3.3.2 Plant Defence Responses

Plants have evolved a range of physical, chemical, and mechanical defences to protect themselves from invading pathogens and predators.

Physical Defences: Preventing Microorganism Invasion

• Waxy Cuticle: The tough, waxy cuticle on the surface of leaves acts as a barrier, preventing pathogens from entering the plant.
• Cellulose Cell Walls: The cellulose in plant cell walls forms a strong physical barrier that protects the cells from pathogen invasion.
• Layers of Dead Cells: Plants develop layers of dead cells around their stems, such as bark, which act as a shield against pathogens. These dead cells eventually fall off, taking any attached pathogens with them.

Chemical Defences: Deterring Predators and Killing Bacteria

• Poisons: Certain plants produce poisons, like those found in foxgloves, tobacco plants, deadly nightshade, and yew, to deter herbivores from eating them.
• Antibacterial Compounds: Some plants, such as mint and witch hazel, produce antibacterial compounds that kill bacteria and protect the plant from infections.

Mechanical Defences: Protecting Against Predators

• Thorns and Hairs: Many plants have thorns or hairs that make it difficult and painful for animals to consume them, though these defences are less effective against insect pests.
• Drooping or Curling Leaves: Some plants have leaves that droop or curl when touched. This movement can dislodge insects or move the plant away from harm.
• Mimicry:
  • Unhealthy Appearance: Some plants droop to mimic the appearance of an unhealthy plant, discouraging animals from eating them.
  • Butterfly Egg Mimicry: Certain plants have patterns that resemble butterfly eggs, deterring butterflies from laying their eggs on the leaves to avoid competition.
  • Stone and Pebble Mimicry: Plants in the 'ice plant family' have evolved to resemble stones or pebbles, making them less likely to be eaten by herbivores.