Aster? I hardly know 'er!

The flowers of the plants in the family Asteraceae are instantly recognizable as the many sunflowers, daisies, black-eyed susans, and dandelions that populate the world around us. The characteristic that distinguishes all of the plants in this family is their composite blossoms, which consist of numerous tiny flowers (florets) clustering to create the illusion a single large one. The family is especially prominent in the Rocky Mountains and in the Sierra Nevada Mountains(1), suggesting that perhaps something about the Asteraceae's adaptive features is particularly well-suited to alpine environments.

This same adaptive feature can make flowers in this family somewhat difficult to identify, as many of their identifying structures are quite small or not intuitively examined in the field. This has earned the yellow members of the family the affectionate moniker Damned Yellow Composites (DYC's), as wildflower enthusiasts despair of figuring out fine distinctions buried deep in dichotomous keys. I do not think that there is a similar acronym for the purple or white Asteraceae which grow so well here in Colorado, but they certainly can be very confusing. Fortunately all that is needed is a little leg work to learn the particulars of Asteraceae anatomy, and most of the flowers can be readily identified. The usual field characters such as leaf shape, environment, etc. are also useful, but with these plants I've found a slightly deeper dive to be helpful.

Helianthus pumilus (Little Sunflower) is a classic DYC. It closely resembles a number of 

related species also blooming in the Spring on North Table Mountain.

Anatomy of a Composite Flower

Basically, the blooms of the Asteraceae are many, many flowers arranged on a pedestal which in turn is embraced by distinctive bracts called phyllaries. Individual plants, in turn, may have as few as one or as many as a multitude of these blossoms. I've posted some illustrations of a common foothills flower, Senecio integerrimus below to enumerate the different parts, followed by some pictures of actual flowers. This section was compiled with information from the Colorado Flora, Eastern Slope(2) and from a very handy guide to plant anatomy terminology(3).

A cutaway view of a flower head as seen in S. integerrimus (click to enlarge).

• Disc and ray florets make up the head of the Asteraceae flower. Disc florets are the smaller, ligule-less florets in the middle, while ray florets have long extrusions called ligules which appear as regular petals from above. In a common daisy, the yellow area consists of disc florets while the white "petals" are ray florets (see illustration below).
• Phyllaries are bracts peculiar to Asteraceae which create the involucre, or whorl of bracts which subtend the flowers. The characteristics of the phyllaries are invaluable for identifying Asteraceae species. A familiar example of phyllaries is the "petals" of an artichoke, although referring to them as phyllaries in a culinary context strikes me as unwieldy.
• The receptacle is the pitted disc in which the florets are ensconced. I haven't noted that it's particularly helpful for identification, but it is peculiar to this family. A familiar example is the nub at the top of the stem which is left over after all of a dandelion's seeds have been blown into the wind.

The two types of florets as seen in S. integerrimus (click to enlarge).

• The ligule is only present in the ray floret, and extends from the corolla (petals and sepals). In the disc floret, a united or near-united corolla is common, but never a ligule. The ligule is the "petal" of a sunflower, daisy, or dandelion. 
• The anthers and the style are the male and female reproductive organs of each floret, respectively. They may be helpful for identification, but they present somewhat unusually in Asteraceae. The styles always come in pairs and can often be seen peeking above the surface of a composite flower like hundreds of tiny antennae. The anthers are fused or united into a tube around the style. Male and female structures are not always both present within a given composite flower.
• The pappus is a fluffy, modified calyx which appears as a sheathing poof of hairs bellow the corolla. The pappus may be absent in some species. When it is present it may be composed of simple (unbranched) hairs, or plumose (feather-like) hairs. A familiar pappus is that which crowns dandelion's seeds after the flower has bloomed; it is the fluffy parasol which allows the seeds to become airborne. 
• The achene is a more precise term for the seed. It indicates that there is but a single ovule contained within a single locule, and the ovule is attached to the ovary wall at only one point. Any differences between achenes are going to be best observed with magnification. I have not had to use seed differences for any Asteraceae identification, but I imagine under the right circumstances they could be helpful.  

Going Full Montanoa: Pictures of Bare Asteraceae Anatomy

I don't have photographs to illustrate all the variations that occur between all the structures in flowers in this family, but in my experience the most useful field characteristics are the properties of the phyllaries and the absence or presence of ray vs. disc florets. These are what I have added below:

This Carduus nutans (Musk Thistle) flower head is not yet even showing any 

florets; all the scale-like bracts in this picture are rank upon rank of phyllaries. I think the family resemblance to artichokes is especially evident here. 

C. nutans is listed as an invasive weed in Colorado.

Grindelia squarrosa, the Curleycup Gumweed, has reflexed (down-curving) phyllaries (foreground). Some species are distinguished by characteristics as fine as whether or not their phyllaries curve down. G. squarrosa is a common high planes and lower foothills species which blooms late in the season.

Tonestus pygmaeus (Pigmy Serpentweed) has both ray and disc florets. This minute 

species was found at high elevation on the slopes of Mt. Evans.

Tonestus pygmaeus (Mountain Dandelion) has only ligulate florets, similar to the 

common dandelion. This flower, seen at around 9500 ft on Pikes Peak, is one of 

few flowers I've seen in CO which is truly a brilliant orange.

Brickellia grandiflora (Large-flowered Bricklebush) has only disc flowers (click to enlarge). 

You can also see the pappi of older florets as well. This flower is another late bloomer, 

this photo was taken a couple days before September in the mountains near the Devil's Head fire lookout.

Thanks for checking out this quick guide to damn (pick a color) composites! When Spring rolls around I hope those of you who have been troubled by DYC's in the past find this information helpful.

References

1. Kartesz, J.T., The Biota of North America Program (BONAP). (2015). Taxonomic Data Center. (http://www.bonap.net/tdc). Chapel Hill, N.C. [maps generated from Kartesz, J.T. 2015. Floristic Synthesis of North America, Version 1.0. Biota of North America Program (BONAP). (in press)] Retrieved from http://bonap.org/2015_SpecialtyMaps/Density%20Gradient%202015/Density%20Gradient%202015.html 
2. Weber, W., & Wittman, R. (2012). Colorado Flora: Eastern Slope. Boulder, Colorado: University Press of Colorado.
3. Harris, G., & Harris, M. W. (1997). Plant Identification Terminology: An Illustrated Glossary. Payson, Utah: Spring Lake Publishing.