Composite Flowers: A Guide to Understanding Their Unique Beauty

Maximilian Sunflower, HELIANTHUS MAXIMILIANI

Of all the world’s flowering-plant families, the Orchid and Composite Families are the two largest and most diverse. It’s a matter of debate which family takes the crown as the biggest. These families are not only expansive but also considered among the most “modern” and “successful” due to the sheer number of species they contain. The Composite Family, in particular, boasts around 23,000 recognized species worldwide, with 2,413 found in North America alone.

The Huge Composite Family

If you have a typical backyard garden, chances are you’ll find some well-known Composite or Aster Family members flourishing among your plants. These include chicory, dandelion, chrysanthemum, yarrow, coreopsis, sunflower, Spanish needle, dahlia, zinnia, goldenrod, fleabane, aster, sneezeweed, groundsel, eupatorium, ageratum, lettuce, thistle, ironweed, cosmos, and Black-eyed Susan.

To truly appreciate the beauty of composite blossoms, you need to familiarize yourself with their unique flower structure.

Composite Family Flower Structure

When examining a Composite Family flowering head, such as that of the Cowpen Daisy (Verbesina enceliodes), you’ll notice two views: a top view and a bottom view. However, these views don’t represent one flower but are actually two views of flowering heads composed of multiple flowers called florets. Technically, these flowering heads are referred to as “capitula.”

There are two types of florets in the Composite Family: disc flowers and ray flowers. Disc flowers have cylindrical corollas, while ray flowers have flat, blade-like ones. These florets are surrounded by a collection of modified leaves, known as bracts, which form the involucre. Each individual bract is called a phyllary. Identifying the species within the Composite Family often relies on examining the different forms and sizes of these bracts.

To better understand disc and ray flowers, take a look at a marigold blossom. You’ll notice how the ray flowers surround the disc flowers. Another unique feature of the Composite Family is the presence of cypselae, which are special one-seeded fruits stacked neatly on top of the receptacle. In their immature state, cypselae are white but turn black as they mature.

Three Kinds of Composite Flower

When identifying a composite flower, the first step is to determine which of the three basic flower-head types you have:

  1. Heads composed of only ray flowers, like dandelions, chicory, endive, and wild lettuce.
  2. Heads composed of only disc flowers, like eupatorium, ageratum, thistles, and burdock.
  3. Heads composed of both disc and ray flowers, with disc flowers tightly packed in the center and enlarged ray flowers radiating outward. Sunflowers, asters, Black-eyed Susans, chrysanthemums, dahlias, and zinnias fall into this category.

Floret Structure

Once you grasp the basics of composite blossoms, it’s time to examine a flowering head up close. Remove a tiny disc or ray floret and observe its structure. Each floret contains stamens and a pistil. In disc and ray florets, the stamens are fused together by their anthers, forming a cylinder around the style. The pistil, comprising stigma, style, and ovary, extends from the base. The style branches bear stigmatic papillae, where pollen grains germinate.

Composite Family ovaries are “inferior,” situated below the corolla and sexual parts. As the pistils develop into fruits, the ovary matures into a unique dry fruit called a cypsela, also known as an achene. Cypselae are one-seeded fruits that do not split open during seed dispersal. They are commonly found in sunflowers, often mistaken for seeds.

Pappuses

On top of many cypselae, instead of sepals, you’ll find special projections called pappuses or pappi. These pappuses come in various forms, including hairlike bristles or awns, cup-like crowns, or feathery bristles. The pappus acts as a parachute, helping the mature cypsela be dispersed by the wind or cling to animal fur.

Identifying composite blossoms requires close attention to the pappus, as different species within the family possess unique pappus variations.

Paleae

One more feature that can aid in identifying Composite or Aster Family members is the presence of paleae. These thin, papery, scale-like structures wrap around the bottom of each floret in some genera but not others. Paleae are sometimes referred to as “chaff” and add a distinctive touch to the overall appearance of the flower.

The Composite Family showcases unparalleled diversity and beauty. Whether you’re a backyard naturalist or a flower enthusiast, understanding the unique structure and characteristics of composite flowers will deepen your appreciation for their extraordinary beauty and complexity.

Sunflower Goldeneye, VIGUIERA DENTATA, paleae or scales around achenes
Dandelion, TARAXACUM OFFICINALE, close-up of broken open flower head