Double Fertilization of Flowers
Double fertilization is a unique event in the biological world, occurring predominantly among the angiosperms, a group more commonly known as flowering plants. In most advanced living organisms, one sperm unites with one ovule to produce an embryo, which is the beginning of a new individual. However, in flowering plants this event is compounded by the presence of a second sperm cell, which does not fertilize an egg cell but instead creates unique cellular material that will nourish the seed after it departs the parent plant. Does this Spark an idea?
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The Pollen Tube
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Fertilization in flowering plants begins normally with the formation of diploid cells in the pollen-producing anthers and in the ovule, which forms inside the ovary at the base of the pistil. Through a complex process of cellular division, haploid cells are left with half the genes of regular cellular material. Because these are the cells that will combine and reunite the genetic material to form a new individual with its own set of genes, they need to go through this process of genetic reduction. In many advanced biological organisms, sexual cell division is necessary to produce diverse offspring.
The Pollen
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Once the pollen is transferred to the tip of pistil (usually by bees), the large pollen cells undergo morphological changes and then begin their migration down the pollen tube. These changes that occur at the top of the pistil are brought on by chemicals released by the female part when the pollen arrives. Eventually, several nuclei, which are pollen cells without their cell walls, arrive at the ovule, where one nuclei will join the egg cell.
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The Embryo
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Nonetheless, in advanced seed plants one pollen cell or sperm still unites with one egg cell to form one embryo. This is basically the same process of reproduction that in occurs in many organisms of higher order, including man. However, the presence of a second male haploid nuclei, which recombines with non-sexual plant material, is a situation unique to the advanced plants.
The Endosperm
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The fascinating part about double fertilization is the union between one male cell and a normal diploid cell to create cells with triploid genetic material. These cells are not created to produce another embryo, but instead they act to nourish and sustain the embryo as it begins its journey into the new world. This event does not happen right away, but will occur only after the embryo and endosperm have grown into a more mature state. Sometimes a triggering effect is needed to start a new life cycle, and many times the embryo will also enter a dormant stage until growing conditions are optimum.
Significance
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The significant part of double fertilization is in the endosperm cellular material that is created to nourish the new embryo. Not only is this process important to the plant, but it is highly beneficial to many birds and animals (including man) that feed on nuts and fruits. While the endosperm layer may only provide a small amount of nutritional food in some cases, in other fruits this layer can become quite thick and thus popular as a food source.
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References
- Photo Credit flowers image by pearlguy from Fotolia.com
