Plant Defences

Pathogens can cause diseases in plants, for example, rose black spot and tobacco mosaic virus (TMV). Plants are vulnerable, so they are constantly fighting off pathogens and pests. Just like humans, plants have both physical and chemical defences to fight and prevent infection and disease.

Physical Defences

Some of the plant’s physical characteristics provide defence against pests and pathogens.

  • All plant cells have a cellulose cell wall which is difficult for pathogens to penetrate
  • Bark is a thick, tough layer, made up of dead cells, that surrounds some plants. It acts as a barrier that prevents pathogens from entering
  • The waxy cuticle is a thin, oily layer that coats the leaves and provides a barrier against microbes that try to enter the plant

Chemical Defences

Physical barriers are sometimes not enough to protect a plant. Some plants produce chemicals that have antimicrobial properties.

  • For example, plants such as mint and witch hazel produce antimicrobial chemicals to protect them from pathogens. However, these chemicals are also used in antiseptic products

Some plants protect themselves from herbivores. For example, stinging nettles contain long, thin hairs that can inject toxins into the skin of a herbivore when touched. The toxins can cause irritation or pain, so their purpose is to deter herbivores from eating the plant.

Farmers can also genetically engineer plants to make them resistant to infections.

Mechanical Plant Defences

The structural features of plants can help to protect them from herbivores. For example:

AdaptationHow they protect the plantExample
Thorns and hairsMakes it unpleasant or painful for herbivores to touch or eat the plant Brambles and cacti
MimicryPlants can manipulate their appearance to confuse herbivores or pollinatorsThe hammer orchid mimics a female wasp to lure males into depositing and picking up pollen
Drooping leavesSome leaves droop or curl up when touched. This can dislodge insects and protect the plant from being eatenThe Mimosa pudica plant closes its leaves and droops its stem when touched, making it more difficult for insects to feed on the leaves

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