The most obvious and outstanding general feature of Asteraceae is that the flowers are grouped characteristically into compact inflorescences (heads) that superficially resemble individual flowers. Each such head is ordinarily subtended by an involucre of small modified leaves (bracts). Furthermore, in more than half the members of the family, the flowers in the outermost row or rows of the head have a modified, mainly flat and elongate corolla that resembles an individual petal of most other flowers. Thus, the “petals” of a daisy or sunflower are actually the outermost flowers of the head. An inflorescence of this family can have more than 1,000 individual flowers (florets), and the heads can be grouped into more complex, secondary arrangements called capitulescences.

sunflowerSunflower (Helianthus annuus).Tom Brakefield—Photodisc/Getty Images

eastern purple coneflowerEastern purple coneflower (Echinacea purpurea).Ulf Eliasson

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When present, the flower petals of Asteraceae are joined together by their margins, forming a tubular or strap-shaped corolla (a sympetalous corolla) that often has apical teeth representing the petal tips. The other floral parts are attached to the top of the ovary rather than beneath it. The calyx (sepals) of Asteraceae is so highly modified, in contrast to that of other families, that it is given a different name, the pappus. The pappus consists of one to many dry scales, awns (small pointed processes), or capillary (hairlike) bristles; in some the scales may be joined by their margins to form a crownlike ring at the summit of the ovary. In a few genera (e.g., Marshallia) the calyx consists of five regularly placed scales that are obviously homologous with sepals.

The pistil (female structure) is composed of two carpels, which are united to form a compound ovary with a terminal style. There is usually a nectar-producing region (nectary) in the form of a minute ring surrounding the style atop the ovary. The ovary has only one locule (seed cavity), with a single ovule arising from the base. The fact that the ovule is basal is the best single feature distinguishing Asteraceae from the related Calyceraceae, which also has involucrate heads with a similar pollen-presentation mechanism but has the ovule pendulous from the top of the ovary.

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The sequence of flowering within individual heads is always centripetal, characteristic of a racemose (indeterminate) inflorescence. This means that the outer flowers bloom first, with a progressive spiral of flowering toward the centre of the head. The secondary arrangement of heads (capitulescences) of Asteraceae is typically cymose (determinate). The terminal head on the main axis blooms first, followed by the terminal heads of the main branches. After that the sequence is mixed, with both cymose and racemose components. Only rarely, and then clearly as a derived condition, is the secondary inflorescence racemose throughout, with the lowest heads blooming first and the terminal ones last.

Individual heads of most members of Asteraceae are said to be discoid, radiate, disciform, or ligulate, according to the kinds of flowers they contain. The simplest type is the discoid head, in which the flowers have a regular, tubular corolla, with generally four or five apical teeth representing the tips of the petals. This kind of flower is called a disk flower. Ordinarily, the flowers in a discoid head are all perfect (bisexual) and fertile. Thistles and ageratums are examples of Asteraceae species with discoid heads.