Pollen tube

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)

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Lipids at the surface of the stigma stimulate pollen tube growth for compatible pollen. Plants that are self sterile inhibit the pollen grains from their own flowers from growing pollen tubes. The presence of multiple grains of pollen has been observed to stimulate quicker pollen tube growth in some plants. The Pollen Tube is very useful in reproduction of the flower.

The number of pollen grains needed for pollination/syngamy is equivalent to the number of ovules. One can look at a cut watermelon to identify the mature seeds that resulted from syngamy, and the white, undeveloped seeds that resulted from a failure of syngamy. The proportion of mature seeds to undeveloped ones is proportional to the quality of the fruit, which can then be tested by taste.

The trace mineral boron is vital to pollen tube growth. While too much boron is toxic, a complete lack of boron in the soil can cause a crop failure.

Ferns, algae, and other lower order plants (including Ginkgo Biloba and cycads, oddly enough) do not have their pollen directly deposited to the egg through pollen tubes: they have flagellate sperm, which swim through a watery fluid to pollinate the egg cells.