YOUR DAILY DOSE OF BOTANY – SEPTEMBER 2012
Composite Flowers
by Scott Namestnik, snamestnik@orbisec.com
The composites (members of the family Asteraceae, formerly Compositae) are an astonishing group of plants. In addition to having great economical value because of their use as foods such as lettuce, artichokes, and sunflower-based products, composites are amazing because of their highly evolved reproductive structures. When many people think of a typical daisy “flower,” they think of white “petals” around a yellow “center.” However, daisies are composites, and those white structures are actually individual flowers themselves. So what is the yellow “center?” That, too, is a cluster of flowers! Using the daisy as our example, the yellow structures in the center of the flower head are what are known as disc florets, flowers that are radially symmetrical (meaning that two or more longitudinal sections of the flowers will look like mirror images). The white structures in our example are known as ray florets, each with a tubular base and flattened strap-like fused petals that form the ray. Thus, the daisy “flower” is not just a flower but a “composite” of flowers clustered together on a receptacle (a platform formed by the expanded upper part of the flower stalk) to mimic a single flower. The composites are also a diverse bunch, with some species having both ray and disc florets (like the daisy), some having only ray florets (such as chicory [Cichorium intybus]), and some having only disc florets (such as blazing star [Liatris spp.]).
Although composites can be found in flower year-round (yes, even in the depths of winter, you can find a blooming common dandelion [Taraxacum officinale] if you look hard enough), many, such as asters (Aster spp.), goldenrods (Solidago spp.), and sunflowers (Helianthus spp.) are coming into glory right now. Get out there soon to see the magnificent composite floral structures of plants in the family Asteraceae!
Flower head of prairie dock (Silphium terebinthinaceum), showing radially symmetrical disc florets and strap-like ray florets with fused petals.
If you have a question about plant terminology or morphology that you would like answered in a future edition of this column, send me an email at snamestnik@orbisec.com. I may not be able to address all requests given the space allotted for this column, but I will answer those that I can.
