Pollination and Fertilization

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Flowers contain the structures necessary for sexual reproduction. The male component, or stamen, consists of a thin stalk called the filament, capped by the anther. The female component, the pistil, includes the stigma, a sticky surface that catches pollen; the ovary, which contains the ovule and embryo sac with its egg; and the style, a tube that connects the stigma and ovary (A). Pollen is produced in the anther (B), and is released when mature (C). Each mature pollen grain contains two sperm cells. In self-pollinating plants, the pollen lands on the stigma of the same flower, but in cross-pollinating plants—the majority of plants—the pollen is carried by wind, water, insects, or small animals to another flower. If the pollen attaches to the stigma of a flower from the same species, the pollen produces a pollen tube, which grows down the neck of the style, transporting the sperm to the ovule (D). Within the embryo sac of the ovule, one sperm cell fertilizes the egg, which develops into a seed. The second sperm cell unites with two cells in the embryo sac called polar nuclei, and this results in the development of the endosperm, the starchy food that feeds the developing seed. The ovary enlarges (E) and becomes a fruit.

Pollen consists of thousands of microscopic pollen grains. A tough pollen wall surrounds each grain. In most flowers, the pollen grains released from the anthers contain two cells. If a pollen grain lands on the stigma of the same species, the pollen grain germinates—one cell within the grain emerges through the pollen wall and contacts the surface of the stigma, where it begins to elongate. The lengthening cell grows through the stigma and style, forming a pollen tube that transports the other cell within the pollen down the style to the ovary. As the tube grows, the cell within it divides to produce two sperm cells, the male sex cells. In some species, the sperm are produced before the pollen is released from the anther.

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