Asteraceae

General distribution:

The Asteraceae are herbs, shrubs, or less commonly trees and are arguably the largest family of flowering plants, comprising about 1,100 genera and 20,000 species that are characterized by having the flowers reduced and organized into an involucrate pseudanthium in the form of a head or capitulum.

The family has a cosmopolitan distribution, with species ranging from subpolar to tropical regions, colonizing a wide variety of habitats.

Asteraceae is an economically important family, providing products such as cooking oils, leaf vegetables like lettuce, sunflower seeds, artichokes, sweetening agents, coffee substitutes, and herbal teas.

Botanical description:

Inflorescence

The inflorescence consists of several small flowers, called florets, that are crowded together, sessile, on a receptacle. This inflorescence is often mistaken for a single flower. Compositae refers to the superficial resemblance of the head to a single, large flower. What looks like an ordinary flower is really a composite of small florets.

The inflorescence is called a capitulum, or head.

The flower cluster is surrounded by bracts, called involucral bracts, or phyllaries, and the whole structure is referred to as an involucre.

Some members of the family have only one row (uniseriate) of involucral bracts (Senecio); others have several rows.

The arrangement (how many rows; whether the bracts are overlapping or side by side) and the texture of bracts are often important in identifying the genus.

In some species, some or all the florets are subtended by their own bracts. These bracts are attached to the receptacle and are referred to as receptacular bracts, or chaff. They are generally not visible unless you pull apart the capitulum. Species without chaff are said to have a ‘naked’ receptacle. The receptacle is the enlarged portion of the peduncle upon which the florets are borne.

Flower

Disk florets - In most members of Asteraceae, the central florets have a radially symmetrical, tubular corolla, with 5 short lobes. These florets are called disk florets. They form the central disk of the capitulum in typical daisies. Disk florets are often perfect flowers. The entire inflorescence may be composed of disk florets only (a condition referred to as homogamous); when so, the inflorescence is said to be discord.

Ray florets - Surrounding the disk florets are sometimes an outer ring of ray florets. These have zygomorphic symmetry, and are usually either sterile or pistillate, with 3 apical teeth. Ray florets are strap-shaped, imperfect, and never occur alone in the inflorescence (a condition referred to as heterogamous). They are always associated with disk florets (although disk florets may occur without ray florets) and form a circle around the margin of the head, the center filled with disk florets.

Ligulate floret – the best known weedy composite, the common dandelion, has a third kind of floret, a ligulate floret. These resemble ray florets with their zygomorphic symmetry, but ligulate florets are perfect (bisexual) and have 5 apical teeth at the end of the strap.

Bilabiate floret – These florets have zygomorphic symmetry, but not so much as the ray or ligule. They have 3-4 lobes on the lower (long) lip, and 1-2 lobes on the upper (shorter) lip. They are also bisexual, can be represented by thistles, and are relatively rare in the U.S.

Apart form differences in corolla shape and sexuality, the 4 types of florets are very similar to one another in many family-unifying characteristics:

• The ovary is inferior and encloses a single seed.
• The fruit is an achene, with the ovule basally attached to the seed pericarp.
• The calyx is so highly modified it is given a different name – pappus.
• The pappus may consist of capillary hairs (fine hairs) that may be plumose (have yet finer hairs on them, like down); or they may have bristles, awns, scales, or no pappus at all (epappose). Think of a dandelion that has gone to seed, and you’ll recognize the function served by the pappus in most  Asteraceae: seed dispersal. The “parachute” of a dandelion consists of capillary hairs that form the pappus. The “ropes” connecting the parachute to the portion of the ovary with the seed is an extension of the upper part of the ovary (beak).
• Androecium consists of  5, epipetalous stamens. In almost all species the anthers are fused together, forming a ring (tube) around the style (syantherous condition). The tube of the anthers is an important part of the pollen presentation of composites.
• The anthers and pollen mature before the stigmas (protandry), and shed their pollen into the cylinder formed by the anthers.
• There is only I style, but it ends in two style branches (stigmas) which are stigmatic only on the inner, facing surface.
• At the time when anthers are shedding pollen, the style is short and the two style branches are pressed together. As the style elongates, it grows up through the ring of anthers, with the style branches still pressed together. The pollen is then pushed out of the anther cylinder by the tops of the style branches. This makes the pollen available to passing insects. Because the style branches are still pressed up against one another, there is little chance any pollen will land on the floret’s own stigmas. Once the style has elongated to the point where the branches are completely above the anther ring, the style branches arch back, exposing the stigmatic surfaces. By then, most of the floret’s own pollen will have been dispersed. This pollen presentation mechanism is a means of preventing self-pollination and requiring cross-pollination in many composite species.

Seasonal development:

Pollination is effected by diverse agents, most commonly various sorts of insects.

The members of Asteraceae produce a type of fruit called an achene, which is dry and single-seeded and does not open at maturity.