A word to conclude the celebration of Pollinator Week (along with three pollinator videos below)
Any discussion about pollinators and pollination should include anthers, the pollen-producing portion of the stamen, a plant’s male reproductive organ. To understand how plants and pollinators have coevolved and what pollinators we can expect to see on a particular plant, it is helpful to understand the structure and function of anthers.
An anther usually has two lobes called thecae joined by the connective, which also attaches the anther to the filament. Within each theca are two sac-like structures called microsporangia in which pollen grains develop. They are separated by a stomium, which is where the anther dehisces or opens to present the pollen grains. [Two plant families, Orchidaceae (orchid) and Asclepiadaceae (milkweed), do not produce individual pollen grains. Instead the pollen is coalesced in sacs called pollinia and in Asclepias the anther is split in half. For more information, read The Perils of Pollinia and More About Milkweed.]
Click on images on this page for larger views and captions.
The dehiscence of an anther is timed to synchronize with the maturation of the flower and its opening; and the presentation of pollen ceases when the flower closes. An anther can dehisce along its length or width, or through pores at its tip or under tissue flaps.
Pollen from the anthers can be released explosively via vertical catapulting with or without the aid of an animal (as in natives Kalmia latifolia and Cornus canadensis). It can be released in small clumps through sonic vibrations, known as sonication or buzz pollination, which are created by the rapid movement of certain bees’ flight muscles (as in azaleas and tomatoes). In some flowers the movement of animals on the flower or brushing against the anther dislodges the pollen. In grasses like Bermuda and rye, some research has shown that while up to 20% of the pollen breaks away from the anther during dehiscence, a minimum wind speed of 5.6 miles per hour is required to remove the rest. (Lu, 2004)
Flower traits (structure, color, fragrance, nectar, and pollen) have evolved to attract specific pollinators and the anther placement and design help ensure that these pollinators will be successful. For example:
In native Monarda punctata (spotted beebalm), two fertile stamens and a style/stigma are tucked under the upper petal lip. When the anthers are mature with pollen, they become visible and dispense pollen on visiting insects like the Polistes dominula (European paper wasp) in this video. Beneficial wasps are one of the most effective pollinators of spotted beebalm and will be attracted to gardens in which it is planted.
Azaelas require sonication or buzz pollination by particular bees to release pollen from the poricidal anthers. Larger bees, like Bombus impatiens (common eastern bumble bee), that buzz the entire flower are more effective pollinators than small anther-buzzing bees. Though efficient in collecting large amounts of pollen, Andrena cornelli (azalea miner bee) tends to do so without touching the stigmas, making it an ineffectual pollinator. According to entomologist John Ascher, “This is the only bee species oligolectic on azalea. Other Andrena cannot handle its pollen, but this species does so efficiently as it has long, widely spaced scopal hairs.”
Butterflies, consumers of nectar, are usually weak pollinators. However, a 2015 research paper, Reproduction in Flame Azalea (Rhododendron calendulaceum, Ericaceae): A Rare Case of Insect Wing Pollination, found Papilio glaucus (eastern tiger swallowtail) transferred pollen efficiently between the anthers and the stigmas of native flame azalea due to its “wing-flapping behavior.” The wings of the eastern tiger swallowtail in this video carry pollen, which also may have transferred to stigmas while the butterfly searched for nectar on an unidentified azaela species.
Carr G. Images and Descriptions of Flowering Plant Families Alphabetical Listing. University of Hawaii Botany Department. [accessed 13 May 2021].
Lu, G. 2004. The Effect of Wind on the Emission of Grass Pollen. Research Science Institute.
Mesquita‐Neto JN, Blüthgen N, Schlindwein C. 2018. Flowers with poricidal anthers and their complex interaction networks—Disentangling legitimate pollinators and illegitimate visitors. Functional Ecology. DOI: 10.1111/1365-2435.13204.