bract [ brakt ] noun: a modified leaf or scale
Bracts, modified leaves or scales, come in many forms, colors, and textures; and terminology may vary by type and species. Their main function is to protect young flower buds from the elements by enclosing the floral organs. They may also serve to attract pollinators or protect fruits. Some bracts may persist on the plant; others fall or fade away as the plant matures. Often bracts resemble and are mistaken for other flower parts.
Foliaceous Bracts
Foliaceous bracts resemble the leaves of a plant. For example, one might assume that the green to mottled bracts of trillium are leaves. Likewise, the conspicuous bracts at the base of the flowers of some monardas and of southern native Rhynchospora colorata (narrow-leaf whitetop sedge) are leaf-like.
Petaloid Bracts
Petaloid bracts are usually bright or colorful and like the flower petals they resemble, aid in attracting pollinators. For example, the showy bracts of native Cornus florida (flowering dogwood) and non-natives bougainvillea and poinsettia are often mistaken for petals, but they actually lie beneath the plants’ real, somewhat insignificant flowers.
Involucral and Related Bracts
- An involucre is a whorl of bracts at the base of some inflorescences, especially composite flowers like coneflowers and others in the Asteraceae (aster family).
- A phyllary is one of the involucral bracts subtending the flower head of a composite plant, like fireweed or thistle.
- An epicalyx is an involucre that resembles the calyx but consists of a whorl of bracts exterior to the calyx, as found in hibiscus and strawberry.
- A cupule is a modified woody involucre or a whorl of bracts that have hardened and coalesced into a cup at the base of a fruit, as found in oak and hazelnut.
Hibiscus moscheutos, Hibiscus coccineus, Quercus alba, Corylus avellana ‘Contorta.’
Spathes
When one thinks of spathes, native Jack-in-the-Pulpits, invasive arums, or tropical anthuriums may come to mind. In these plants, the spathe sheaths the minute flowers of the spadix. But a spathe is also the covering that protects emerging iris buds and iris ovaries.
Scaly Bracts
In conifers, a bract may subtend each scale of a seed cone (female strobilus). In Humulus lupulus (common hops), each pair of flowers in a cluster is subtended by a bract. These floral bracts enlarge to form the cone-like strobili, which contain the seeds. Pistillate (female) catkins of native Betula nigra (river birch) have one to three flowers (comprising an ovary and two styles) per bract. These are referred to as scaly bracts.
Left to right: Pseudotsuga menziesii, Humulus lupulus, Betula nigra.
Bracts in Grasses and Sedges
In grasses, the bract below a spikelet is called a glume. Within a spikelet are florets, each protected by an inner bract called a palea and an outer bract called a lemma. However, some research has shown that the palea and lemma may actually be modified sepals rather than modified leaves.
“In sedges, the word bract is used in a restricted sense, to mean only the one (or rarely more) bract that occurs on the culm just below the inflorescence.” (Johnson 2001) Some consider the perigynium, which encases the sedge achene, to be a bract whose margins have fused together to form a sac.
Diagram © Mary Free
References
Johnston BC. 2001. Field guide to sedge species of the Rocky Mountain Region: The genus Carex in Colorado, Wyoming, western South Dakota, western Nebraska, and western Kansas. Renewable Resources R2-RR-01-03. Denver, CO: U. S. Department of Agriculture, Forest Service, Rocky Mountain
Region. page 4.
Lombardo F. Yashida H. 2015. Interpreting lemma and palea homologies: a point of view from rice floral mutants. Frontiers in Plant Science. doi: 10.3389/fpls.2015.00061
Grasses, Sedges and Rushes. Herbaceous and Woody Vines. Minnesota Wildflowers. (accessed October 17, 2012).
Snell RS. 1936. Anatomy of the Spikelets and Flowers of Carex, Kobresia and Uncinia. Bulletin of the Torrey Botanical Club, 63(5): 278–279. doi: 10.2307/2480780
Weakley AS, Ludwig JC, Townsend JF. 2012. Flora of Virginia. Botanical Research Institute of Texas.
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