Plant Reproduction
What Are the Reproductive Structures in Plants?
The Reproductive Organs of Flowering Plants
Flowers are the reproductive organs of flowering plants. Flowers are shoots modified to produce gametophytes that will, in turn, produce gametes. At some time in the sporophyteglossary term (opens in a new window) stage of a flowering plant’s life cycle, the plant develops flowers. The production of flowers is usually determined by the length of daylight. This phenomenon is called photoperiodism. Plants that bloom in spring or fall need short days to flower. These are called short-day plants. Plants that bloom in the long days of summer are called long-day plants. Some plants bloom in response to other factors. It is possible to control when some plants flower using artificial lighting to adjust the photoperiod to fit the plant. Flowering is initiated within the plant by the hormone florigen.
Flowers generally have four parts: stamens, carpels, petals, and sepals. The flowers of some species are missing some of these parts.
Stamens
The male organ of a flower is called a stamenglossary term (opens in a new window). Stamens produce pollenglossary term (opens in a new window) grains. Pollen grains are the gametophyteglossary term (opens in a new window) (1n) generation of flowering plants. A stamen is composed of a filament and an anther. The filament is a thin stalk. The anther, a small, bean-shaped structure, is attached to the top of the filament. The anther produces and stores the pollen. The pollen is produced by meiosis. A mature anther contains thousands of pollen grains. A single mature pollen grain contains two cells. One of these cells will later divide to make sperm. The other cell becomes the tube cell. The tube cell is involved in the fertilization process.
Carpels
The basic female organ of a flower is called the carpelglossary term (opens in a new window). The carpel has three parts: a style, stigma, and ovary. The style is a long, tube-like structure leading to the ovary. The style is tipped by the stigma, where pollen is captured. The stigma has a sticky surface to hold pollen that lands on it. The surface contains substances that stimulate pollen grains from the same species to form pollen tubes. Carpels may be single or combined to form a pistil.
The ovary is where the female gametophytes are produced. They are produced in structures called ovules. Typically, an ovary will contain many ovules. Meiosis occurs inside the diploid ovule, producing four haploid megaspores (1n). Only one of these divides by mitosis to produce the female gametophyte, called an embryo sac. An egg (1n) develops inside the embryo sac.
Teacher Note: Misconception
Students may assume that plants are either male or female. In fact, in most plants, the male and female sexual organs develop on the same plant.
Petals and Sepals
Petals are often the showy part of the flower. Many flowers have brightly colored and scented petals that have evolved to attract pollinators. Flowering plants have coevolved their flowers with specific pollinating species. As a result, the color of a plant's flowers is specifically targeted at the spectrum that is visible to the pollinator species. Bees are attracted to yellow and see well in the blue to ultraviolet part of the spectrum. Flowers that use bees as pollinators are therefore commonly yellow, blue, or purple. Their petals are commonly marked with lines that are only visible in ultraviolet and are invisible to humans. Hummingbirds are attracted to red flowers. This is why most hummingbird feeders are red.
Petals are usually arranged in a circular formation called a corolla. The petals that make up a corolla can be fairly uniform or vary widely in their size and shape. The size and shape of the petals is an evolutionary mechanism for ensuring pollination. Petals also protect the plant’s reproductive organs. Nectaries are often found at the base of petals. Nectaries produce a sugary solution called nectar that attracts pollinators to the flower.
Plants that are wind pollinated, such as grasses and many trees, do not need to attract pollinators. Their petals are usually small and green, or may be absent altogether. They do not produce nectar.
Sepals are small, leaf-like structures that surround and protect the flower bud before it opens. When the flower opens, they may fall off or form a ring around the base of the opened petals called a calyx.
