Compositae

Compositae

In number of species, the family Compositae or Asteraceae, commonly known as the sunflower family, is among the largest families of flowering plants.

The Compositae family consists of more than eleven hundred genera worldwide and possibly as many as twenty-three thousand species. Representatives of the family are found on every continent except Antarctica.

Species diversity is high in the southwestern United States and Mexico, in southern Brazil and along the South American Andes Mountain range, along the Mediterranean region, in Southwest and Central Asia, in South Africa, and in Australia.

The plants typically are found in open, sunny habitats, although some species are found in lightly forested areas and at the edges of woodlands.


Examples of common genera found in North America are goldenrod (Solidago), sunflower (Helianthus), daisy (Chrysanthemum), Aster, and ironweed (Vernonia). The predominant life-formswithin the family are perennial herbs and shrubs, but some species exist as annuals, vines, lianas (woody vines), and trees.

Leaf arrangement is most commonly alternate, although some genera do exhibit opposite leaves. Despite this gross morphological diversity, the feature that unites all the members of the family is the unique form of the compound inflorescence, or flowering head.

Inflorescence

At first glance, the flowering head of many Compositae, such as that of the sunflower, resembles a single large flower with what appears to be a set of petals surrounding a cluster of anthers and stamens.

Inflorescence

Upon closer inspection, this compound inflorescence, or capitulum, is really a collection of highly reduced and modified flowers borne on a surrounded by a series of leaflike structures termed bracts that collectively comprise the involucre.

Two types of reduced flowers can be present within a capitulum, either ray or disc flowers, or both. The ray flower superficially resembles a petal and occupies the outer circumference of the capitulum. It can be either pistillate or sterile. The corolla is fused at the base, gradually expanding in width and eventually tapering to a tip.

At maturity the tip of the ray flower corolla is oriented away from the center of the capitulum. The inner disc flowers can be either perfect (all reproductive parts present) or functionally staminate and possessing a corolla tube consisting of fused petals. At maturity the anthers and the stigmas will protrude from the corolla tubes.

At the base of the corolla tube is a set of modified sepals termed the pappus. The pappus can consist of fine hairs, scales, or bristles, or it may be absent in some species. The seed is an achene and upon maturity is wind-dispersed with the assistance of the pappus.

Pollination Biology

Pollination Biology

A variety of mechanisms account for successful pollination in the Compositae, wind pollination being a very common mode. “Hay fever,” which in humans is an allergic reaction to plant pollen and other allergens, is greatly exacerbated in the late summer and early fall by the wind-pollinated ragweed (Ambrosia).

Although often falsely accused, the goldenrods (Solidago) are not a major cause of hay fever, as they are pollinated by insects, not wind. By far, the majority of the species in the family are pollinated by insects, which are often attracted flowering stalk (peduncle).

The expanded and flattened top of the peduncle is the receptacle and is to the inflorescences by brightly colored ray flowers. The insect-pollinated flowers are typically generalists, attracting a variety of insects rather than a specific insect species. In some tropical taxa, birds or other animals are largely responsible for pollen exchange.

Economic Uses

Economic Uses

Genera that contain species used for human consumption include endive and chicory (Cichorium), artichoke (Cynara), sunflower seeds and oil (Helianthus), lettuce (Lactuca), and dandelion greens (Taraxacum).

Pyrethrumis a naturally occurring pesticide obtained from tansy (Tanacetum). Numerous genera are used as ornamental plants, such as marigolds (Calendula), Zinnia, bandana daisy (Gaillardia), and Dahlia.

A number of plants are used for medicinal purposes. Coneflower (Echinacea) and fireweed (Liatris) are often used as herbal teas. Extracts from species of the Compositae are available from many health food retailers and practitioners of folk medicine.

A current focus of pharmacological research is in determining those plant compounds responsible for producing beneficial effects in humans. Some species, however, contain compounds that at sufficient levels are quite toxic to animals, such as humans and livestock.

Evolutionary Success

Evolutionary Success

Current opinion is that the family probably originated in South America or in the Pacific region, perhaps as early as the Tertiary period. Several reasons have been proposed for the biological success of the family.

First, the pappus is a very efficient agent for wind-borne dispersal of mature seeds, serving as a lofting device (similar to a parachute) to carry the small seeds on air currents over great distances. Second, the agents of pollination are diverse, ranging from mechanical (wind) to biological (insect or bird).

Third, many members of the family exhibit unique chemical compounds (such as sesquiterpene lactones and polyacetylenes) that may discourage large herbivores and insects from foraging on the leaves, thus allowing plants to mature and engage in reproduction. It is the successful production of viable offspring that constitutes the major measure of evolutionary success in the family.