Pollen consists of tiny grains that are produced in the male organs of flowering and cone-bearing plants. Seeds develop after pollen is transferred from the male part of a plant to the female part. This transfer of pollen is called pollination.
Most pollination is carried out by birds, insects, and the wind. Most flowers pollinated by birds and insects have colorful blossoms and an odor that attracts the animals. When they come into contact with a flower, pollen clings to their bodies, and they then carry the grains to other flowers. The wind blows pollen from one flower or cone to another. Most flowers pollinated by the wind have neither bright colors nor a fragrant odor.
A flowering plant produces pollen in its stamens, the male parts of a flower. After pollination, seed development occurs in the female part, called the pistil. A cone-bearing plant produces pollen in its male pollen cones. Pollination occurs when the wind carries pollen from the male pollen cones to the female seed cones.
Many people are allergic to pollen. Large amounts of pollen in the air cause them to develop hay fever. This allergy results in headaches, red and itching eyes, a runny nose, and sneezing. Ragweed pollen is the most common cause of hay fever in the United States.
Fossilized pollen grains are often preserved in sediments from lakes and bogs. By studying these grains, scientists can learn much about the plant life and climate of earlier ages.
Pollen grains
Pollen grains vary in shape, size, and surface features. These variations make the grains of each species of plant different. Most pollen grains are round or oblong and range from 15 micrometers to more than 200 micrometers wide. (About 25,000 micrometers equal 1 inch.) Every pollen grain has an outer shell, which may be smooth or wrinkled, or covered with spines or knobs. This shell prevents the inner cells from becoming dry.
Such plants as corn, wheat, and pine trees, which are pollinated by wind, produce huge amounts of pollen. A corn plant can produce more than 18 million grains. But some plants that are pollinated by birds and insects produce only a few thousand grains.
Most pollen grains live only several days or weeks after being released. However, the cells of date palm pollen live for as long as a year.
Methods of pollination
There are two methods of pollination, cross-pollination and self-pollination. Cross-pollination is the transfer of pollen from the stamens of one flower to the pistil of a flower of another plant. Self-pollination occurs when pollen is transferred from the stamens of one flower to the pistil of the same flower, or to another flower on the same plant.
Cross-pollination
is the most common method. For seeds to develop, cross-pollination must occur between flowers of the same or closely related species.
Honey bees carry out more cross-pollination than any other kind of insect. They make honey from nectar and use pollen for food. Honey bees collect pollen in small cavities on their hind legs and carry it back to the hive. However, some pollen clings to their bodies and is carried to other flowers. Other insects that carry pollen include ants, beetles, butterflies, and moths.
Among birds, hummingbirds are the most important pollinators. They insert their long, thin beak into flowers and drink the nectar. Pollen sticks to the beak and is carried to the pistils of other flowers.
The wind pollinates many plants, including birches, corn, grasses, cattails, oaks, and ragweeds. It may carry pollen grains 100 miles (160 kilometers) or farther.
Botanists have used artificial cross-pollination to create new varieties of corn, cotton, wheat, and other plants. They use special brushes to transfer pollen from one plant to another.
Self-pollination.
Many plants, including beans, cotton, oats, peas, and wheat, normally pollinate themselves. Certain cross-pollinating plants, such as pansies and some violets, can also self-pollinate.
The growth process and the structure of some flowers prevent self-pollination. In plants called crane’s-bills and spiderworts, for example, the stamens ripen earlier than the pistils. Therefore, the pollen is shed from the stamens before the pistils of the same plant become ripe. Willow trees and other species have imperfect flowers. In such species, each plant bears flowers with either stamens or pistils, but not both.
Fertilization
All flowering and cone-bearing plants produce seeds through fertilization. In fertilization, which occurs after pollination, a male sperm cell unites with a female egg cell.
In flowering plants, the egg cells develop in the ovary, the base of the pistil. The sperm cells are produced by the pollen grains. After pollination, a pollen grain swells as it absorbs water, sugar, and other materials from the stigma, the top of the pistil. The pollen then germinates—that is, it grows a tube downward to the ovary, where one or more ovules are located. The ovules are the structures that contain the egg cells. After the pollen tube reaches an ovule, it releases two sperm. One sperm fertilizes an egg cell. The second sperm fertilizes two female structures called the polar nuclei. The union of the second sperm with the polar nuclei produces the endosperm, the food-storage tissue of the new seed. All flowering plants form seeds through such double fertilization.
In cone-bearing plants, the sperm and egg cells develop in the male cones and female cones. Usually, after pollination, one of the two sperm fertilizes an egg, and the other sperm disintegrates. However, some species of cone-bearing plants undergo a double fertilization process similar to that of flowering plants.
