18.4–18.6

Flowers have several roles in plant reproduction.

A water lily flower.

Take a look at FIGURE 18-8, which showcases the diversity of flower shapes and colors. If we investigated more closely, though, and dissected some flowers, we’d see that with only a few exceptions, all flowers have the same fundamental structures. The consistency we find there brings order to the seemingly infinite array of flower forms.

Figure 18-8The captivating and varied colors, textures, and shapes of flowers.

Specifically, flowers have four distinct parts (FIGURE 18-9).

Figure 18-9Most flowers are organized on the same general body plan.

1. Sepals. These leaf-like structures are located at the point where the flower is connected to a main support structure of the plant, such as a branch, stem, or stalk. Sepals grow in a ring surrounding the outside of the flower, and they protect the flower bud during its development. Although sepals are usually green, in some flowers they are brightly colored and closely resemble the petals.

2. Petals. Just inside the ring of sepals is another ring of leaf-like structures, the petals. They are usually brightly colored—helping the flower to attract pollinators—and frequently have unusual shapes, from long and thin to short and broad.

Figure 18-10Saffron: the world’s most expensive spice.

3. Stamens. Moving inward toward the center of the flower, the next structures are the stamens, the male reproductive parts. There are usually several stamens, each of which appears like a head on top of a long, thin stalk. The stalk is called the filament and the head-like top structure is called the anther. The anthers are the sites where the pollen grains—the structures that contain the male gametes, or sperm—are produced. The typical anther on a cornflower produces about 3,000 pollen grains.

4. Carpel. In the very center of the flower is the carpel, the female reproductive structure. There is usually only one carpel, and it is usually shaped like a long-necked vase. At the very top of the carpel is a flat, sticky surface called the stigma, which functions as a landing pad for pollen. The stigma is held high, out of the center of the flower, by the style, a long thin structure that leads down to an enclosed chamber, the ovary. Within the ovary are one or more ovules, which produce the female gametes (eggs). After an egg is fertilized, the ovule develops into a seed.

The prize for the most valuable stigma probably goes to Crocus sativus, a flowering plant native to Southwest Asia that is more commonly known as the saffron crocus (FIGURE 18-10). Collected and dried, the stigma and style produce a deep yellow powder, valued by food lovers for its taste, sometimes described as slightly bitter and earthy. Common in Persian and Indian dishes, saffron is the world’s most valuable spice: a pound (requiring the stigma and style from about 75,000 flowers) sells for more than $2,000.

Not all flowers have all four structures described above. We’ll see later in this chapter that some plants produce flowers that have only male or only female reproductive parts. Still others may lack sepals or petals.