Plant Hormones and Responses

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.

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Plant Responses to the Environment

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)