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This lesson covers plant hormones (growth regulators) and how plants respond to environmental stimuli as required by the Edexcel A-Level Biology specification (9BI0), Topic 9 -- Control Systems. You need to understand the roles of auxins, gibberellins, and other plant hormones, the mechanisms of tropisms, and how plant responses are coordinated.
Unlike animals, plants cannot move to escape unfavourable conditions. Instead, they respond to environmental stimuli by changing their growth patterns. These growth responses are called tropisms.
| Tropism | Stimulus | Direction of Growth |
|---|---|---|
| Phototropism | Light | Shoots grow towards light (positive phototropism); roots grow away from light (negative phototropism) |
| Gravitropism (geotropism) | Gravity | Roots grow towards gravity (positive gravitropism); shoots grow away from gravity (negative gravitropism) |
| Thigmotropism | Touch | Tendrils grow towards the surface they touch (positive thigmotropism) |
| Hydrotropism | Water | Roots grow towards water (positive hydrotropism) |
| Chemotropism | Chemicals | Pollen tubes grow towards the ovule (positive chemotropism) |
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