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Determine if the Head of a Sunflower Is Made of Complete or Incomplete Flowers

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Author: Janice VanCleave

What You Need to Know

A complete flower is one that has sepals, petals, stamens, and pistils. The pistil is the female part of a flower and the stamen is the male part. Petals are leaf-like structures, often brightly colored, which surround and protect the pistil and stamen. Sepals are leaf-like structures, usually green, that surround and protect the flower before it opens. An incomplete flower is one that lacks one of the four basic flower parts. A special petal-like leaf is called a bract. A composite flower is made of many separate flowers that together are called a head.

How Do Composite Flowers Work?

Composite flowers are one of the largest groups of flowering plants. Each composite flower, such as the sunflower, the daisy, the goldenrod, and the dandelion, looks like a single flower, but each one is really many separate flowers grouped together into the flower head. The head of many composite flowers is made of two kinds of flowers. In the center of the head are disk flowers, and around the disk flowers are the ray flowers, which often look like petals. The heads of some composite flowers, such as dandelions, are made of groups of ray flowers only, and some are groups of disk flowers only.

What Does This Have to Do with a Flower Being Incomplete or Complete?

A complete flower, such as a rose, has the four parts: sepals, petals, stamens, and a pistil. The poinsettia, on the other hand, is an incomplete flower because it lacks petals. What look like bright red petals are actually bracts. The dogwood is a special flower. What look like white leaves are bracts, but the flower is complete because each flower has its own tiny petals, sepals, stamens, and pistil.

What Does This Have to Do with a Flower Being Incomplete or Complete?

Fun Fact

The largest flower in the world, the Rafflesia arnoldi, grows in Indonesia. It weighs up to 15 pounds (7 kg), and its petals grow to 1.7 feet (0.5 m) long and are 1 inch (2.5 cm) thick. The world's smallest flower is the Wolffia. A bouquet of a dozen Wolffia blooms would easily fit on the head of a pin. A type of duckweed, Wolffia grows on the surfaces of ponds and slow-moving streams.

Real-Life Science Challenge

Poinsettias are popular flowers that are sold during the Christmas season. People who grow these plants need them to bloom at a certain time of the year. Scientists have helped solve this problem by studying what makes a flower bloom. One of the main factors influencing when a flower blooms is the length of time it is exposed to daylight. Poinsettias are short day plants, meaning they require short periods of daylight and long periods of night in order to prepare for flowering. If the plants are kept in greenhouses where the lights are on for part of the night, the plants' natural blooming schedule is delayed. When growers are ready for the plants to bloom, they turn off the lights, and the plants bloom naturally.

Experiment

Now, start experimenting with ways to study the different flowers that make up the head of a sunflower.

Hints

  • Create a way to separate the flowers in the head of the sunflower without destroying their structure.
  • Try using a magnifying lens to identify the parts of the separate flowers.
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