2. Tropisms and Plant Responses

Plants Respond to Their Environment

Plants may appear motionless, but they constantly detect and respond to changes in their surroundings.

Plants can respond to:

• light
• gravity
• water
• touch
• temperature
• chemicals

Unlike animals, plants cannot usually move their entire bodies from one location to another.

Instead, they often respond by changing:

the direction or rate of growth

A directional growth response to an environmental stimulus is called a:

tropism

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What Is a Stimulus?

A stimulus is a detectable change in the environment that produces a response.

For example:

light from one direction → stimulus

shoot bends toward light → response

Other stimuli include:

• gravity
• moisture
• physical contact

Plants can detect these conditions and adjust their growth accordingly.

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What Is a Tropism?

A tropism is a directional growth response of a plant to a stimulus.

The direction of the stimulus determines:

the direction of growth

For example, if light comes from the left, a young shoot may curve toward the:

left

If the light source is moved to the right, new growth can eventually curve toward the:

right

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Positive and Negative Tropisms

A tropism can be:

positive

or:

negative

A positive tropism means growth:

toward the stimulus

A negative tropism means growth:

away from the stimulus

For example:

shoot grows toward light → positive phototropism

root grows away from light → negative phototropism

The terms positive and negative do not mean:

good or bad

They describe only the direction of growth.

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The Four Major Tropisms

Four important plant tropisms are:

A useful way to remember them is:

photo = light

gravi = gravity

hydro = water

thigmo = touch

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Phototropism

Phototropism is a directional growth response to:

light

Young shoots usually show:

positive phototropism

This means they grow:

toward light

This response can easily be observed in houseplants placed near windows.

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Why Grow Toward Light?

Plants require light for:

photosynthesis

Photosynthesis allows plants to produce glucose.

Therefore, growth toward light can increase:

light absorption

which can improve:

• photosynthesis
• growth
• energy availability
• reproductive success

Positive phototropism is therefore closely connected to a plant's ability to obtain an essential resource.

---

How Does Phototropism Work?

Imagine light reaching a young shoot mainly from one side.

The plant detects the unequal light conditions.

A plant growth hormone called:

auxin

becomes unevenly distributed.

In shoots, more auxin accumulates on the:

shaded side

Auxin stimulates cells there to:

elongate more

The shaded side therefore grows faster than the illuminated side.

The shoot bends:

toward the light

This visualization shows how one-sided light changes auxin distribution and produces curvature.

Unequal Growth Produces Bending

This is an important idea.

The plant does not bend because light physically pulls it.

Instead:

one side grows faster than the other

Imagine the shoot receiving light from the left:

Light → 🌱

The right side is more shaded.

More auxin accumulates on the right.

Cells on the right elongate more.

Therefore, the shoot curves:

left, toward the light

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Auxin

Auxin is an important plant growth regulator.

It is produced mainly in actively growing tissues, including:

shoot tips

Auxin influences:

• cell elongation
• root development
• shoot growth
• tropisms
• apical dominance
• fruit development

In tropisms, auxin helps create:

unequal growth on different sides of an organ

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Phototropism Experiment

A simple experiment can demonstrate phototropism.

Place young seedlings inside a box with:

one opening on one side

Light enters through the opening.

After several days, the shoots usually:

bend toward the opening

The direction of the stimulus predicts the direction of the response.

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What If Light Comes from Directly Above?

If light reaches both sides of the shoot approximately equally:

auxin distribution remains more balanced

Both sides elongate at similar rates.

Therefore, the shoot grows:

relatively straight upward

There is little reason for sideways curvature.

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Gravitropism

Gravitropism, also called geotropism, is a directional growth response to:

gravity

Roots and shoots usually respond differently.

Roots generally show:

positive gravitropism

Shoots generally show:

negative gravitropism

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Positive Gravitropism in Roots

Roots generally grow:

in the direction of gravity

This is positive gravitropism.

Growing downward helps roots enter the soil where they can obtain:

• water
• mineral ions
• anchorage

Therefore:

gravity → downward root growth → improved resource acquisition

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Negative Gravitropism in Shoots

Shoots generally grow:

against the direction of gravity

This is:

negative gravitropism

Growing upward increases the chance that leaves will reach:

light

This is especially important when a seed germinates underground.

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What If a Seed Is Planted Sideways?

Suppose a germinating seed is placed horizontally.

Initially, the root and shoot may point sideways.

Over time:

root curves downward

while:

shoot curves upward

The plant can therefore orient itself even when the seed begins in an unusual position.

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How Do Plants Detect Gravity?

