Pollen is the male part of a plant. Every pollen grain
carries two sperm cells inside it. Pollen is produced in the anther, the male part of
the flower. In some species of plant, the male and female parts are right next to each
other (example: apple blossoms). In others there may be separate male and female flowers
on the same plant (ex: squash plants), and in still others the entire plant may be male
or female (ex: holly). In any of these cases, it's generally considered desirable for
pollen to be transferred to a different, unrelated plant if possible, since this
maximizes genetic diversity in the resulting offspring, which increases their odds of
survival.
Pollen grains are dispersed a variety of ways.
Dispersal is simply the term for moving the pollen to the female parts of the plant.
Bees and other insects are very common pollinators; they are efficient because they tend
to stick to the same type of flower, and so carry pollen from one flower to others of
the same species. Hummingbirds also do this, but they tend to visit flowers of different
size and shape than the ones the bees like. The plant often "pays" these pollinators by
producing nectar to attract them and reward them.
Other
plants, including most members of the grass family, use wind pollination. In this case
the plant must produce very large amounts of pollen, because only a small percentage of
it will actually hit the target. A few aquatic plants have their pollen carried by
water, which is similar.
After pollen reaches the stigma,
which is the receptive surface of the female part of the plant, the pollen sprouts,
almost like a little seed, and grows a hollow tube into the female plant's ovary. The
sperm then swim down this pollen tube and find the egg
cell.
In short, pollen dispersal is any mechanism a plant
uses to transfer pollen, which contains the male genetic contribution, to the female
part of the plant, so that sperm can reach egg.
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