David Brandenburg, Taxonomic Botanist
Looking out at your backyard one morning, you count some dozen and a half dandelions in bloom. The actual number of individual dandelion flowers in your yard is surprisingly much greater than your visual estimate. That’s because dandelions belong to the “composite” family of plants (Asteraceae, a.k.a. Compositae), sometimes called the daisy, aster or sunflower family. What looks to be a solitary flower in plants of the composite family is in reality a flower cluster.
That’s right, a “single” dandelion in bloom is, in fact, a cluster of many small yellow flowers—these so closely arranged as to have the appearance of but one larger flower. In the case of the common dandelion, the number of smaller flowers is typically between 40 and 100. The superficial resemblance of this cluster of 40—100 small flowers to a solitary large flower is not immediately obvious in a dandelion. A much easier composite-family representative to study is the zinnia (we’ll get back to those dandelions!).
As with the dandelion, a zinnia “flower” is really a tight grouping of smaller flowers—the whole cluster is termed a “head.” Figure 1 depicts the flower head of a zinnia as seen from the top.
A zinnia head has two different types of small flowers: in the center are yellow, tubular five-lobed flowers called “disc flowers.” Arranged radially around the center of the head are numerous small flowers that look like petals. In a zinnia, these outer small flowers—termed “ray flowers”—may also be yellow although they are commonly red, orange or pink. A close-up, side-by-side view of a ray flower (Figure 2, top) and a disc flower (Figure 2, bottom) from a zinnia head shows the contrast between these two types.
There is quite a bit of variation within the composite family. In zinnias, all of the flowers in the head are fertile; upon pollination and subsequent fertilization, each small flower will form its own seedlike fruit. In sunflowers only the disc flowers are fertile, the familiar black-and-white sunflower “seeds” forming in the central portion of the flower head (the outer yellow ray flowers in a sunflower are sterile and function simply to attract pollinating insects).
Although most members of the composite family have heads with both disc and ray flowers (such as the aster in Figure 3, which has yellow disc flowers and violet-blue rays), some composite species have only disc flowers. The latter is the case with ironweed (Figure 4), where the head is made up entirely of tubular disc flowers.
In the head of a dandelion, all of the individual yellow flowers are raylike (Figure 5), and each small flower will give rise to a seedlike fruit topped with a fluffy tuft of hairs that can launch the propagule into the breeze (Figure 6).
Over 250 species of composites—both native and introduced—are found in Ohio, including yarrow, thistles, tickseeds, coneflowers, fleabanes, blazing-stars, chicory, & sneezeweed. Various composites are in bloom throughout the growing season, although they are most conspicuous in the autumn when fields become awash in the fall colors of goldenrods, sunflowers and asters.