Q1. Explain the difference between tropic and nastic movements in plants, giving suitable examples.
Answer:
Tropic movements are growth movements of plants towards or away from a stimulus. These are directional – the direction of response depends on the direction of the stimulus, such as light or gravity. Example: When a plant stem bends towards sunlight (phototropism).
Nastic movements are non-directional responses. Here, the movement does not depend on the direction of the stimulus, but only on its presence. Example: The leaves of the Mimosa pudica fold up when touched (thigmonasty).
Another example: Tulip flowers opening or closing with changes in light (photonasty).
Both kinds of movements help plants respond to changes in their environment, but they do so in different ways.
The main difference lies in whether or not the direction of movement is linked to the stimulus.
Q2. How do auxins help a plant bend towards sunlight? Describe the process in simple steps.
Answer:
Auxins are plant hormones that encourage cell elongation.
When only one side of a plant stem is exposed to light, auxins move away from the light and collect on the shaded side.
Cells on this shaded side grow longer due to more auxin presence.
This causes the stem to bend towards the light, a process called positive phototropism.
So, auxins play a direct role in enabling plants to get maximum sunlight for making food.
Without auxins, this bending movement would not occur as strongly.
Q3. Why do roots grow downwards and stems grow upwards? Explain using the concept of geotropism and plant hormones.
Answer:
Roots show positive geotropism because they grow towards the force of gravity.
Stems show negative geotropism as they grow away from gravity (upwards).
In roots, auxin accumulates on the lower side due to gravity but, here, high auxin concentration inhibits cell growth, causing the upper side to grow faster, bending roots downward.
In stems, more auxin on the lower side makes those cells grow longer, so the stem bends upwards.
This coordinated movement helps plants stay anchored and reach sunlight for photosynthesis.
The response to gravity ensures proper orientation for survival.
Q4. Describe two real-life examples where plant tropisms are beneficial for human activities such as agriculture or gardening.
Answer:
Phototropism is used by gardeners when placing plants near windows – stems grow towards light, ensuring better growth and more leaves for food production.
Hydrotropism helps in farming dry areas. Roots naturally grow towards water, so seeds sown in the right spots can reach underground moisture, improving crop survival.
Thigmotropism is seen in vines (like grapes or peas) winding around supports; this allows vertical growth and space-saving in gardens or vineyards.
By understanding these responses, farmers train plants for optimal space and resource use.
Such knowledge leads to increased yields and healthier plants.
Q5. How do plant hormones interact to control the growth and development of a plant? Give examples of at least two hormones.
Answer:
Plant hormones rarely act alone; they interact with each other for balanced growth.
For example, auxins promote stem growth while abscisic acid (ABA) can stop growth and promote dormancy.
Cytokinins encourage cell division and delay aging, but when auxins are also present, they work together to allow shoots to form and branches to grow.
Gibberellins promote stem elongation and seed germination, while ethylene causes fruit to ripen and leaves to fall.
The balance between these hormones ensures a plant grows, flowers, fruits, and survives stress.
If one hormone level changes, it affects the actions of others, leading to coordinated plant behaviour.
High Complexity (Analysis & Scenario-Based)
Q6. Imagine a farmer wants faster fruit ripening and healthy bushier plants. Which plant hormones should he use or regulate, and how would it affect the plants?
Answer:
To speed up fruit ripening, the farmer should use ethylene. Ethylene is a gaseous hormone that triggers fruit ripening and can be used to ripen bananas or mangoes quickly.
For bushier and healthier plants, the farmer should increase the use of cytokinins. Cytokinins stimulate cell division and growth of side branches (lateral buds), making plants fuller and more productive.
By using cytokinins, the plants' leaves stay green longer and do not age quickly.
Using these hormones in the right amount ensures better quality fruits and more yield per plant.
However, too much hormone can have negative effects, so a correct balance is essential.
Thus, the right combination and timing of hormones directly impact the harvest and plant health.
Q7. A scientist finds that a plant's leaves close at night and open in the morning, but this does not depend on the direction of light. Identify the type of movement and explain which hormone might be involved.
Answer:
The movement described is a nastic movement because it is non-directional; the leaves respond to the presence or absence of light but not to its direction.
The specific type here is photonasty—opening and closing of leaves or petals in response to light.
This is usually controlled by the internal biological clock of the plant and can also be regulated by hormones such as abscisic acid (ABA), which helps in the closing of leaves.
Photons received trigger chemical changes leading to movement, but there is no bending or directional growth as in tropism.
This movement helps the plant conserve water at night and ensure best use of sunlight during the day.
The ability to time these changes is important for plant survival.
Q8. Describe a scenario in which an increase in abscisic acid (ABA) in a plant would be beneficial. Explain its impact on the plant’s physiology.
Answer:
During a drought or dry weather, an increase in abscisic acid (ABA) is beneficial for plants.
ABA causes stomata (tiny pores on leaves) to close, which greatly reduces water loss by slowing down transpiration.
It also triggers dormancy in seeds and buds, so the plant does not waste resources growing when water is not available.
ABA can lead to leaf fall (abscission), reducing the total leaf area and saving water.
This survival response ensures the plant conserves as much water as possible until the environment becomes favorable again.
Thus, ABA is crucial for managing stress and increasing survival chances during harsh conditions.
Q9. Suppose you observe that a young plant in a dark room grows tall, thin, and bends towards a small hole where light enters. Analyze and explain the hormonal and movement-based reasons for this occurrence.
Answer:
The plant is showing positive phototropism—it bends towards the light source in search of sunlight.
In the dark, plants produce more auxins, which accumulate on the side away from light (the shaded side).
Auxins cause the cells on the shaded side to elongate more than those on the lighted side, leading to bending.
The plant also grows tall and thin (a process called etiolation) as it "reaches" for available light, caused by high auxin levels stimulating vertical growth.
This growth pattern helps the plant maximize its chances to capture sunlight for photosynthesis.
If sunlight becomes available, normal, stronger, and less "leggy" growth happens.
Q10. Analyze how the misuse or excessive artificial application of gibberellins or ethylene might adversely affect plant growth or agriculture.
Answer:
Excessive use of gibberellins might cause plants to grow abnormally tall and weak, making them prone to lodging (falling over) in wind or rain. This leads to loss of yield and poor crop quality.
Overuse of ethylene could result in fruit ripening too quickly, making fruits soft before they reach the market or before people can buy them.
High ethylene levels might also cause premature leaf drop (abscission) and flower wilting, reducing the beauty and lifespan of plants or crops.
Such misuse could disrupt the plant’s natural growth cycle, reduce productivity, and increase waste.
These examples show why the balanced and controlled use of plant hormones is vital in agriculture.
Responsible application improves crops, but too much causes harm.
