How plants defend themselves


The bracken is a fern which is found in many habitats around the world. It is so successful because it has many ways of defending itself. Its sap contains insect repellents, and its fronds contain poisons which prevent many kinds of animals from eating it. The fronds are also tough and hard for animals to digest. Even when the bracken's habitat is destroyed by fire it still survives because it has an underground stem. This stem also helps the bracken survive in habitats which have cold winters. The stem stays sheltered from the harsh conditions above the ground.

Do animals always die when they eat a poisonous plant?
No. The poison may give the plant an unpleasant taste and make the animal spit out the leaves. The animal will quickly learn to avoid the plant. This is why you may see cattle or sheep grazing in a field which is full of buttercups. The animals have learned to avoid this poisonous plant. The poison in a buttercup may cause skin rashes in humans. In the past beggars used to rub buttercups on their feet so that the skin would become inflamed. This would make people take more pity on the beggar and he or she would receive more money.

How can poisons in plants affect humans?
They can affect them in a variety of ways. Some poisons just produce an unpleasant taste or cause the mouth to go dry. Many can affect the digestive system and cause people to be sick or have diarrhoea. Other poisons can affect the liver or the nervous system and slow down the breathing rate and the heart beat. A few poisons can cause a person to go into a deep sleep and eventually kill them.

Are some common plants poisonous?
The yew is a poisonous evergreen tree found planted in churchyards as well as in the wild. Another common poisonous tree is the laburnum tree, which produces yellow flowers and is commonly planted in parks and gardens in the summer. The flowers form pods which look like pea pods. Children have been killed by mistaking the seeds for peas and eating them. Holly, ivy and mistletoe berries are also poisonous.

Are there any trees which sting like a nettle?
Yes. The shiny leaf stinging tree, or nettle tree, grows in Australia. If you touch its leaves they give you a very painful sting.

Can some animals be unaffected by a plant's defences?
Yes. Acacia trees live on the African plains and have spiky twigs to protect their leaves. The Giraffe can move its tongue round the spikes to pull off the leaves. Deer are not affected by the poison in the yew. Some caterpillars like the caterpillar of the monarch butterfly, feed on poisonous milkweed and dogbane plants and store the poisons in their bodies. The caterpillars have bright colours which warn birds that they are poisonous and should not be eaten.