capitulum [ kuh-PICH-uh-luhm ] noun, plural capitula: a compact head of flowers on a very short axis; a composite flower characteristic of the Asteraceae
disk [ disk ] noun: (in the Asteraceae) the central part of the involucrate head, bearing disk flowers
disk [ disk ] floret noun: one of the often tubular flowers in the disk of a composite plant
floret [ FLAWR-it, FLOHR– ] noun: a small flower; one of the small flowers forming the head of a composite or the spikelet of a grass
ligule [ LIG-yool ] noun: the strap-shaped corolla of a ray floret in a composite head; a membranous outgrowth on the adaxial side of a leaf at the junction between the leaf blade and sheath in many grasses and some sedges
ray [ rey ] floret noun: one of the strap-shaped marginal flowers of the head in a composite plant
Can you identify which of the plants pictured below have the same flowering structure?
Left to right: Helianthus annuus, Lactuca sativa, Coreopsis verticillata, Tanacetum vulgare.
Each of the plants—sunflower, lettuce, tickseed, and tansy—have a capitulum, a type of inflorescence characteristic of the Asteraceae (the aster or composite family), of which they are all members. The word capitulum, first used in the 18th century, means “little head” in Latin, and some flora use the term head interchangeably with or instead of capitulum.
Image © www.cronodon.com
Though it may appear to be a solitary flower when it stands alone on a stem, the common sunflower (Helianthus annuus) is a capitulum or composite flower. Its head comprises numerous (150–1000+) tiny, 5-petaled, tubular, epigynous flowers or florets surrounded by 13–100+ strap-like florets on a short axis—a common receptacle—subtended by bracts called phyllaries and borne on a peduncle. (The heads of other composites may be sessile.)
The central part of the head is called a disk (sometimes spelled disc) and the florets, disk florets, which are arranged in clockwise and counterclockwise spirals. Each of the ray florets that encircle the disk has a corolla consisting of a single strap-like ligule of five fused petals forming a tube at the base. In sunflowers, ray florets are sterile, but in other composite flowers they may be pistillate (female). The modified caylx is a pappus of two lanceolate scales, but other composites may have pappi made of bristles, awns, setae, or coronas. Capitula with both disk and ray florets are referred to as radiate heads (or bilabiate for Gerbera with its two-lipped florets or radiant for Centaurea in which the peripheral florets are enlarged).
Composite heads are indeterminate, so the florets on the disk rim open first followed by the opening of each successive whorl of florets moving toward the center of the disk. The bisexual flowers—those having both male and female organs within the same structure—are protandrous, where the flowers are functionally male in the first phase of their development and functionally female in the second phase. This nonsynchronous phasing promotes cross-fertilization in order to create genetically diverse offspring. If cross-fertilization fails to occur, then they will self-pollinate toward the end of the female phase to produce seeds.
The anthers (male parts), which are fused in a column, appear first. When they dehisce, the pollen released inside the column is pushed outward by the developing style and appears as a yellow star between the five anther tips. This is the cue to pollinators that nectar and pollen are ready to harvest. When the anthers lose their reproductive activity, the column retracts and the style and stigma (female parts) elongate from the center of the column. The two stigma lobes open and arch downward becoming receptive to pollen. (Maier, 2014)
Video © Mary Free
You can imagine how attractive composite flowers are to pollinators—bees, wasps, flies, lepidopterans. With a receptacle holding dozens to hundreds of florets with their nectaries and pollen all compressed into a small space, an insect can just walk from one floret to the next, taking its fill without expending much energy.
The ray florets with their UV-reflecting parts signal to pollinators from afar and guide them to the nectaries and pollen once they land. Since pollinators tend to approach the outside of the disk first, they deposit pollen from other inflorescences on the receptive stigmas. As insects forage around the disk toward the center, they pick up new pollen grains from the younger florets still in the male phase to transport to other inflorescences.
Some composites, like ageratums, gayfeathers, Joe-pye-weeds, thistles and thoroughworts and bonesets, do not have ray florets. Instead, they only have bisexual disk florets and are described as discoid. A variation of this type of capitulum is seen in American burnweed and some asters that have disciform heads in which a ring of sterile or female florets surround male or bisexual disk florets. Chicory, dandelions and lettuce exemplify the ligulate head—one with only bisexual ray florets. Globe-thistles add another modification—they have secondary heads, known as capitulescence.
Capitula can be borne singly (like some sunflowers, coneflowers, dandelions, daisies, gaillardias, ragworts, tickseeds, and zinnias) or may be arranged in a second inflorescence and referred to as: corymbiform (Achillea, Coreopsis, Dahlia, Erigeron, Eutrochium, Helianthus, Liatris, Packera, Rudbeckia, Vernonia); cymiform (Liatris, Packera); paniculiform (Helianthus, Liatris, Rudbeckia, Silphium, Solidago, Symphyotrichum, Vernonia); racemiform (Liatris, Silphium, Solidago, Symphyotrichum); spiciform (Helianthus, Liatris); or some variation thereof.
Radiate and Bilabiate Capitula (Sometimes) Borne Singly
Radiate and Radiant Capitula Borne in a Secondary Inflorescence
Disciform, Discoid, and Ligulate Capitula and Capitulescence
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One cannot overstate the importance of the composites of the Asteraceae to pollinator well-being and to the ecosystem. Two genera in the Asteraceae—Solidago (goldenrod) and Symphyotrichum (aster), each of which supports about 100 lepidopteran species—are considered the top perennial/annual keystone plant species, which are critical to the food web. Plants in the Asteraceae also are among the top plant genera supporting specialist bees—those that feed only on specific plant species. The following genera support bee specialists in the eastern United States (insect numbers in parentheses): Solidago (39), Helianthus (35), Symphyotrichum (32), Rudbeckia (26), Coreopsis (17), Packera (12), and Cirsium (10).
Learn more about sunflower spirals and the Fibonacci sequence and about the fruit of the Asteraceae, a cypsela, and its dispersal mechanism, a pappus.
Ligules on Grasses and Sedges
A ligule occurs at the junction between the leaf blade and sheath—lower part of a leaf that surrounds the stem—in many grasses and some sedges. Like stipules in dicots, to which some researchers have compared ligules, their appearance varies greatly among species. Ligules may be long or short or absent altogether and their shape ranges from jagged to rounded to squared to pointed or even split. Their membranous tissue may be translucent, glabrous, or shredded and can be fringed with hairs. What is the function of a ligule? It thwarts detritus and water from getting between the leaf sheath and stem to prevent rotting. It also aids in the identification of the species.
![The ligules of Bromus ciliatus (fringed brome) are short, usually glabrous, rarely pilose, and jagged. Photo © Matt Lavin CC BY-SA 2.0 [rotated]](https://i0.wp.com/mgnv.org/wp-content/uploads/2025/10/Bromus_ciliatus_3828240402.jpg?ssl=1)
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