- Published: 28 December 2008
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George Hull, age 10, of Helena, Mont., for his question:
WHAT IS CROSS POLLINATION?
Many flowers have to depend on wind, insects, birds or some other means to carry their pollen from one flower to another. Flowers of this type are called cross pollinating flowers.
The things that we often consider beautiful in flowers are often aids for cross pollination. The blossoms, fragrant scent and sweet nectar attract various insects which have the important job of carrying the pollen from plant to plant.
If you look into a flower, you'll see the tiny yellow grains that are called pollen. This pollen helps to form seeds.
Plants make the pollen in the saclike anthers of their flowers. The anthers are considered the male organs of reproduction. The female organs include the pollen receiving stigma leading to the ovary, which is the egg bearing part of the plant.
Pollination is simply the transfer of pollen from the anther to the stigma. Pollen that reaches the stigma goes down through pollen tubes to the ovules, or tiny egg cells, which are located in the ovary. When the pollen reaches an egg cell, it causes the cell to divide.
Each divided cell then becomes a seed embryo and develops all the structures of the seed. This process is called fertilization and it is necessary for the reproduction of some plants.
Self pollination happens in flowers that can transfer pollen from their own anthers to their own stigmas. Sometimes the pollination even happens before the blossom opens.
But other flowers must have cross pollination. And the most important carriers of pollen for cross pollination are honeybees. They gather nectar for honey and pollen for food. They have small cavities on their hind legs to carry pollen to the hive. But some pollen clings to their bodies and is carried to other flowers.
Ants, beetles, moths and butterflies are also pollen carriers.
Red, blue or pink flowers often attracts bees and other insects by their color. Yellow or white flowers usually attract them with nectar.
Flowers that give off their fragrance at night, like night blooming jasmine or some honeysuckles, are pollinated by night flying moths. Those that are most fragrant in the daytime attract bees and butterflies.
Sometimes specific insects are necessary to cross pollinate certain flowers. The fig, for example, cannot produce seeds that will grow unless it is pollinated by a certain small wasp. And the yucca flower is pollinated only by the yucca moth.
Almost all common trees and shrubs are cross pollinated by the wind. Wind can carry pollen grains as high as three miles into the air and deposit them 100 miles from their original plant.
Hummingbirds, sunbirds and honeysuckers, with long, thin beaks, also transfer pollen.