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 negative gravitropism, growing upwards.
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False. Roots show positive gravitropism, growing downwards towards water and anchorage. Shoots show negative gravitropism.
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)
Write from a shoot in one-sided light
- A shoot is lit from the left only.
- Auxin gathers on the shaded side.
Which way does the shoot bend, and which way do roots grow?
More elongation on the shady side pushes the tip toward the light.
Where does auxin gather in a one-sided lit shoot?
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The shaded side elongates more.