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The pollen tube acts as a conduit to transport sperm cells from the pollen grain, which has landed on the stigma, to the ovules at the base of the pistil. The pollen tube germinates from the pollen grain and grows the entire length through the stigma, style and ovules to reach the eggs. In maize, this single cell can grow longer than 12 inches to traverse the length of the pistil. The sperm cells by themselves are not motile and are carried within the tube. As the tip of the tube reaches an egg it bursts and releases two sperm cells leading to a double fertilization. One sperm unites with the ovum (egg) to produce the embryo of a new plant while the second sperm unites with the central cell to produce the endosperm of the seed. The endosperm is composed of starch, proteins and oils and is a major source of food (wheat, barley, rye, oats, corn)