26.3A: Angiosperm Flowers
- Describe the main parts of a flower and their purposes
Flowers
Flowers are modified leaves, or sporophylls, organized around a central stalk. Although they vary greatly in appearance, all flowers contain the same structures: sepals, petals, carpels, and stamens. The peduncle attaches the flower to the plant. A whorl of sepals (collectively called the calyx) is located at the base of the peduncle and encloses the unopened floral bud. Sepals are usually photosynthetic organs, although there are some exceptions. For example, the corolla in lilies and tulips consists of three sepals and three petals that look virtually identical. Petals, collectively the corolla, are located inside the whorl of sepals and often display vivid colors to attract pollinators. Flowers pollinated by wind are usually small, feathery, and visually inconspicuous. Sepals and petals together form the perianth. The sexual organs (carpels and stamens) are located at the center of the flower.
Styles, stigmas, and ovules constitute the female organ: the gynoecium or carpel. Flower structure is very diverse. Carpels may be singular, multiple, or fused. Multiple fused carpels comprise a pistil. The megaspores and the female gametophytes are produced and protected by the thick tissues of the carpel. A long, thin structure called a style leads from the sticky stigma, where pollen is deposited, to the ovary, enclosed in the carpel. The ovary houses one or more ovules, each of which will develop into a seed upon fertilization. The male reproductive organs, the stamens (collectively called the androecium), surround the central carpel. Stamens are composed of a thin stalk called a filament and a sac-like structure called the anther. The filament supports the anther, where the microspores are produced by meiosis and develop into pollen grains.
Key Points
- Sepals, petals, carpels, and stamens are structures found in all flowers.
- To attract pollinators, petals usually exhibit vibrant colors; however, plants that depend on wind pollination contain flowers that are small and light.
- Carpels protect the female gametophytes and megaspores.
- The stigma is the structure where pollen is deposited and is connected to the ovary through the style.
- The anther, which comprises the stamen, is the site of microspore production and their development into pollen.
Key Terms
- sepal : a part of an angiosperm, and one of the component parts of the calyx; collectively the sepals are called the calyx (plural calyces), the outermost whorl of parts that form a flower
- corolla : an outermost-but-one whorl of a flower, composed of petals, when it is not the same in appearance as the outermost whorl (the calyx); it usually comprises the petal, which may be fused
- stamen : in flowering plants, the structure in a flower that produces pollen, typically consisting of an anther and a filament
- carpel : one of the individual female reproductive organs in a flower composed of an ovary, a style, and a stigma; also known as the gynoecium