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How Do Pollen Grains on a Stigma Cause the Fertilization of Egg Cells?
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1- Flower Anatomy
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The sticky female part in the center of the flower is called the stigma. A long tube called the style connects the stigma to the ovary, which contains a number of ovules, each of which contains one egg cell. The ovule is surrounded by a protective layer of tissue called the integument. The male part of the flower, which produces pollen grains, is called the anther or stamen. A mature pollen grain contains two cells: a tube cell, which develops into the pollen tube, and a generative cell, which develops into sperm. The pollen grain consists of a tough spore wall on the outside, the tube cell contained within the spore wall, and the generative cell inside the tube cell.

2- Pollination
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Pollen grains can be transported from one flower to another by animals such as bees, or carried by wind currents. Flowering plants can also self-fertilize, when pollen is transferred from the anther to the stigma within a single flower.

3- Germination
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When a pollen grain lands on a stigma, it sticks, germinates and grows a long pollen tube down through the stigma and the style all the way to the ovary. A chemical attractant produced in the ovule directs the growth of the pollen tube to the micropyle, a gap in the integument surrounding the ovule. Pollen tubes grow rapidly, approximately one centimeter per hour. As the pollen tube is growing, the generative cell of the pollen grain divides to become two sperm within the tube; the pollen tube then carries the sperm through the style into the ovary, and through the micropyle to the egg cell within the ovule.

4- Fertilization and Development
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Two sperm are released into the ovule through the pollen tube. One sperm fertilizes the egg cell, forming a zygote which develops into the plant embryo, while the other sperm fuses with two other nuclei in the ovule to create a triploid cell, which undergoes mitosis and develops into endosperm, the tissue that provides nutrition for the developing embryo within the seed. The ovule eventually becomes a mature seed when the integument hardens into a tough seed coat. A complete seed consists of the developing embryo and the endosperm tissue contained within the seed coat; the ovary develops into a fruit, which contains the seeds.

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