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COMPOSITE FLOWERS
(Blossoms of the Composite or Aster Family, the Asteraceae)

SunflowerOf all the world's flowering-plant families, the two with the most species are the Orchid and Composite Families. It's debated which of these has the biggest family.

These big families are also considered among the most "modern," or recently evolved, as well as some of the most "successful," in terms of being commonly encountered. Probably it surprises you  that orchids make this list. If you visit the tropics and see how many orchid species occupy really specialized habitats, you'll understand.

About 23,000 Composite Family species are recognized. The online Flora of North America includes 2413 species found just in North America.

THE HUGE COMPOSITE FAMILY

IDENTIFICATION KEYS IN
FLORA OF NORTH AMERICA

If you can handle the Composite Family's specialized terms and concepts you can "key out" your unknown finds in the Asteraceae section of the Flora of North America site. Choose a key among those near the bottom of the long page you arrive at when you click the above link.

Here are some of the best-known Composite Family members that might appear in a typical backyard garden: 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.

In other words, if you're going to understand all the blossoms you encounter as a backyard naturalist, at some time or another you must come to terms with the composites' very unique flowers.

COMPOSITE FAMILY FLOWER STRUCTURE

ray and disc flowers, and involucre

Above you see a typical Composite Family flowering head, top view at the left, bottom at the right. The big thing about Composite Family flower structure is that the head brings together several to very many tiny flowers, or florets, so that they look like just one larger flower. The two flower types are disc flowers and ray flowers. Disc flowers form the flower head's "eye," while ray flowers look like petals on a simpler flower. Beneath the flower head there is a collection of modified leaves, or bracts, forming the involucre, in the above picture seen at the right. Each individual  bract is technically known as a phyllary (FIL-uh-ree).

Involucral bracts or phyllaries of Chrysanthemum flowerAt the right you see phyllaries of a Chrysanthemum flower, which are somewhat different.  When you are identifying species in the Composite Family, noticing the bracts is very important. You might enjoy looking over our page examining the varied the world of Composite Family involucre bracts.

Marigold from my gardenTo give you some practice with disc and ray flowers, at the left you see a marigold blossom plucked from my garden and broke apart. Notice how the ray flowers surround the disc flowers.

Also, note the cypselae (SIP-suh-lee), which are special kinds of one-seeded fruits found in the Composite Family (singular cypsela, pronounced SIP-suh-luh). The cypselae are neatly stacked beside one another atop the low, conical, platform-like receptacle. The cypselae at the very top of the receptacle are white because they are still immature. Later they will turn black.

THE THREE KINDS OF COMPOSITE FLOWER

When you begin trying to figure out what kind of composite flower you have, the first thing you should notice is how the heads are configured, in terms of disk and ray flowers. Here are the three basic flower-head strategies:

  • ray flowers and disk flowers in composit blossomsHeads composed of only ray flowers, as in dandelion (shown at the right), chicory, endive, and wild lettuce.
  • Heads composed of only disc flowers, as in eupatorium (the white flowers at the right), ageratum, thistles, and burdock.
  • Heads composed of both disc and ray flowers, with disc flowers tightly packed together in the head's "eye," while enlarged ray flowers function as petals radiating outward from the eye. Species in this group include sunflowers, asters, Black-eyed Susans, chrysanthemums, dahlias, and the red zinnia pictured at the right. In the zinnia, the yellow, five-lobed disc flowers in the head's center are clearly visible, surrounded by red ray flowers, which most people would incorrectly call "petals."

ANALYZING DISK & RAY FLOWER STRUCTURE

Once composite-blossom basics are understood, you should seek out a composite flower, remove a tiny disk or ray flower from the head, and see if you can find the flower's basic parts. If you don't understand terms like style and stamen, you may want to consult our Standard Blossom Page  as you read through the following. Here are a couple of composite-family peculiarities to keep in mind:

  • STAMENS: Stamens in disk and ray flowers number four or five, and are fused together by their anthers to form a cylinder around the style.
  • PISTILS: In each flower type a thread-like style passes up through the anther-cylinder to two long style branches projecting above the blossom. The ovary is situated below the point of attachment for sepals and petals, not in the center of them as with the Standard Blossom. Technically such an ovary is referred to as being inferior.

Pistils develop into fruits. The Composite Family's inferior ovary transforms into a unique kind of fruit called a cypsela (SIP-suh-luh). In other words, technically speaking, sunflower "seeds" are actually one-seeded fruits of the cypsela type. Of course in standard English, speaking with people whose minds are on other things than botany, just let people call sunflower fruits "seeds." By the way, in older books, cypsela are referred to as "achenes," but nowadays the two fruit types are considered a little different.

PAPPUSES

pappus atop an achene of a EupatoriumAtop the achenes of many members of the Composite Family sepals have become reduced to special projections known collectively as the pappus. At the bottom of the picture at the right you see the achene of a Eupatorium plant. Atop the achene, the slender, radiating items are the pappus. Later the pappuses on these mature achenes will serve as parachutes, enabling the achenes to be blown by the wind into new territory. The purple item in the picture's center is the disk flower.

Pappuses come in an amazing variety of forms, not just the hairlike bristles shown at the right.  Below you see achenes of one of several species of "beggar-ticks" or "sticktights," Bidens bipinnata. On these achenes the pappus bristles have stiffened into barbs that can stick into animal fur (or hikers' pants), and thus travel into new territory to be planted.awned achenes of Bidens bipinnata

Echinacea pappuses are cup-like crowns. Sunflower pappuses consist of two sharp scales that fall off when the achene is mature. Common thistles have feathery bristles united into a ring at the base. Many flowers have no pappuses at all.

On and on pappus variations go, typically all the members of a genus possessing similar types. Needless to say, when identifying composite blossoms, the pappus is one of the most important features to note.

BOOKS ABOUT COMPOSITES:

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Conrad, Jim. Last updated . Page title: . Retrieved from The Backyard Nature Website at .