Which way does the plant grow?
Watch ten plant parts bend, twist and grow, find out what each is responding to, and sort them into phototropism, geotropism, hydrotropism, thigmotropism and chemotropism.
Core concept
Plants respond to stimuli by growing towards them (positive) or away from them (negative): light (phototropism), gravity (geotropism), water (hydrotropism), touch (thigmotropism) and chemicals (chemotropism)
Topic: Control and coordination
Plants cannot walk, but they still move: they grow in a chosen direction. A tropism is growth towards or away from a stimulus coming from one direction. If the plant grows towards it, the tropism is positive; away from it, negative. Shoots bend towards light (phototropism). Roots grow down with gravity and shoots grow up against it (geotropism). Roots grow towards water (hydrotropism). Tendrils coil round anything they touch (thigmotropism). A pollen tube grows towards chemicals made by the ovule (chemotropism). Plant hormones such as auxin cause much of this growth.
What you will learn
- Define a tropism and tell positive from negative.
- Name the stimulus for each of five tropisms.
- Explain why a shoot and a root of the same plant respond differently to gravity.
Before you start
- Know the parts of a plant.
Materials
- A potted seedling, a cardboard box with a hole, wet cotton and a few bean seeds
Equipment
- A window sill with light from one side
Procedure
- Pick a card and find out what the plant is responding to and which way it grows.
- Do this for at least six cards.
- Sort every card into its tropism.