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Pollen may refer to the microspores of either angiosperms (flowering plants) or gymnosperms (conifers and cycads). These two types of pollen are distinct because they undergo different processes when uniting with the female gamete. Gymnosperm pollen grains are dispersed by wind.
Angiosperm pollen grains naturally dry to about 20% moisture before being released from the anthers of a flower. Angiosperm pollen is carried to the stigma of the same, or another flower in the process of pollination, and deposited on its sticky surface. Each pollen grain hydrates and germinates, then begin to grow a pollen tube down the style, through the wall of the ovary and into an ovule (incipient seed). As the pollen tube grows, two sperm are produced. One unites with the ovum (egg) within the ovule. This union of male and female gametes is traditionally called fertilization, but, since it has little to do with fertility, is more properly called syngamy. The second sperm unites with the central cell of the ovule, and this union produces the endosperm of the seed, which is composed of starch, proteins and oils. The endosperm of grains (wheat, barley, rye, oats, corn) is a major source of human food.
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 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.
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