Plant defences.
Plants have two main types of defences to protect them against infection by pathogens
1.Physical defences
2.Chemical defences
Physical defences
These are physical barriers that prevent microorganisms from entering plant tissues.
| Physical defence | Description | How it prevents infection / damage |
|---|---|---|
| Cellulose cell walls | Plant cell walls contain cellulose | Strong and rigid making it difficult for microorganisms to penetrate the cells. Cellulose cell walls act as a physical barrier to prevent pathogens entering plant cells. |
| Waxy cuticle on leaves | Leaves are covered by a tough, waxy cuticle. | This layer: Reduces water loss Prevents microorganisms from entering the leaf surface |
| Layers of dead cells (bark) | Stems and trunks are covered in dead cells (e.g. bark). | These cells: Form a protective outer layer Falls off, removing microorganisms growing on them |
Chemical defences
Chemicals produced by plants to kill or deter organisms.
Some plants produce antibacterial chemicals. Humans have used substances from plants to make medicines such as antibiotics
| Chemical defence | Description | How it prevents infection or damage |
|---|---|---|
| Antibacterial chemicals | Plants produce antibacterial chemicals | These: Kill bacteria Stop bacteria from growing |
| Poisons to deter herbivores | Some plants produce toxic chemicals | These poisons: Make the plant unpleasant or dangerous to eat Reduce damage from animals |
Mechanical adaptations (defence responses)
These are structures or movements that reduce grazing by animals.
| Mechanical adaptations (defence responses) | Description | How it prevents damage |
|---|---|---|
| Thorns and hairs | Thorns or hairs growing on the stem of the plant such as cacti thorns. | Thorns and hairs make plants difficult or painful to eat. They deter herbivores physically. |
| Leaves that droop or curl when touched | Some plants move in response to touch. | This movement: Startles animals Makes the plant less attractive to eat |
| Mimicry | This is how a plant tries to replicate features of another organism | Some plants mimic other organisms or features. This tricks animals into avoiding them (e.g. appearing poisonous or already damaged). |
Linking to adaptations
All these defences are adaptations.
They:
Increase the plant’s chance of survival
Improve reproductive success
Reduce damage from pathogens and herbivores
Practice Questions
1. Define the term plant defence response.
2. Name two physical defence responses found in plants.
3. Explain how the waxy cuticle acts as a defence.
4. Why are cellulose cell walls an effective defence against pathogens?
5. Explain one way bark helps protect a plant.
6. Name two mechanical defences used by plants.
7. How do thorns reduce damage from herbivores?
8. What is meant by a chemical defence in plants?
9. Some plants produce antibacterial chemicals. Why is this useful?