Beard

beard [ beerd ] noun: a tuft or line of usually dense, long hairs. adjective: bearded

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When one mentions the word beard in casual conversation, one usually visualizes facial hair. However, if a botanist uses the word beard, the reference may be to a tuft or row of hairs on a plant. The appearance and location of that beard on a plant differs according to the species.

You are likely familiar with the showy flowers of hybrid Iris x germanica (bearded iris). With thousands of cultivars available in a stunning array of color combinations, these irises add vibrancy to the spring landscape. Striking, often contrasting, beards run down the center of their falls (sepals), enhancing their allure. Just like the patches of contrasting color (signals) on native Iris virginica (Virginia blue flag) or the ridges (crests) of native Iris cristata (dwarf crested iris), beards provide a visual element to attract pollinators to the flower, as well as a tactile element to guide pollinators into the tight space between the sepal and style arm where the anthers and nectar are located. Other species with a perianth beard are the late summer-bloomers in the genus Chelone (turtlehead), whose bilabiate flowers have white to yellow beards located on the lower lip of their corollas, partially hidden by the flower’s hooded upper lip.

Left to right: Iris x germanica, Chelone glabra, Chelone lyonii ‘Hot Lips’

Beards are not always located on the petals or sepals however.  The flowers of Penstemon (beard-tongue) have tufts of hair located on their staminodes while those of Tradescantia (spiderworts) have hair on the filaments of their stamens. If these hairs are dense enough, they are referred to as beards. For example, Flora of Virginia describes the staminodes of Penstemon digitalis (foxglove beard-tongue) as “pubescent apically ,” and those of Penstemon laevigatus (smooth beard-tongue) as “bearded distally for ⅓–½ of their length.” It describes Tradescantia filaments as either villous or bearded.

Left to right: Penstemon digitalis (with pubescent staminode), Penstemon laevigatus (with bearded staminode), Tradescantia virginiana (with villous filaments and with bearded filaments)

These beards may serve a variety of functions: attracting pollinators to the flowers, guiding pollinators to the nectar and pollen, or serving as deterrents to unwanted crawling insects. In the late 1970s, Japanese scientists thought that the beards of cloned spiderworts might be useful indicators of abnormal radiation when experiments found that their filament hairs turned “from blue to pink when exposed to as little as 150 millirems of radiation.” Alas, they were not sensitive enough to perform that function.

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References

Pennell FW, Wherry ET. 1927. The Genus Chelone of Eastern North America. Bartonia.10: 12–23. http://www.jstor.org/stable/41609564

Weakley AS, Ludwig JC, Townsend JF. 2012. Flora of Virginia. Bland Crowder, ed. Foundation of the Flora of Virginia Project Inc., Richmond. Fort Worth: Botanical Research Institute of Texas Press.

Webster B. April 25, 1979. Flower is Detector of Slight Radiation. The New York Times. Section A, page 24.