Word of the Week: Stamen

stamen [ stey-muhn ] noun, plural stamens, stamina: male reproductive organ of a seed plant, typically consisting of a stalk (filament) bearing a pollen producing anther at its tip; the angiosperm microsporophyll

Adapted by Mary Free from a diagram by Mariana Ruiz

When we speak of stamens and pistils, we are talking about the way flowering plants reproduce. A stamen is the male reproductive organ. Its filament supports typically a 2-lobed anther that produces pollen from four saclike structures called microsporangia. Together, all the stamens on a single flower are called the androecium.



Stamens Vary Widely in Number and Appearance by Species

Plants have characteristics that favor the distribution of pollen by wind, water, or animal pollinator. These include color and fragrance to attract and nectaries at the base of stamen filaments that secrete food (nectar) to reward insect, bat, and bird pollinators, who may then carry the pollen to the pistils of other flowers to fertilize them.

Some flowers are structured to help prevent self-pollination. One way they do this is through a phenomenon called “stamen irritability,” or, more formally, seismonasty or thigmonasty, meaning response to movement, or tactile sensitivity. The movement can come from insects, humans, rain, or even strong winds. Contact with a stamen will cause it to tilt toward the pistil, in the process depositing the pollen on any visiting insect or the rim of the pistil, but not the cushiony main pad, thus lessening the likelihood of self-fertilization. Cross fertilization is more desirable for ensuring vigorous offspring. The motion toward the center does not bend or change the stamen’s curvature, researchers say; instead it seems to come from a spring-like change in cell structure at the base of the filament. The filaments return to their original position, ready to repeat the process.

Stamen irritability has been noted and written about since the time of Carl Linnaeus, (1707-1778), known as the father of taxonomy. Linnaeus described stamen motility in the genus Berberis in his writings. Modern researchers, including Bob Harms at the University of Texas, have further explored and written about the subject in recent years. In the 18th Century, Erasmus Darwin, the naturalist grandfather of the evolutionary theorist Charles Darwin, wrote a more anthropomorphic description: “The approach of the anthers in many flowers to the stigmas, and of the pistils of some flowers to the anthers, must be ascribed to the passion of love, and hence belongs to sensation, not to irritation.” Besides Berberis, plants that may exhibit stamen irritability include Opuntia polycantha (prairie prickly pear) and some members of the Portulaceae, Tilliaceae, and Asteraceae.

Other plants exhibit a different pollen dispersal method from their stamens: explosive pollination via vertical catapulting, for which the particulars vary among species. In Cornus canadensis (bunchberry dogwood), the opening of the flower releases the stamens. Within the first .2 milliseconds the petals clear the stamens, which launch their pollen more than ten times the height of the flower. If an insect triggered the flower to open, then it will be covered with pollen; if not, the pollen will be carried on the wind. Kalmia latifolia (mountain laurel) anthers are secured in pockets at the edge of the petals, so that when the flower opens, the filaments of the stamens arch upward under tension. When a visitor of a certain weight seeks nectar in the center of the flower, its weight dislodges the stamens, which spring up flinging pollen onto the visitor’s body at a speed of about 11.5 feet/second. Why and how plants respond to different stimuli are questions that offer many avenues for research into diverse flower forms.

References

Darwin E. 1731–1802. Zoonomia: or, the laws of organic life. (pt.1, p. 103)
Darwin, Erasmus, 1731-1802. https://quod.lib.umich.edu/e/ecco/004874881.0001.001/1:7.13?rgn=div2;view=fulltext. [accessed 2021 Feb 20].

Edwards J, Whitaker D, Klionsky S, Laskowski MJ. 2005. A record-breaking pollen catapult. Nature 435(164). https://doi.org/10.1038/435164a.

Harms B. Irritable Stamens. A Phenological and Morphological Study of Berbis in Northern Hays County (Central Texas). Billie L. Turner Plant Resources Center. The Universitty of Texas at Austin. http://w3.biosci.utexas.edu/prc/DigFlora/BERB/IrritableStamen.html [accessed 2021 Feb 20].

Switzer CM, Combes SA, Hopkins R. June 2018. Dispensing Pollen Via Catapult: Explosive Pollen Release in Mountain Laurel (Kalmia latifolia). The American Naturalist. 191(6). https://dx.doi.org/10.5061/dryad.65sd4.

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