Seedling responses: unequal growth makes a bend
| English | 中文 | Pinyin |
|---|---|---|
| phototropism/ˌfəʊtəʊˈtrəʊpɪzəm/ | 向光性 | xiàng guāng xìng |
| auxin/ˈɔːksɪn/ | 生长素 | shēng zhǎng sù |
What would explain this observation?
- A seedling bending towards light has not simply moved its whole stem sideways. Different growth on its two sides changes its direction.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Plants produce hormones coordinating growth and responses to light and gravity. Phototropism 向光性 is a growth response to light; gravitropism is a growth response to gravity. Unequal auxin 生长素 distribution causes unequal growth in roots and shoots. In the usual shoot model, more auxin on the shaded side increases elongation there, bending the shoot towards light.
- phototropism: A directional growth response to light; auxin: A plant hormone involved in coordinating growth responses.
Why does the stated shoot bend towards light?
Roots and shoots respond differently. In a horizontal root, more auxin on the lower side inhibits elongation relative to the upper side, so the root bends downwards. In a shoot, greater lower-side elongation bends it upwards. Explain the growth difference rather than claim that gravity pulls all the cells into a new shape.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Roots and shoots respond differently. In a horizontal root, more auxin on the lower side inhibits elongation relative to the upper side, so the root bends downwards. In a shoot, greater lower-side elongation bends it upwards. Explain the growth difference rather than claim that gravity pulls all the cells into a new shape.
- For RP8 use teacher-approved newly germinated seedlings. Compare a planned light direction or orientation with a suitable control, keeping species, initial age, water, temperature and observation time comparable. Record lengths and careful labelled drawings showing direction. Do not test hormone effects on people; avoid touching growing tips or changing light and gravity conditions simultaneously.
Which two habits make the investigation or model in this case more defensible?
For RP8 use teacher-approved newly germinated seedlings. Compare a planned light direction or orientation with a suitable control, keeping species, initial age, water, temperature and observation time comparable. Record lengths and careful labelled drawings showing direction. Do not test hormone effects on people; avoid touching growing tips or changing light and gravity conditions simultaneously.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: a fictional shoot model grows 6 mm on its shaded side and 4 mm on the illuminated side during one interval. Growth difference = 2 mm. The longer side helps explain bending towards light, but this difference alone does not calculate the bend angle without geometry and tissue information.
A supplied shoot model has growth of 9 mm and 5 mm on opposite sides. Calculate the difference. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A supplied shoot model has growth of 9 mm and 5 mm on opposite sides. Calculate the difference.
The result is 4 mm. Known: a fictional shoot model grows 6 mm on its shaded side and 4 mm on the illuminated side during one interval. Growth difference = 2 mm. The longer side helps explain bending towards light, but this difference alone does not calculate the bend angle without geometry and tissue information.
Check the conclusion and its limits
- A tropism is growth in relation to a directional stimulus, not every plant movement. Auxin does not have the same elongation effect in every root and shoot situation. Gibberellin and ethene roles are Higher Tier and are taught separately.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Auxin always has an identical elongation effect in roots and shoots. This claim is false: A tropism is growth in relation to a directional stimulus, not every plant movement. Auxin does not have the same elongation effect in every root and shoot situation. Gibberellin and ethene roles are Higher Tier and are taught separately.
Seedling responses: unequal growth makes a bend: Roots and shoots respond differently. In a horizontal root, more auxin on the lower side inhibits elongation relative to the upper side, so the root bends downwards. In a shoot, greater lower-side elongation bends it upwards. Explain the growth difference rather than claim that gravity pulls all the cells into a new shape.
Auxin always has an identical elongation effect in roots and shoots.
A tropism is growth in relation to a directional stimulus, not every plant movement. Auxin does not have the same elongation effect in every root and shoot situation. Gibberellin and ethene roles are Higher Tier and are taught separately.
A directional growth response to light: write the technical term.
phototropism means A directional growth response to light.