Before understanding what a bract is, it is helpful to understand what it is not. First, it is not a part of the "flower" by definition. Complete flowers are composed of four parts: sepals, petals, stamens, and pistils. A bract is none of these.
Neither is a bract a leaf, although it is sometimes defined as a leaflike structure. Bracts usually differ in shape or color from leaves, and they function differently. Leaves may be anywhere along the stem while bracts are generally located on a stem just below a flower, a flower stalk, or an inflorescence.
Flowers may arise between bracts, flowers may be separated from bracts, and bracts can be present without flowers. Bracts can be leaflike as in poinsettias, scalelike as on pinecone gingers and bromeliads, or even petal-like as on dogwoods. They can be brightly colored like those on Bougainvillea or green like those on sunflowers. The inflorescences of the carrot family have two sets of bracts; one beneath the entire flower cluster (involucre) and one beneath each sub-cluster (involucel).
Bracts serve several purposes. When some flowers (sunflowers and other composites, for instance) first bloom, they are surrounded by thick, green bracts that protect the flowers from pests and harsh weather. The bracts form the base of these composites and remain present for the life of the inflorescence. Some brightly colored bracts attract pollinators, sometimes in place of colorful petals or tepals (combination of petals and sepals, comprising the perianth). Other bracts, such as those of the passion flower, are coated with a sticky substance that traps insects. Acid on these sticky bracts helps to dissolve the insect so that it becomes a source of nutrients for the flower. Some bracts serve as insulators, protecting the delicate flowers within from freezing weather.
Several familiar flowers have bracts. The dogwood inflorescence, for instance, consists of a cluster of tiny flowers. Surrounding the actual flowers are showy, petal-like bracts. Another familiar example is the poinsettia. Actual flowers are small and light green to yellow and grow at the center of the brightly colored bracts. Bougainvillea flowers are usually small and white and are surrounded by three or six 1- to 2-inch brightly colored bracts. The bracts of composite flowers such as daisies and sunflowers are the green parts underneath that hold the disk and ray flowers to the stem.
The flowers of grasses and grains have inner bracts called the palea and outer bracts called the lemma. Aroids, such as Anthurium and Spathiphyllum have a specialized type of inflorescence called a spadix that is subtended by a bract called a spathe. Euphorbia milii (crown of thorns) has colorful bracts that resemble petals, and another Euphorbiaceae, Dalechampia dioscoreifolia (bowtie or butterfly vine) has a tiny central flower subtended by colorful purple to fuchia-colored bracts that resemble butterfly wings. Members of the Justicia genus have colorful bracts out of which the flowers emerge. Some conifers, such as Pseudotsuga (Douglas fir), have bracts that extend beyond the cone scales.
The Zingiberaceae (ginger) family has many species that produce flowers from bracts. In most cases, the flowers bloom while the bract is young, and mature bracts may remain colorful well past the time the flowers bloom. Bromeliaceae family members (bromeliads) often have colorful bracts, most of which are produced from the center of a rosette. The real flowers emerge from among the scalelike bracts.
Once observers are able to identify the basic flower parts, (sepals, petals, stamens, and pistils), they can figure that if other plant parts that are neither these nor leaves, fruits, needles, or cones, they just might be bracts. Recognizing specific plants that have bracts and being able to identify them will help to identify bracts in unfamiliar flowers. All it takes is careful inspection and a bit of basic understanding of plant parts.
What are the differences between a flower and an inflorescence? A flower is solitary as in a single flower, while an inflorescence is a cluster that contains multiple flowers. The individual flowers of an inflorescence are usually called florets, and the arrangement of these florets is different in the different types of flower clusters. Some inflorescence types include cymes, panicles, spikes, capitulums, racemes, umbels, corymbs, and spadices.