In angiosperms the pollen tube germinates from the pollen grain and grows the entire length through the stigma, style, ovary 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 egg cell to produce the embryo of a new plant, while a second sperm unites with the central cell (polar nuclei) to produce the endosperm of the seed. The endosperm is rich in starch, proteins and oils and is a major source of human food (e.g., wheat, barley, rye, oats, corn)