambers and stored in the hoods. It also has a large supply of pollen. However, the pollen is encapsulated in the pollinium and is not accessible to flower visitors. So nectar is the only reward that the milkweeds offer to pollinating insects.
Common milkweed, with its complicated flower structure, requires that its pollinating insects be compatible with the flower structure. The insects land on the slick and waxy hoods and horns to get nectar that is around the base of the hoods. Then one of its legs may slide into one of the slits.
In the process of removing its leg, the corpusculum gets stuck on the leg. Then entire pollinarium gets pulled out; the 2 translators, and pollinium (pollen sac) pair out of the anther pouches.
Sometimes insects will get stuck and die on the flower because they can't get their leg out or they would lose a leg, especially smaller insects. Larger insects like bumble bees and honey bees are more likely to be successful at extracting the pollinium apparatus without loss of legs. Then the insects may continue to collecting nectar and inadvertently collect more pollinaria.
In a visit, an insect may collect many pollinaria before leaving. With the pollinaria dangling from their legs, the insect will fly around and some pollinaria will just drop off. However, some of the pollinaria will be taken to another milkweed flower. When the insect is oriented properly over the flower, the translator arm will orient a pollinium (pollen sac) so that it slides into the stigmatic slit and into the stigmatic chamber! Amazing! The pollinium breaks off and remains in the stigmatic chamber, the translator arm is broken, and the corpusculum remains on the insect. If an insect is inspected, one can count the remaining corpuscula to determine how many deliveries it had made. This is an amazing process.
Inside the stigmatic chamber, the pollinium makes contact with the stigmatic surface and the pollination processes occur. The plant is not self-fertile, so the pollinia must come from a genetically different plant in order to have seed set. Since this milkweed plant sends out rhizomes, nearby plants may be clones and the pollinating insects may need to come from further away.
Fruit
Since seed production requires a lot of energy and there would not be enough resources for all the pollinated flowers to set seed. So, only one of the pollinated flowers in an inflorescence develops into mature seed pod (follicle). All the other flowers are aborted.
The seed pods (follicles) are 3-4" long, wider at the base and coming to a point at the tip. They are covered with conspicuous soft prickles and short woolly hairs. At maturity, each seed pod splits along one side to release many seeds that have fluffy tufts of white hairs (comas). The comas aid the seeds as they are dispensed by wind. Common milkweed can be distinguished from other milkweeds by its prickly follicles.
Plant - Leaves - Habitat
The common milkweed plant is a large upright perennial 2 to 6 ft. tall and not branched until near the top where the flowers occur. The stem is sturdy, pale green, and usually short-pubescent or smooth. The plant, grows a network of thick rhizomes (horizontal underground stems) which send out shoots and roots for new plants.
The leaves are simple and opposite. The leaves mainly have short petioles (leaf stalks). Some leaves appear to have very short petioles or are almost sessile.
They are oblong (rectangular with rounded ends) to elliptic (broadest in the middle). The thick leaves are large, up to 8" long and 3.5" across. The edges are smooth but frequently undulate(wavy).
The upper surface is light to dark green and hairless or slightly hairy and the lower surface lighter green and densely hairy.
The leaf has a prominent midrib. The midrib may be red-brownish sometimes. The many smaller veins extending from the midrib to almost the edge have a distinctive pattern with the veins almost parallel to each other and perpendicular to the midrib.
When the leaves or stems are broken, a milky latex liquid is released. This liquid contains some cardiac glycoside compounds that makes the plants toxic to most insects and animals.
Leaf shapes and venation pattern
Lighter green underside and dark green top surface (bottom leaves) Petiole (leaf stalk) is easily seen.
Fuzzy hairs on the underside.
There are 12 species of milkweed in NJ. Some features that distinguish common milkweed (Asclepias syriaca) from the others are:
- Follices (fruits) are broad and covered with bumps with soft prickles and short woolly hairs
- Leaf blades are broad-oblong, broad-elliptic, or ovate. They have soft hairs on the under side.
- Leaves are mostly opposite and have short stalks. Some leaf stalks are very short or almost sessile.
- Corona hoods are taller than the gynostegium, the horns shorter or same height as the hoods.
-
Corona hoods whitish - light purple.
The corolla is pale purple-green to pale pinkish purple - The flower stalks (pedicels) are 1-2 inches long;
On this site we have:
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Milkweed: Butterfly Weed | Asclepias tuberosa |
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Milkweed: Common | Asclepias syriaca |
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Milkweed: Green Comet | Asclepias viridiflora |
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Milkweed: Poke | Asclepias exaltata |
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Milkweed: Purple | Asclepias purpurascens |
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Milkweed: Swamp | Asclepias incarnata |
The habitat includes meadows and sunny areas.
It has been found that over 450 species of insects feed on some portion of common milkweed. It provides nectar for flies, beetles, ants, bees, wasps, and butterflies and other insects.
Even though the milkweeds contain various levels of cardiac glycoside compounds which make the plants toxic to most insects and animals, some insects can store these chemicals in their tissue so that they are not edible by other animals. The Monarch butterflies do this, so that birds do not eat them or their caterpillars.
Some birds use the milkweed fiber to make their nests. Some birds use the milkweed silky seed tufts to line their nests.
Text by Millie Ling and all photos by Hubert & Millie Ling. Micrographs and dissections by Hubert Ling. Photos: Summer, Sourlands, Whittingham WMA, Delaware & Raritan Canal, Kittatinny Valley State Park, cultivation. Common milkweed can be found in many places.
Additional information / References
Additional information / references:
- USDA plant of the week: https://www.fs.usda.gov/wildflowers/plant-of-the-week/asclepias_syriaca.shtml
- USDA Milkweed Pollination Biology. https://www.nrcs.usda.gov/plantmaterials/nvpmctn12764.pdf
- Minnesota Wildflowers: descriptions. https://www.minnesotawildflowers.info/flower/common-milkweed
- Milkweed - For More Than Monarch Butterflies. https://www.fws.gov/story/2016-07/milkweed-more-monarch-butterflies
- A Close-up View of the Unusual Wildflower Common Milkweed http://www.microscopy-uk.org.uk/mag/artfeb05/bjmilkweed.html
- The Intricate Dance of Milkweed Pollination https://www.hachettebookgroup.com/articles/the-intricate-dance-of-milkweed-pollination/
- Key to NJ Milkweeds: GoBotany. It includes most of NJ milkweeds. https://gobotany.nativeplanttrust.org/dkey/asclepias/#all
- The USDA website shows Asclepias syriaca distribution in the US and other information: https://plants.sc.egov.usda.gov/plant-profile/ASSY
- Crossection of 2 stigmatic chambers of Asclepias syriaca https://www.researchgate.net/figure/Cross-section-of-the-area-of-two-stigmatic-chambers-of-a-flower-of-Asclepias-syriaca-CP_fig2_238192722
- Cultivation information: Jersey Friendly Yards searchable plant database: Asclepias syriaca