Carried through the air by wind, insects, or birds, a plant's pollen travels to other plants of the same kind or from one part of a plant to another. Pollen can also be carried by water. In all plants that produce seeds, pollen is the substance that causes a plant to become fertilized and reproduce.

Pollination

A pollen grain is very tiny and is best viewed through a microscope. These grains come from the male part of a plant, called the stamen, and produce sperm cells. The female part of a plant, called the pistil, contains egg cells. The pollen grains must reach the eggs to fertilize them and thus produce new plants. The process by which this happens is called pollination.

The pistil typically has a swollen base, called the ovary, that contains the potential seeds, or ovules. Inside the ovules are the egg cells. The pistil also has a stalk with a tip called the stigma, which receives pollen. When a pollen grain is deposited on the stigma, it forms a pollen tube that grows down through the pistil and into the ovules. Fertilization occurs when a sperm cell in the tube fuses with an egg cell. The fertilized cell then develops into a seed.

Fertilization can take place either on one plant or between plants. When pollen goes from stamen to pistil on the same plant, the process is called self-pollination. When the pollen travels from one plant to fertilize another plant, it is called cross-pollination.

How pollen is carried

Many plants have fragrant or colorful flowers to attract insects or birds to the flower's nectar. Nectar is a sweet liquid on which the animals feed. When an animal comes to drink the nectar, the flower's pollen sticks to its body. The animal carries the pollen to other flowers when it moves on for more food. Among the insects, bees are the main pollinators.

The pollen of grasses and many types of trees is spread by the wind. Some water plants have flowers that float on the surface, where pollen is washed into them.

Hay fever

Plants throughout the world produce so much pollen that it makes up a significant portion of the material floating in the air. A substance present in the pollen grains of many plants (namely, ragweed and many grasses) causes the allergic reaction known as hay fever. Sneezing, stuffy nose, and itching of the eyes and nose are symptoms of hay fever. Pollen counts, which are estimates of the amount of pollen in the air, are published so that people who are allergic will know that they may have trouble with hay fever on certain days.