Flowering Plants & Pollen Grain

Pollen grains are vital in the reproductive process.

There are more than 250,000 identified flowering plants in the world. Flowering plants, also classified as angiosperms, make up about 90 percent of the Kingdom of Plantae. They can be found in various aquatic and dry habitats. Flowering plants come in a wide variety of shapes and sizes but they can all produce flowers that, in turn, create pollen grains that are vital in the reproductive process.

Flowers

Flowers are the sexual reproductive component of flowering plants. Flowers can be unisexual or bisexual. Sexuality is determined by the presence of the stamen, which is the male sex organ, and the pistil, which is the female sex organ. Flowering plants can also have both separate male and female flowers (monoecious) or can have either female and male flowers within a population (dioecious). Ninety percent of flowering plants produce bisexual flowers.

Parts

Flowers can have four major parts. These are the petals, sepals, stamens and pistils. The pistil is composed of the main reproductive parts of the flower. These parts include the stigma, style and ovary. Located on the uppermost part of the pistil is the stigma. This is where the pollen lands and germinates. The style is the long tube above the ovary and below the stigma. The style separates the stigma and the ovary. The ovules, also called embryo sacs, contain the female gametophyte. These ovules are located inside the flower's ovary; only one ovule can develop into a seed.

Pollen Grain

Pollen grains are produced along the inner edge of the anther, which is the tip of the flower's stamen. The name "pollen" comes from the Greek word palynos, which means dust or pollen. Pollen grains contain two or three cells that carry the male gametophyte, which is the equivalent of the human sperm. The outer layer of the pollen is called the exine. The germ cell splits to produce two sperm cells. This division can take place before or after pollination.

Pollination

Flowers need to be pollinated to bear seeds. Pollination refers to the transfer of pollen grains from the male anther to the female stigma. Pollination can be accomplished on the same plant. This process is called self-pollination. This only occurs if the flower is unisexual. Cross pollination takes place if the transfer of pollen grains is between two different plants. The elements responsible for the process of cross pollination are wind, insects, birds and mammals such as rodents, marsupials and bats. Entomophyly is the transfer of pollen by insects while anemophyly is the term given to the transfer of pollen by wind.

Reproduction

When the pollen grain germinates on the stigma, the pollen tube crawls down through the style. The tube then penetrates the ovule. To locate the ovule, the tube is believed to use chemical signals from synergid cells. The two male nuclei then are released through the pollen tube. One nucleus fuses with the egg to form the embryo or zygote, which will later becomes a seed. The other nucleus fuses with two polar nuclei in the egg to create the triploid endosperm, which is a nutritive tissue.

References

• Emc: Flowering Plant Reproduction: Fertilization and Fruits
• waynesword: Diversity Of Flowering Plants
• cavehill: introduction to flowering plants
• enchanted learning: Label the Flower

Resources

• microscopy: studying pollen