4527. How are crown flowers pollinated?

 HOW ARE CROWN FLOWERS POLLINATED?

The crown flower (Calotropis gigantea) is primarily pollinated by insects, specifically large bees and butterflies, through a specialized and complex mechanism. The flower structure ensures that the pollinators transfer pollen from one individual plant to another, as the plant is not self-fertile. 

The Pollination Process

The crown flower, a type of milkweed, has a unique floral anatomy that facilitates this specific interaction with pollinators. 

• Pollen in Pollinia: Unlike most plants that have loose, powdery pollen, the crown flower's pollen is contained within sticky, waxy sacs called pollinia.

• The Trap Mechanism: The flower has narrow slits (stigmatic slits) between the anthers. When an insect, such as a large bee or a butterfly, visits the flower to feed on nectar, its leg or other appendage can slip into one of these slits.
• Pollen Attachment: As the insect struggles to pull its leg out of the narrow opening, a sticky disc attached to the pollinia gets hooked onto the insect's leg, and the entire pollen sac is yanked free.
• Pollen Transfer: For pollination to occur, the insect must then visit another crown flower. If its pollen-laden appendage slips into the stigmatic slit of the new flower, the pollinia can be deposited onto the stigma, thus fertilizing the new flower.

 

 

 

Key Pollinators

• Bees: Large and powerful bees, such as bumblebees and carpenter bees (Xylocopa spp.), are the primary and most effective pollinators because they are strong enough to free themselves from the flower's grip with the pollinia attached.
• Butterflies: These insects also visit the flowers for nectar and can aid in pollen transfer, though the mechanism can sometimes trap smaller insects, which may struggle to escape. 

This intricate system, while effective with the right pollinators, can be a challenge for smaller insects, which sometimes get stuck in the flower's mechanism and perish or lose limbs. 

 

 

 Milkweed flowers have a fascinating pollination process. They do not have loose pollen, but they have nectar. When a bee or other insect visits to drink its nectar, it may stick its leg in the stigmatic slits. As the pull away, pollinia sacs attach around their legs and travel with them to the next flower. If all goes well, the pollinia sacs gets inserted and the cycle continues. Occasionally, honeybees, and other insects, are too weak to pull away and wind up "shackled" to the plant. You may find them hanging from the flowers.

 

 

 

 

https://collection.ento.vt.edu/2016/08/05/milkweed-pollinia-revisited/

 

 Milkweeds (Asclepias spp.) are herbaceous perennials that tend to have brightly colored clusters of flowers with abundant nectar. There are over 100 described species and subspecies of milkweed in North America. The monarch butterfly, Danaus plexippus (L.), may be the most well-known visitor to milkweed flowers, but milkweeds attract a large suite of butterflies, flies, beetles, bees, and wasps. Many people grow milkweeds for pollinators and other beneficial insects that feed on the plant’s copious nectar

 

Unlike most flowers, milkweeds do not produce loose pollen. Waxy masses of milkweed pollen are grouped into sacs called pollinia. Orchids are the only other group of plants known to use pollinia. Bees don’t collect pollinia to use as food for their larvae the way they do with the loose pollen of other flowers. Nectar is a pollinator’s only reward for visiting milkweed flowers, but there’s a lot of nectar to be had on milkweed. The design of the milkweed flower is all about attracting pollinators with nectar and ensuring that the pollinators pick up and move pollinia to a different flower to complete pollination. Let’s take a look at the unusual structure of the milkweed flower to understand that process.

Each individual milkweed flower has an attractive star-shaped corona on top of a short central column and with outward flaring petals below the column. In the photo below of common milkweed (Asclepias syriaca L.), the corona is a lighter pink than the lower petals. Insects will find nectar on the top of the flower in those stars.

 The botanical illustration below shows a milkweed flower from a lateral perspective. The star-shaped corona of a single flower (A) has five hoods (h) with a  corolla (c) of lower reflexed petals. A closeup of a flower (B) shows a slit-like opening (f) between each hood formed by the fusion of the anthers at their top and bottoms. Each slit leads to the stigmatic chamber, where the pollinia are housed. Each pollinium (C) is a paired, winged structure with a central body (d).

 

 

 

 

Botanical illustration of individual milkweed flower.
By Internet Archive Book Images [No restrictions]
via Wikimedia Commons.

 

 

 

 Closeup of a Melissodes trinodis Robertson (Hymenoptera: Apidae)
leg with attached pollinia. By USGS Bee Inventory and Monitoring
Lab from Beltsville, Maryland, USA [CC BY 2.0], via Wikimedia Commons.

 

A visiting insect trying to reach the nectar offered at the top of the star-shaped corona will slip one of its legs or another appendage inside the anther slits between the hoods. The pollinia inside the stigmatic chamber sticks to the insect’s setae or tarsal claws. By pulling its leg out of the slit, the insect extracts the pollinia and carries them off to another milkweed flower. Again the insect’s leg or another appendage will likely slip inside a slit while feeding. The flower will be pollinated successfully if the donor pollinia remain in the recipient anther slit. [For more information on this process and in greater detail, see Betz et al. (1994) and Borders and Lee-Mäder (2014). Both Eye on Nature and Robert Klip at Ohio State have nice blog posts on milkweed pollination and great closeup photos of pollinia, too.]