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Coordination in Plants – CBSE Class 10 Science

Plants coordinate their functions without a nervous system or muscles—unlike animals. Instead, they use chemical means for control and communication. Their responses to the environment are mainly through slow movements, regulated by plant hormones. These movements include tropisms (directional) and nastic movements (non-directional).

Let's break down the key ideas with details, examples, and some fun insights!

Key Point 1: Movements in Plants

A. Tropic Movements (Directional Responses) B. Nastic Movements (Non-directional Responses)

A. Tropic Movements (Directional Responses)

Tropic movements are growth movements determined by the direction of a stimulus in the environment. The main types of tropisms are:

  1. Phototropism (Response to Light)
  2. Geotropism / Gravitropism (Response to Gravity)
  3. Hydrotropism (Response to Water)
  4. Thigmotropism (Response to Touch)

1. Phototropism (Response to Light)

  • What is it?
    The plant's growth in response to light — it moves towards or away from the light.

  • Important points:

    • Positive phototropism: Shoots grow towards light.
    • Negative phototropism: Roots grow away from the light.

  • Examples:

    1. Place a potted plant near a window. The stem will bend towards the window to get more light.
    2. Sunflowers seem to “follow” the sun across the sky (this tracking is also known as heliotropism).
    3. In a science experiment, if you cover one side of a plant, the uncovered side will elongate faster, causing the stem to bend towards the uncovered, sunlit side.

2. Geotropism / Gravitropism (Response to Gravity)

  • What is it?
    It is the movement of a part of the plant in response to gravity.

  • Important points:

    • Positive geotropism: Roots grow downward, in the direction of gravity.
    • Negative geotropism: Shoots/stems grow upward, against the force of gravity.

  • Examples:

    1. If you plant seeds sideways, roots will always turn and grow downward, even if the seed's position changes.
    2. Take a transparent box, fill with soil, and put a bean seed in horizontally. After a few days, the root curves down and the stem curves up.
    3. In hanging pots, roots will find their way down, regardless of the pot's position.

3. Hydrotropism (Response to Water)

  • What is it?
    Plant roots respond to the presence of water and grow towards areas where moisture is higher.

  • Important points:

    • Roots show positive hydrotropism by growing towards the water source.

  • Examples:

    1. If one side of the soil is moist, roots will bend and grow towards the moist side.
    2. When you water only one side of a plant, the majority of new roots will grow in that direction.
    3. In deserts, deep roots will grow towards underground water sources.

4. Thigmotropism (Response to Touch)

  • What is it?
    The response of plants to touch or physical contact.

  • Important points:

    • Tendrils or other parts of the plant curl or wrap around objects they touch for support.

  • Examples:

    1. Pea plant tendrils wrap around a stick or support.
    2. Cucumber vines attaching themselves to a fence.
    3. Grape vines twisting along wires in a vineyard.

Activity: Observing Tropic Movements

Activity:
Grow a potted plant (like balsam or sunflower) near a window with light coming from one direction.

Step-by-Step Instructions:

  1. Take a young plant in a small pot.
  2. Keep it on a window where light comes from only one side.
  3. Mark the original direction of the plant's stem using a marker or a ribbon.
  4. Observe daily for a week.

Observations:

  • The stem gradually bends towards the light.
  • If you rotate the pot, the stem will slowly adjust and bend towards the new direction of light.
  • This shows phototropism (stem moves towards light), a coordination response by the plant.

B. Nastic Movements (Non-directional Responses)

Nastic movements are responses to stimuli, but the direction of movement does NOT depend on the direction of the stimulus.

Examples:

  1. Mimosa pudica (“touch-me-not” plant): Leaves fold up when touched.
  2. Dandelion / Lotus flowers: Petals close at night and open in daylight, regardless of light direction.

Key Point 2: Role of Plant Hormones in Coordination

Plants produce hormones (chemical messengers) for growth, development, and responses to stimuli. Let's look at the main hormones and their functions.

  1. Auxins
  2. Gibberellins
  3. Cytokinins
  4. Abscisic Acid (ABA)
  5. Ethylene

1. Auxins

  • Function: Promote stem elongation; responsible for most tropic responses.

  • Key facts:

    • They collect on the shaded side of the plant, making cells there grow longer, causing bending towards light.
    • Also involved in root development and preventing early dropping of leaves/fruits.

