Plant Defence Responses

Plant Defences

Just like animals, plants are constantly defending themselves from attack from pathogens. Again like us, plants have physical and chemical defences which stop pathogens from infecting them and causing disease.

Physical Defences

Many plants are covered with a thick bark. This is an external layer of dead cells which forms a physical barrier against infection. In this way it is very much like our skin. Beyond bark, each plant cell has a cellulose cell wall which acts as another barrier against infection. Leaves are covered with a waxy cuticle which also stops their cells from becoming infected.

Chemical Defences

• Some plants such as mint and witch hazel produce antibacterial chemicals. These limit the spread of bacteria that were not stopped by physical defences. These chemical defences are now used in antiseptics for humans.
• Other plants, like stinging nettles, have developed poisons to stop themselves being eaten by herbivores. These do not defend plants from infection by pathogens.

Other Defences

Farmers can now genetically engineer crop plants to be resistant to infections.

Mechanical Plant Defences

Plants have also evolved other defences from attack. These defences are often to avoid being eaten by herbivores.

Thorns and Hairs

Plants like roses have evolved large thorns to avoid being eaten, while plants like cacti have evolved thin spines. These adaptations protect plants from damage and keeps their vital water stores safe.

Drooping leaves

The Mimosa pudica plant is very special. It has evolved to close its leaves and then point its stems towards the ground when touched by an insect as it lands on it, making it difficult for the insect to feed on the leaves.

Mimicry

Some plants have evolved the ability to mimic what is around them. The passion flower vine has evolved small yellow spots on its leaves. These look like the eggs left behind by butterflies. Female butterflies are unable to tell the difference between these and real eggs, so they lay eggs on other leaves to avoid competition when they 'might' turn into caterpillars. Therefore, the passion flower vine suffers less damage to its leaves from feeding caterpillars.