How Does Fertilization Occur in Flowering Plants?
Production of Pollen
Fertilization occurs in flowering plants through the process of pollination. A flower's center contains long, thin filaments, topped by small oblong anthers. Together, the filaments and anthers are called stamen.
Each anther houses four pollen sacs, which produce pollen. The pollen sacs contain microspore mother cells, or MMCs. Each pollen sac will produce eight sperm cells from the MMCs, or pollen grains. When the pollen grains in the sacs become ripe, the anthers burst open, releasing the sticky pollen onto the stamen. This bursting of the pollen sacs, called dehiscence, sometimes happens with enough force that the pollen propells forcefully out of the flower and onto the wind, which carries it to other flowers.
The bright colors of the petals lure insects, where they alight and partake of the flower's nectar. Glands located at the flower’s base produce nectar. To reach the nectar, insects must climb all the way into the flower itself. During the process, pollen grains located on the anthers either become stuck to the insect's feet (where they are transferred to another flower, in a process called cross-pollination) or moved from the stamen to the pistil of the same flower (self-pollination). This occurs in many instances, because of the pistil's location in the center of the flower surrounded by the stamen.
The pistil (sometimes called a carpal) houses the female organs of the flower and comprises the stigma (a thin hollow tube) and the ovary. At the base of the tube lies the ovary, which houses the ovule. Many flowers have more than one carpal, and some have more than one ovule (egg) in each ovary.
When a pollen spore lands on the stigma, it sends a tube down to the ovum. At the tip of the tube lie the male gametes. Once the tip reaches the ovule, it bursts, and two male gametes are released. One of the gametes fuses with the egg, and the other with the polar nuclei. This is called double-fertilization. The ongoing division of the polar nuclei nourishes the developing egg. The ovum then begins to develop into a seed, and the ovary grows a protective fleshy barrier around it, which will become the fruit.