Pollination is the placement of the pollen on the stigma of the carpal. This pollen transfer can be accomplished by wind, insects, built in mechanical discharge, and man. Once the pollen lands on the stigma, a series of chemical reactions takes place allowing the pollen grain to begin producing a structure called the pollen tube. As this is happening, the generative nucleus will divide and produce 2 sperm nuclei. This pollen grain with the pollen tube and 3 nuclei is considered the mature gametophyte. The pollen tube will work its way through the style of the carpal and touch the micropyle of the ovule. Here the sperm nuclei will enter the embryo sac and fertilize the egg and the two polar nuclei; hence the term double fertilization. The fertilized egg (2N) will develop into the immature seed plant, while the (3N) central cell will develop into the endosperm or food storage area of the seed.
How does a pollen grain get to the stigma of a pistil?
This depends on the plant. Plants the have flowers use an intermediary. This could be an insect like a bee, butterfly, moth, fly...etc. Could also be a hummingbird. Flowers have evolved specific traits to attract a particular pollinator, so you can tell just from looking at them what would visit.
Plants that do not have flowers use the wind on land, or water currents in aquatic environments.
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