The Science of Botanical Art

The Aster Family

By Dick Rauh

Originally appeared in The Botanical Artist, Volume 13 – Issue 4


I have written about the orchid family, generally acclaimed as the largest in the world (22000-35000 species). The family purported to run a close second has a much more plebeian reputation and is estimated to run to about 19000-23000 spp. You notice that I am being cagey about the numbers here, because depending on which source you rely on, there seems to be some disagreement, or at least a very wide range of estimated numbers. Suffice it to say both these families are extremely successful, if one counts success on the number of species. 

Asteraceae is largely temperate or subtropical, preferring drier habitat and is far more down to earth. Ask a child to draw a flower, and nine out of ten times you will get a daisy. The problem of course, is that the daisy is not a flower at all, but a collection of many small flowers or florets, gathered together to look like a solitary bloom. It is known as a pseudanthium, a false flower, and that’s exactly what it is. The earlier name for the family, Compositae, which describes this arrangement, is still accepted as an alternate, and in England it supercedes the World Congress preference of Asteraceae. This means that if you are submitting a painting of a member of this family for consideration in a Royal Hort Show, you’d better call it Compositae! It is the inflorescence that gives this huge family its common composite character and makes it a “natural group.” 

The head or capitulum consists of two distinctly different types of flowers. The ones that look like petals are ray or ligulate flowers. I repeat, they are not petals, they are individual flowers. They are irregular, with a flat corolla of three to five lobes that folds at the top of the inferior ovary. Tucked into this fold, and visible depending on its age, is the style with two stigma arms opening in graceful curves. Ray flowers are mostly pistillate, sometimes sterile, in the typical daisy form. There are some genera that consist of only ray flowers. Chicory and dandelions are examples, and in these cases the ray flower, while still remaining zygomorphic, is perfect containing both the stamens and pistils. Sepals in this family are absent, although many scientists insist that they have become modified into chaff; bristles, or hairs that we call pappus. Personally I find the irregularity and randomness of these forms where the sepals should be indicates that this series is missing, for all intents and purposes, and in many genera it is indeed absent. 

In the center of the flower, disguised as stamens, are the tube or disc flowers. They are regular and perfect, lacking only sepals (my prejudice) and sometimes with pappus. The arrangement of the disc flowers follows the double spiral of the Fibonacci sequence. It is obvious in the large sunflowers, but you can find it, perhaps less obviously showing, in cone flowers, cosmos and chrysanthemums. As with the composites that consist solely of ray flowers, there are genera made up solely of disc or tube flowers. The thistles are examples of this variation. 

Looking like a calyx are arrangements of bracts or phyllaries. The resulting structure has the name of involucre. Remember, a calyx is by definition one of the four organs that makes up a flower. This pseudanthium is made up of hundreds of flowers, and so this has, if you will, a false calyx made up of bracts. These are largely green and photosynthetic and are often overlapping and imbricate. The spiky or fringed bract forms in the involucres of thistles and centaureas are fascinating in themselves. They are well worth detailing and emphasizing when rendering these genera. 

The vegetative form and habits of this family are extremely variable, so that it is indeed the flower heads that produce the common character. Although even there, the huge number of the species have heads or capitula that range from the huge daisy like flowers of the Helianthus to the disc flower buttons of pussytoes, and the branching heads of goldenrods. 

The fruit of this family is an achene with attached pappus called a cypsela. An achene is a dried fruit with a hard or leathery pericarp and usually a single seed attached at one point inside. Here is where the “modified sepals” finally come into their own. In many cases the silvery hairs of the pappus are attached directly to the top of the achene, and act to catch the wind to disperse the seeds. In some cases there is a slender single thread that leads from the fruit to the cluster of hairs above and the ‘beaked’ pappus looks a bit like a parachute and proves an even more effective means of floating the fruit away from its mother plant. The unbroken seed heads of some of these genera are things of beauty on their own. Wind is not the only vector the family employs to disperse its seeds. Some of the pappus is made up of small hooks that attach to fur or clothing and thus uses mammals (including us) to spread around the species. 

A word of caution about American asters. The common name aster is still viable, but the Latin genera are all over the place. Even though this is the genus that gives the family its name, the latest taxonomic thinking places all our familiar species in many new genera, and the Aster genus is relegated to Old World species. The New England aster is now Symphyotrichum novae-angliae, and the common white heart-leaved aster is Eurybia divaricata. Note that it is generally the genus that changes, the old specific epithet hangs on. The new Wildflowers in the Field and Forest by Steve Clemants and Carol Gracie is a wonderful and up-to-date guide to all the aster name changes. 

  • Aster, watercolor, ©Dick Rauh 1989