Information about Compositae or Asteraceae

Dahlia 'Rio Rojo', a red single type

The family Asteraceae or, alternatively, Compositae, is a group of dicotyledonous plants including asters, sunflowers, daisies, and dahlias. The family name is derived from the genus Aster, meaning "star," and refers to the star-shaped flower head of its members, epitomized well by the daisy. The Asteraceae is the second largest family in the Division Magnoliophyta (flowering plants), with over 20,000 recognized species. Only the orchid family (Orchidaceae) is larger, with about 25,000 described species.

All Compositae share the following characteristics:

  • The inflorescence is an involucrate capitulum (see below for more information).
  • Tubular or disc florets are actinomorphic (radially symmetrical); ligulate/ray florets are zygomorphic (laterally symmetrical)..
  • Anthers are syngenesious (stamens are fused together at their edges, forming a tube).
  • Ovules are basally arranged around the ovary.
  • Each ovary contains a single ovule.
  • The calyx (sepals) of the florets are modified to form a pappus, a tuft of hairs, which often appears on the mature fruit.
  • The fruit is an achene (a thin-walled fruit which does not dehisce).

The most common characteristic of these plants is the presence of a capitulum, or floral head, containing a number of small individual flowers, termed florets. There are two types of florets: disc florets and ray florets. The inner portion of the capitulum is comprised of florets with tubular corollas; these are the disc florets, and they represent the "center" of a dahlia flower. The outer, more showy portion of the capitulum is comprised of florets with a single ligulate, or strap-like petal; these are the ray florets, and they represent the "petals" of a dahlia flower. The composition of Compositae capitula can range from all ray flowers (such as dandelions, Taraxacum sp.) to all disc flowers (such as thistles, Centaurea sp.).

This diagram shows typical parts of a Compositae (or Dahlia) flower. The following examples are photographs from a dissection of Dahlia 'Alpen Diamond', a collarette type. Please click on the images below to see a larger version.

This is the "flower," actually a capitulum composed of multiple types of florets.

This is the reverse of the capitulum, showing the phyllaries, or leaf-like bracts, subtending the capitulum. These phyllaries, together with the portion of the capitulum holding the individual florets, called the receptacle, comprive the involucre which supports the entire flower. Phyllaries do the job of sepals (or sepals and bracts) in a normal flower, protecting the more delicate petals and sexual organs inside the developing inflorescence.

To the right of the image you can see ray florets removed. Notice the yellowish subtending floral bracts still attached to the receptacle. In dahlias, ray florets are sterile, so no normal stamens or pistils are evident. The collarette "petals" distinguishing this type of dahlia are in fact sterile stamen filaments, called staminodes, which have become petaloid through selective breeding.

Here the capitulum has been cut in half, showing the ray florets and disc florets on the left, and just the disc florets on the right. Notice the very large number of disc flowers, tightly packed in the center of the capitulum.

A few disc florets have been removed from the capitulum. Each is subtended by a floral bract. Ray florets have been included for comparison purposes.

Here the disc florets are shown without ray florets. Each individual disc floret is about 7 mm in length.

This is an individual disc floret, somewhat dissected. This is as much as can be seen with the naked eye. The bifurcate stigmatic surface is clearly visible at the top of the photograph. Just below this are 5 stamens, which have at their bases fused to form a syngenesious structure. To the middle left is the corolla, the petals of this floret, which have been reflexed to allow us to see the stamens, which are usually inside the corolla. If you look carefully, you can see that the corolla tube has 5 points at its tip. These are all that remain of what used to be 5 seperate petals, which have fused and become much smaller than what is considered an ancestral floral type. The whitish section at the base of the floret is what remains of the calyx, the sepals of the flower. This whitish area, when mature, will become a set of bristles called "pappus", which aids in seed dispersal.