Composite Family (Compositaceae)

Members of the Asteraceae are most readily characterized by their unique floral structure. The flowers of members of this family are aggregated within a composite grouping known as an inflorescence, which in this family is known as a head. In the head, the small, individual flowers, called florets, are attached to a basal structure known as a receptacle. The latter is surrounded by one or more rows of bracts, that make up the involucre.

Artichokes in Salinas, California. © 1983 Lawrence Midgale, National Audubon Society Collection/Photo Researchers, Inc. Reproduced with permission.

The heads may be present singly, or they may occur in various sorts of aggregated groupings. Typically, each head gives the visual impression of being a single, large flower, even though the structure is actually a composite of several to many, small florets. This visual display is best developed in insect-pollinated species of the Aster-aceae, and is ultimately designed to attract pollinators.

In many cases, the individual flowers may occur as disk florets that have functional stamens and pistils but lack petals, or as ray florets that have an elongate, strap-shaped petal known as a ligule or ray. In some species, the head is entirely composed of disk florets, and is known as a discoid head. Discoid heads occur, for example, in the tansies (Tanacetum spp.). In other species the head is entirely made up of ray florets, and is referred to as a ligulate head, for example, in the dandelions (Taraxacum spp.).

In other species, the disk florets occur in the center of the head, and ray florets on the periphery, giving a particularly striking resemblance to a single, large flower. The ray florets of these latter, relatively complex inflorescences are commonly sterile, and are only designed to aid in attracting pollinating insects. One familiar example of this head structure is the ox-eye daisy (Chrysanthemum leucanthemum), which has a central packing of bright-yellow disc florets, and a white fringe of long, ray florets. The ox-eye daisy is the species of wildflower that love-struck young people use to tell whether their adoration is returned by their sweetheart-the petals are picked off one by one, to determine whether "he/she love me, or loves me not." The seeds of plants in the aster family are borne in dry fruits known as achenes. In many cases, the achenes of these plants are small, and have a fine, filamentous attachment known as pappus, which serves to give the fruits aerodynamic qualities that favor their dispersal by the wind. This seed form and dispersal method can be illustrated by the familiar dandelion, whose fruiting heads develop as whitish puffs of pappus-bearing seeds, which eventually disintegrate and blow about on the wind.

In some other cases, such as the sunflowers (Helianthus spp.), the seeds are encased in a relatively hard coat, and are only dispersed locally. The seeds of some other plants in the composite family are specialized to stick to the fur of mammals, and are dispersed in this way. Two common examples of this hitch-hiking strategy are the beggar ticks (Bidens spp.) and the burdock (Arctium spp.).