  • Examples:

    1. When you use rooting powder to propagate plants, auxins are the active ingredient.
    2. Banana stems bending towards light.
    3. Natural auxin: Indole-3-acetic acid (IAA).

2. Gibberellins

  • Function: Promote stem elongation, seed germination, and fruit growth.

  • Key facts:

    • Used commercially for increasing fruit size and for malting barley in brewing.

  • Examples:

    1. Spraying grapes with gibberellins makes them grow larger and longer.
    2. Sugarcane sprayed with gibberellins becomes taller.
    3. Used to speed up germination in seeds.

3. Cytokinins

  • Function: Promote cell division, delay aging in leaves, and help in lateral (side) bud growth.

  • Key facts:

    • Can keep vegetables and cut flowers fresh longer.
    • Promote bushier growth by stimulating sideways shoots.

  • Examples:

    1. Florists use cytokinins to keep bouquets fresh.
    2. Applying cytokinins makes plants bushier by growing more side branches.
    3. Used in tissue culture for rapid plant multiplication.

4. Abscisic Acid (ABA)

  • Function: Inhibits growth, causes seed and bud dormancy, helps leaves fall, and closes stomata during water stress.

  • Key facts:

    • Often called the "stress hormone".

  • Examples:

    1. Leaves fall off trees in autumn due to abscisic acid.
    2. During drought, abscisic acid closes leaf pores to save water.
    3. Seeds remain inactive until the right growth conditions appear.

5. Ethylene

  • Function: Gaseous hormone that ripens fruits, causes leaves to fall, and flowers to wilt.

  • Key facts:

    • Used commercially to ripen bananas, tomatoes, and mangoes.

  • Examples:

    1. Farmers use ethylene to ripen green mangoes before market sale.
    2. A ripe banana can make nearby tomatoes ripen faster—thanks to ethylene gas.
    3. Causes petals to drop when flowers get old.

Activity: Observing Nastic Movement in Mimosa pudica

Activity: Touch the leaves of a Mimosa pudica (touch-me-not) plant and watch how they fold instantly.

Step-by-Step Instructions:

  1. Take a healthy Mimosa plant (it's a popular ornamental plant).
  2. Gently touch its leaves with your finger.
  3. Observe the reaction within a few seconds.

Observations:

  • Leaves fold up quickly at the point of touch and sometimes along the same branch.
  • There is no directional requirement for the stimulus—any touch anywhere produces the movement.
  • This is thigmonasty, a nastic movement coordinated by chemical changes in the plant's cells.

Key Point 3: Summary Table – Plant Hormones and Their Functions

HormoneMain Function(s)Example(s)
AuxinsCell elongation, tropismsBanana bending, rooting powders
GibberellinsStem elongation, germination, fruit expansionSeed germination, larger grapes
CytokininsCell division, delay aging, promote lateral growthFresh flowers, bushy plants
Abscisic acidGrowth inhibition, dormancy, stress responseLeaf fall, seed dormancy during dry weather
EthyleneFruit ripening, leaf fall, flower agingRipening mangoes, banana, tomato, petal fall

Fun Fact

  • If you keep a ripe banana with unripe fruits, the others will ripen faster! That’s ethylene at work.

Scenario-Based Questions

1. Scenario: You place a young plant near a window, and after a few days, it starts leaning towards the window.

  • Question: What is the reason for this? Which hormone is responsible for this bending?
  • Answer: This is phototropism. The stem bends towards the light because auxin hormone accumulates on the side away from the light, causing those cells to grow longer.

2. Scenario: During a dry summer, you observe that some plants close their stomata even during the day.

  • Question: Which hormone helps the plant to close its stomata and why?
  • Answer: Abscisic acid (ABA) helps the plant to close stomata to prevent water loss, which is helpful during water stress or drought conditions.

3. Scenario: You want to keep a bouquet of cut flowers fresh for many days.

  • Question: Which hormone would you use or add to the water, and how does this help?
  • Answer: Cytokinins can be used as they delay leaf aging (senescence) and keep the flowers fresh.

4. Scenario: A farmer wants to ripen his unripe mangoes quickly before taking them to market.

  • Question: What should the farmer do?
  • Answer: The farmer should use ethylene gas or keep the mangoes with naturally ripening fruits to speed up the ripening process.

5. Scenario: You notice that a pea plant's tendril w...

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