Coordination in Plants – Long Answer Questions (CBSE Class 10 Science)
Medium Level (Application & Explanation)
Q1. Differentiate between tropic and nastic movements in plants with suitable examples. Explain how these help plants survive.
Answer:
Plants show two broad types of movements: tropic movements and nastic movements.
Tropic movements are directional growth responses to a stimulus. The direction of bending depends on the direction of the stimulus. Examples:
Phototropism: Shoots show positive phototropism by bending towards light.
Geotropism: Roots show positive geotropism by growing downwards (towards gravity).
Hydrotropism: Roots grow towards H₂O.
Thigmotropism: Tendrils curl around a support on touching it.
Nastic movements are non-directional responses; the movement does not depend on stimulus direction. Examples:
Thigmonasty in Mimosa pudica: Leaves fold on touch.
Photonasty in flowers like lotus/dandelion: Petals open in daylight and close at night.
These movements help plants to maximize light capture, water uptake, mechanical support, and protection (e.g., folding leaves discourage herbivores), improving survival in changing environments.
Q2. Describe phototropism in shoots. How do auxins cause bending towards light? Support your answer with an everyday observation.
Answer:
Phototropism is the plant’s growth response to light, typically seen as shoots bending towards a light source (positive phototropism).
The hormone auxin (e.g., IAA) plays a key role. When light falls from one side, auxin redistributes and accumulates more on the shaded side of the stem.
Auxin promotes cell elongation in stems. As cells on the shaded side elongate more than those on the lit side, the stem bends towards the light.
Everyday observation: A potted plant near a window bends towards the window. If you rotate the pot, the stem slowly reorients and bends again towards the new light direction.
This bending ensures better photosynthesis by exposing more leaf area to light.
In contrast, roots may show negative phototropism (grow away from light), helping them remain in the dark, moist soil—another adaptive advantage.
Q3. Explain geotropism and hydrotropism in roots. When the two stimuli conflict, which response dominates and why?
Answer:
Geotropism (gravitropism) is growth in response to gravity:
Positive geotropism: Roots grow downward.
Negative geotropism: Shoots grow upward.
Hydrotropism is growth in response to water (H₂O) availability:
Positive hydrotropism: Roots bend and grow towards regions of higher moisture.
When these stimuli conflict (e.g., moist soil pocket is lateral, not downward), roots often prioritize hydrotropism because securing water is essential for survival, transport, and maintaining turgor.
Examples:
Seeds sown sideways show roots turning down (geotropism) but also curving towards a moist patch (hydrotropism).
In dry areas, deep or lateral roots grow towards underground water sources.
Thus, roots simultaneously obey gravity to anchor the plant and follow moisture gradients to ensure efficient water uptake, with hydrotropism frequently taking precedence when water is scarce.
Q4. What is thigmotropism? Describe how tendrils use it to climb, and contrast it with thigmonasty in Mimosa pudica.
Answer:
Thigmotropism is a directional growth response to touch. In climbing plants, tendrils sense contact with a support (stick, fence, wire) and then curl or coil around it.
Mechanism and advantage:
On touching a support, growth rates on the two sides of the tendril become unequal, causing it to wind around the object.
This provides mechanical support, allowing the plant to climb quickly to better light, improving photosynthesis and reproduction.
Contrast with thigmonasty:
Thigmonasty is non-directional (independent of stimulus direction) and rapid, seen in Mimosa pudica leaves folding on touch.
It occurs due to quick cellular changes (turgor/ion shifts), not long-term growth.
Adaptive value: Leaf folding may reduce herbivory and water loss.
In short, thigmotropism is a slow, growth-based, directional movement for support, while thigmonasty is a quick, non-directional protective response.
Q5. Explain how the five major plant hormones coordinate growth and survival. Give two examples for each.
Answer:
Plants use chemical coordination via hormones:
Auxins: Promote cell elongation in stems; central to tropisms.
Examples: Bending of shoots towards light; rooting powders contain auxins to induce roots in cuttings.
Gibberellins: Stimulate stem elongation, seed germination, and fruit growth.
Examples: Larger grapes after gibberellin spray; taller sugarcane for higher yield.
