Tropic responses
Introduced| English | Meaning |
|---|---|
| tropisms/ˈtrəʊpɪzəmz/ | tropisms |
| phototropism/ˌfəʊtəʊˈtrəʊpɪzəm/ | phototropism |
| gravitropism/ˈɡrævɪtrəʊpɪzəm/ | gravitropism |
| auxin/ˈɔːksɪn/ | A class of plant hormones involved in growth responses, including cell elongation and tropisms. |
| cell elongation/sel iːˌlɒnˈɡeɪʃn/ | cell elongation |
How plants respond
- Plants have no nerves or muscles, yet a shoot still bends towards the light and roots grow downwards.
- These growth responses are tropisms 向性, controlled by plant hormones.
Phototropism is a plant's growth response to:
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Phototropism is growth in response to light.
A plant shoot grows:
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Shoots show positive phototropism — towards light.
Roots show positive gravitropism, growing downwards.
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Roots grow towards gravity for water and anchorage.
Phototropism 向光性 and gravitropism 向地性
- Phototropism: growth in response to light. Shoots grow towards light (positive).
- Gravitropism (geotropism): growth in response to gravity. Roots grow downwards (positive).
Auxin 生长素 gathers on the shaded side and makes those cells grow longer, bending the shoot to the light
Phototropism — bending to light
A shoot grows towards light because a hormone makes one side grow faster than the other.
The plant hormone that controls growth responses is:
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Auxin controls cell elongation and bending.
Auxin gathers on the ______ side of a shoot, making it bend towards light.
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The shaded side grows faster, bending the shoot to the light.
The role of auxin
- Auxin is a plant hormone that controls growth by cell elongation 细胞伸长.
- It gathers on the shaded side of a shoot, making that side grow longer — so the shoot bends towards light.
Why tropisms help
- Growing towards light maximises photosynthesis.
- Roots growing down find water and anchorage.
Worked example. Light shines on one side of a seedling. Auxin moves to the shaded side, which grows faster, so the shoot bends towards the light — a positive phototropism that boosts photosynthesis.
You've got it
- tropisms are plant growth responses: phototropism (light), gravitropism (gravity)
- shoots grow towards light; roots grow down towards gravity and water
- the hormone auxin controls growth by cell elongation (gathers on the shaded side)