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You may think that plants just stand where they are, without doing anything.
But actually, plants are actively responding to any changes in light or gravity so that they can grow towards light and water for photosynthesis 🌱
Plant's response to stimuli is called tropism.
- When plants grow towards a stimulus, it is called a positive tropism.
- When plants grow away from a stimulus, it is called a negative tropism.
For IGCSE Biology, you should understand the response of plants to two stimuli: light and gravity
Before looking at how plants respond to light and gravity, you need to learn about one type of plant hormone: auxin.
Auxin
- Auxin is a type of plant hormone that affects the rate of cell growth.
- Auxin is produced at the tip of growing root or stem cells and can diffuse to other parts of cells.
- High concentrations of auxin have different effects at the stem and at the root.
- At the stem: High concentrations of auxin stimulate cell growth
- At the root: High concentrations of auxin inhibit cell growth
- The movement of auxin and the concentration of auxin at different parts of the plant determines how plants respond to stimuli.
Phototropism
Phototropism is the response of plants to light.
- Stems show positive phototropism: Shoots grow towards light
- Roots show negative phototropism: Roots grow away from light
- Auxin moves to the shadier part of the stem.
- Because auxin stimulates cell elongation at the stem, cells on the shaded side will be elongated, while cells on the lit side will be shorter.
- Therefore, the stem will bend towards the light.
Geotropism
Phototropism is the response of plants to gravity.
- Stems show negative phototropism: Shoots grow away from gravity
- Roots show positive phototropism: Roots grow towards light
- When a plant is placed horizontally, auxin will become concentrated at the lower side of the plant.
- At the stem: Lower part of the cells will become elongated, so the stem will grow upwards.
- At the root: Lower part of the cells will be shorter because of inhibited growth, so the root will grow downwards.