Everything about The Pollen Tube totally explained
The
pollen tube of most
seed plants acts as a conduit to transport sperm cells from the
pollen grain, either from the stigma (in
flowering plants or
angiosperms) to the ovules at the base of the
pistil, or directly through ovule tissue in some
gymnosperms (
conifers and
gnetophytes). In other
gymnosperms (
Ginkgo and
cycads) the pollen tube is involved only in nutrient uptake from ovule tissue by the pollen grain, and doesn't convey sperm cells to the egg.
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 to produce the
endosperm of the seed. The endosperm is rich in starch, proteins and oils and is a major source of human food (for example, wheat, barley, rye, oats, corn)
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.
Ferns, algae, and even some seed plants (including
Ginkgo biloba and cycads) don't have their pollen directly deposited to the egg through pollen tubes: they've flagellate sperm, which swim through a watery fluid to pollinate the egg cells.
Further Information
Get more info on 'Pollen Tube'.
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