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23.5: Anatomy of a Flower

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  • Flowers are composed of sets of highly modified leaves arranged in whorls. The outermost whorl of a flower is called the calyx and is composed of sepals. Inside the calyx is the corolla, which is composed of petals. The sepals are often smaller and less colorful than the petals, but this general rule can be misleading. For example, lilies often have identical sepals and petals. The only way you can distinguish between them is by location: Which whorl is on the outside?

    Together, the calyx and corolla are called the perianth (peri- meaning around, anth- meaning flower). Inside the perianth is the androecium (house of man), a whorl composed of stamens. Each stamen has a long filament holding up pollen sacs called anthers. Inside the androecium is the gynoecium (house of woman), which is composed of carpels. Each carpel has an ovary at the base where ovules are housed. The style emerges from the ovary and is topped by the stigma. Pollen grains land on the stigma and must grow a tube down the style to reach the ovule and complete fertilization.

    All of these whorls attach to an area called the receptacle, which is at the end of the stem that leads to the flower. This stem is called the peduncle. In the case of an inflorescence, where multiple florets are produced in place of a single flower, the stems leading to the florets are called pedicels.

    In the diagram of the flower below, add labels for all of the bolded terms above and assign each whorl a different color. Make a key for the colors and whorls.

    Figure \(\PageIndex{1}\): Floral Structure

    Symmetry and Quantity

    Two other features used to identify flowers are symmetry and the number of parts in each whorl.

    Flowers that have multiple lines of symmetry (like a starfish) are radially symmetrical, also called actinomorphic. Flowers with only a single line of symmetry (like you) are bilaterally symmetrical, also called zygomorphic.

    Figure \(\PageIndex{2}\): Flowers and symmetry

    Flowers with parts in sets of 3 are generally monocots. Flowers with parts in sets of 4 or 5 are generally eudicots. Sometimes these parts are fused together and can be difficult to count. For example, in the diagram of the lily above, there are three fused carpels. You would only be able to determine this by counting the lobes on the stigma or by looking at a cross section of the ovary to count the different compartments (called locules).

    Notes on other terms

    Many wind pollinated flowers have evolved to be either male or female, containing either an androecium or gynoecium, but not both. These flowers are called imperfect, while flowers containing both internal whorls are called perfect. Flowers that contain all whorls are called complete. However, there are also incomplete flowers that have lost other whorls during the course of their evolution. Wild ginger, Asarum caudatum (shown below), has lost its corolla and has a large showy calyx in its place. This strange plant has fly pollinated flowers and ant dispersed seeds.

    Figure \(\PageIndex{3}\): Asarum caudatum

    Dissecting a Flower

    Obtain a fresh flower specimen. Dissect a flower and carefully remove the individual pieces. Draw (or assemble and press) the individual components onto the following table. Label the parts of the stamen and carpel.

    Table \(\PageIndex{1}\): Dissected Floral Whorls

    Drawing with labels:







    Fusion? (yes/no)

    Is this flower a monocot or a eudicot? How can you tell?

    How would you describe the symmetry of this flower?

    What other terms could you use to describe it?

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