Fertilization is a biological process that occurs in the reproductive system of organisms, including plants and animals. It is the fusion of male and female gametes, which results in the formation of a zygote. Fertilization is a crucial step in the reproduction process as it combines the genetic material from both parents to create a new individual with a unique combination of traits.

Fertilization in Plants:
In plants, fertilization takes place within the flower. The male gametes, called pollen grains, are produced in the anthers of the flower. These pollen grains contain the male reproductive cells. The female reproductive organ, called the pistil, consists of the stigma, style, and ovary. The stigma is sticky and captures the pollen grains. The pollen tube then grows down through the style and reaches the ovary, where it enters the ovule. The male gamete fuses with the female gamete present in the ovule, leading to the formation of the zygote. This process is known as double fertilization and is unique to flowering plants.

Fertilization in Animals:
In animals, fertilization takes place internally or externally, depending on the species. In internal fertilization, which occurs in mammals, birds, and reptiles, the male deposits sperm inside the female's reproductive tract. The sperm then travel through the female reproductive system to reach the egg. The fusion of the sperm and egg occurs in the female's oviduct, leading to the formation of a zygote.

In external fertilization, which occurs in fish, amphibians, and some invertebrates, the release of eggs and sperm into the environment coincides. The male and female release their gametes into the water, where fertilization takes place. The sperm swim towards the eggs and penetrate them, resulting in fertilization. This process is highly dependent on environmental conditions, such as water temperature and pH.

Key Points:
- Fertilization is the fusion of male and female gametes.
- It occurs in the reproductive systems of organisms.
- In plants, fertilization occurs within the flower through the fusion of pollen and ovule.
- In animals, fertilization can be internal or external.
- Internal fertilization occurs in mammals, birds, and reptiles, where sperm are deposited in the female reproductive tract.
- External fertilization occurs in fish, amphibians, and some invertebrates, where eggs and sperm are released into the environment.
- Fertilization results in the formation of a zygote, which has a unique combination of genetic material from both parents.