The Open Door Web Site : Biology : Pollinating Mechanisms

Flowers have evolved many different techniques to ensure pollination. Some flowers have developed very special relationships with insects or other animals.

Butterflies are liquid feeders which drink nectar. The butterfly has a long tube-like mouthparts which are normally coiled under its head but which it can extend to reach the nectary at the base of a petal. As it feeds, pollen sticks to the butterfly's body. This pollen is then carried by the butterfly to the next flower that it feeds on.

Bees feed on nectar and also collect pollen. This is why insect-pollinated flowers make lots of pollen; most of it will be used by the bees but some of it will be transferred to other flowers. Many other insects move from flower to flower to feed. Beetles and flies get covered with pollen as they walk over buttercups and daisies. Some of this pollen is brushed onto the female parts of another flower as the insect moves over it.

Moths feed at night so flowers which are pollinated by moths only open at night. These flowers are usually pale-coloured or white so that they can be seen more easily when it is dark. They only give off scent at night when they are open. The flowers of the tobacco plant, evening primrose and honeysuckle are pollinated by moths.

Insects are not the only animals which pollinate flowers. Snails and slugs may slither over flowers and carry pollen in the trail of thick, sticky fluid that they make as they move.

Some birds pollinate flowers. Birds have a weak sense of smell but they can see colour well, particularly red. Many red, odourless flowers are pollinated by birds. North American humming birds are attracted to red, orange and yellow flowers such as fuchsias and columbines.

The Arizona desert in the United States receives less than 25 cm of rain each year. Desert plants have to survive for many months without rain and in very hot conditions. The cactus flowers open just for one night. Their scent attracts moths, humming birds and white doves. By the morning, before the hot sun has risen in the sky, the flowers have been pollinated and have already closed.

The fruit bat is an example of a mammal which drinks nectar and helps to transfer pollen from flower to flower.

Johnny's Fact File No.33
Cross-pollination occurs when pollen is transferred from the anthers of one flower to the stigma of another flower. Self-pollination occurs when the pollen from the anther of a flower is transferred to the stigma of the same flower.
To make sure that the pollen is not transferred to the carpels of the same flower (sell-pollination), the stamens and carpels of hermaphrodite flowers become mature at different times. In the buttercup the anthers are the first to mature and by the time the carpels are mature, the anthers have died and the pollen is too old to be useful. Because of this, pollination between different flowers (cross-pollination) is more likely to occur.
Cross-pollination is better than self-pollination. When flowers are cross-pollinated their seeds germinate into strong, healthy plants. Seeds from self-pollinated flowers produce weaker, less healthy plants.
Most flowers are adapted to prevent self-pollination. This can be achieved in two ways. Some flowers are unisexual which means that self-pollination becomes impossible.

Some types of flowers attract a particular male insect because the flower has a similar shape and colour to that of the female insect. The male insect attempts to mate with the flower and picks up pollen as it does so. Some flowers have an unpleasant scent that resembles rotting meat. They also have dark red petals which make them look like a piece of meat. These flowers are very attractive to flies which then carry the pollen from flower to flower.

One type of flower, called a stapelia, grows in southern Africa and it smells like rotting meat. Its flowers have brown, wrinkled petals covered in short hairs. The flowers look like the skin of a dead animal. The flower even produces heat to make it appear as if it is in the process of decaying. All of these tricks are so convincing that flies not only visit the flower but also lay their eggs in it. When the eggs hatch there is no food for the maggots and they quickly die. The flower, however, was pollinated by the egg-laying flies.

The bucket orchid from Central America drugs visiting insects. Bees get drunk after drinking the nectar and they start to stagger. The bee eventually falls right into the centre of the flower. When the bee recovers its senses and tries to crawl out, it brushes past the anthers which cover the bee with pollen grains.