Examples: Leaf fall in adverse seasons; stomatal closure reduces water loss in drought.
Ethylene (C₂H₄): Gaseous hormone for fruit ripening, leaf and petal drop.
Examples: Ripening mangoes/bananas for market; flower aging and petal fall.
High Complexity (Analytical & Scenario-Based)
Q6. Design an experiment to prove hydrotropism in roots independent of geotropism. Include setup, controls, observations, and conclusion.
Answer:
Aim: To show that roots respond to water (hydrotropism) even when gravity is constant.
Setup:
Take a transparent box with soil having a moist region on one side and drier soil on the other.
Place germinating seeds horizontally so roots can choose direction. Keep the box fixed (gravity constant).
Alternatively, keep the setup on a clinostat (slowly rotating device) to minimize unidirectional gravity effect.
Controls:
Control box with uniform moisture throughout.
Observations:
In the experimental box, roots bend towards the moist side even if this is sideways, not strictly downward.
In the control, roots show normal positive geotropism (mostly downward) because moisture is uniform.
Conclusion:
Bending towards moisture despite constant gravity shows positive hydrotropism is a distinct, dominant response when H₂O availability is uneven, ensuring better water uptake.
Q7. A greenhouse has three issues: (a) seedlings are long and weak, (b) harvested flowers wilt quickly, and (c) tomatoes ripen unevenly. Use plant hormones and environmental tweaks to propose solutions.
Answer:
(a) Long, weak seedlings:
Reduce unidirectional light by providing uniform lighting; turn trays regularly to avoid excessive phototropic bending.
Avoid adding gibberellins, as they promote stem elongation; ensure adequate light intensity to discourage leggy growth.
(b) Flowers wilt quickly:
Add cytokinins to vase water to delay senescence and keep tissues fresh.
Keep flowers cool and hydrated; remove ethylene sources nearby to reduce petal drop.
(c) Uneven tomato ripening:
Use controlled exposure to ethylene (C₂H₄) for batches to synchronize ripening.
Keep ripe bananas away from non-target produce to prevent accidental ripening; ventilate storage to disperse ethylene when uniform ripening is not desired.
Together, these steps apply knowledge of auxins/gibberellins/cytokinins/ethylene and environmental control to achieve sturdier seedlings, longer-lasting bouquets, and predictable fruit ripening.
Q8. During a prolonged drought, a farmer notices reduced growth but better survival in some crops. Explain the role of ABA in this situation and its trade-offs.
Answer:
Abscisic Acid (ABA) is the plant’s stress hormone. Under water deficit, ABA levels rise and:
Induce stomatal closure, reducing transpiration and conserving H₂O.
Promote dormancy in buds and seeds, slowing growth until conditions improve.
Support leaf fall (abscission) to cut surface area and water loss.
Benefits:
Plants maintain turgor longer, avoid wilting, and survive periods of drought.
Trade-offs:
Stomatal closure also limits CO₂ entry, reducing photosynthesis and thus growth and yield in the short term.
Delayed growth and dormancy postpone flowering/fruiting schedules.
Net outcome:
ABA shifts priority from growth to survival. When rains return, reduced ABA permits stomata to reopen and growth to resume, showing how chemical coordination balances immediate survival with long-term productivity.
Q9. A wholesaler stores mixed fruits in one room and finds that all ripen too fast, causing losses. Using your knowledge of ethylene, suggest a plan to control ripening.
Answer:
Ethylene (C₂H₄) is a gaseous hormone that accelerates fruit ripening and leaf/petal fall.
Problem: Storing ripe bananas or other climacteric fruits with unripe fruits leads to rapid ripening of all due to ethylene diffusion.
Plan:
Separate storage: Keep ethylene-producing ripe fruits away from unripe ones.
Ventilation: Improve air exchange to disperse ethylene.
Staggered batches: For planned ripening, place unripe fruits in a smaller chamber and introduce controlled ethylene for the required time, then move to cool storage.
Avoid physical damage, as bruised fruits produce more ethylene.
For display, do not place ripe bananas next to tomatoes or mangoes unless rapid ripening is intended.
