What Produces Pollen Grains?
Even though pollen grains are known as an irritant for their propensity to induce allergies within people, they are integral to the cycle of sexual reproduction in certain plants. These complex structures have evolved a sophisticated pattern of development in order to produce a new life.
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Pollen
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Pollen is defined as the structure of a plant that transports the male gamete. The male gamete is the cell that contains male DNA, somewhat analogous to sperm in animals. The pollen itself is not the reproductive structure, however, but it is the hard, protective external shell that carries the gamete through the environment. Pollen is made out of the carbohydrate cellulose and is microscopic in size; just a little powder contains thousands of pollen grains.
Sexual Reproduction
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Like other life on Earth, plants contain nucleic acids in the form of DNA. DNA is packed into tightly arranged structures known as chromosomes. Individual organisms produced by sexual reproduction have a pair of each chromosome---one from the mother and one from the father. However, gametes contain only a single copy of each chromosome, which is equal to half the total number. Fertilization is the act of combining gametes to reach a pair of each chromosome once again. Sexual reproduction promotes diversity and genetic change. It evolved independently in plants.
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Anther
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Pollen is produced in a sac-like structure known as the anther, which is attached to the top of a thin filament. Together these parts comprise the male-structured stamen. The anther is usually arranged in the open end of the flower so that it can come in contact with pollinating insects or be released into the wind.
Development
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The anther develops simultaneously with the flower. Within the anther, cells differentiate into reproductive cells and a sterile wall that forms the pollen sac and provides nutrition to the inner cells. The gametes are produced by a process called meiosis; cell division renders the gametes haploid--meaning that each chromosome is present as a single unpaired copy--from the paired diploid number.
Walls
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After meiosis occurs within the anther, pollen grains develop a tough outer wall called the exine and a narrow, delicate inner wall called the intine. Upon landing on a compatible flower pistil, the intine produces the pollen tube, which transfers the gametes to the ovary, the female reproductive structure of a flower.
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References
- Photo Credit pollen image by asb from Fotolia.com
