Sometimes a flower may contain both male and female sex organs and is capable of self-pollination. An example of this type of plant is the typical garden pea plant.
The male stamen produce pollen in the anther at the top of this sex organ. The female pistil contains an ovary at the bottom with ovules and eggs inside.
If wind or an insect transfers pollen from the flower's anther and places it on top of the pistil (its stigma) within the same flower, it will stick there. Next, it germinates downward from the stigma, to the style to the ovary by making a pollen tube.
Once the pollen tube enters the ovule, there is a transfer of one sperm nucleus to fertilize the egg cell within to create a fertilized egg (zygote). This is destined to become the embryo plant.
A second fertilization occurs when another sperm nucleus enters the ovule and fertilizes two polar nuclei in the ovule to become endosperm--a triploid material that will serve as a food supply for the developing plant.
The ovule will harden into a seed coat containing both the embryo plant and its food supply. The ovary which contained the ovules develops into a larger fruit.
In self-pollination, the two gametes that are fusing together came from one parent plant. There would be more variation in the offspring if the gametes came from two different plants such as when an insect transfers pollen from a different plant in the garden to another plant, or when wind transports pollen from one plant to another.
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