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Plants cannot move from place to place, but they can respond to their environment by growing towards or away from stimuli. These growth responses are called tropisms and are controlled by plant hormones, particularly auxins. This lesson covers auxin action, phototropism and gravitropism, as required by Edexcel GCSE Biology (1BI0) Topic 6.
Plant hormones (also called plant growth regulators) are chemical substances produced in small quantities in one part of the plant that have an effect on another part of the plant. They coordinate and control growth, development and responses to the environment.
Unlike animal hormones, plant hormones are not produced in specific glands — they are made in various tissues (e.g., shoot tips, root tips, developing seeds).
| Hormone | Where produced | Key effects |
|---|---|---|
| Auxin (IAA) | Shoot tips, root tips | Promotes cell elongation in shoots; controls tropisms |
| Gibberellins | Seeds, young leaves | Promote seed germination, stem elongation, flowering |
| Ethene | Ripening fruits, ageing tissues | Promotes fruit ripening |
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