Specialized cells contain dense structures called:

statoliths

These can settle toward the lower side of cells under gravity.

This helps the plant determine:

which direction is down

The signal influences hormone distribution and growth.

As with phototropism:

unequal growth → bending

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Auxin and Gravitropism

Auxin also contributes to gravitropism.

When a plant is placed horizontally, auxin can become redistributed toward the:

lower side

However, roots and shoots respond differently to auxin concentrations.

In shoots, increased auxin generally promotes:

cell elongation

In roots, higher auxin concentrations can:

inhibit elongation

This difference helps explain why roots and shoots curve in opposite directions.

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Shoot Response to Gravity

Consider a horizontal shoot.

More auxin accumulates on the:

lower side

Those cells elongate more.

The lower side becomes longer than the upper side.

Therefore, the shoot bends:

upward

This produces:

negative gravitropism

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Root Response to Gravity

Now consider a horizontal root.

Auxin also accumulates more on the:

lower side

But roots are more sensitive to auxin.

Higher auxin concentration reduces elongation on the lower side.

Cells on the upper side elongate more.

Therefore, the root bends:

downward

This produces:

positive gravitropism

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Phototropism and Gravitropism Work Together

A young plant may experience several stimuli simultaneously.

A shoot may respond to:

light + gravity

A root may respond to:

gravity + water

Plants therefore integrate information from several environmental conditions.

This allows growth to be directed toward locations that improve:

resource acquisition and survival

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Hydrotropism

Hydrotropism is a directional growth response to:

water or moisture

Roots commonly show:

positive hydrotropism

They tend to grow toward areas with greater water availability.

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Why Is Hydrotropism Important?

Water is essential for:

• photosynthesis
• transport
• maintaining cell turgor
• enzyme-controlled reactions
• mineral uptake
• cooling through transpiration

A root system that grows toward available water improves the plant's ability to:

acquire this essential resource

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Roots Search the Soil

Soil moisture is not always evenly distributed.

One region may be:

dry

while another contains more:

water

Roots can respond to moisture gradients.

Growth toward wetter regions allows the root system to exploit:

patches of available water

This is particularly important during periods of limited rainfall.

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Hydrotropism vs Gravitropism

A root can receive more than one signal.

Gravity encourages:

downward growth

Water may encourage:

growth toward moisture

What happens if the wettest soil is not directly below the root?

The final growth direction can result from interactions among:

multiple plant responses

This shows that plant behaviour is more complex than simply "roots always grow downward."

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Thigmotropism

Thigmotropism is a directional growth response to:

touch or physical contact

This response is particularly important in:

climbing plants

Examples include:

• peas
• grapevines
• passionflowers
• some cucumbers

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Tendrils

A tendril is a thin specialized plant structure used for:

climbing and support

When a tendril touches an object, growth becomes unequal on its different sides.

The tendril begins to:

curve around the object

Continued growth can produce a tightly coiled structure.

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Why Is Thigmotropism Useful?

Climbing plants face a challenge.

They need to reach:

light

but producing a thick supportive trunk requires substantial resources.

Instead, some plants use:

other structures for support

A tendril can attach to:

• branches
• stems
• fences
• rocks
• artificial supports

The plant can then climb upward.

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Climbing Saves Resources

A climbing plant can invest less material in:

large supportive stems

and use surrounding structures instead.

This can allow rapid growth toward:

better light conditions

Thigmotropism therefore helps plants compete for light while reducing the cost of producing massive support structures.

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How Does a Tendril Coil?

When part of a tendril contacts a support, signals cause:

different growth rates on opposite sides

One side elongates differently from the other.

This causes the tendril to:

curve

Continued differential growth produces:

coiling around the support

Once again, the underlying principle is:

unequal growth → directional bending

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Comparing the Four Tropisms

All four involve:

detecting a stimulus and changing growth direction

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Tropisms Are Growth Responses

A key feature of a tropism is:

growth

A tropism is not simply any movement.

For example, a tendril gradually wrapping around a support involves:

differential growth

A shoot bending toward light also involves:

differential growth

These responses generally occur much more slowly than animal movements.

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Plants Can Also Move Without Tropisms

Not every plant response is a tropism.

Some plants make relatively rapid movements that are not determined by the direction of the stimulus.

For example, the sensitive plant Mimosa pudica folds its leaves when touched.

This is not the same as thigmotropism because it is not primarily a directional growth response.

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Tropism vs Nastic Response

A tropism depends on:

the direction of the stimulus

A nastic response does not depend primarily on the direction from which the stimulus arrives.

For example:

vine growing toward and around a touched support → tropism

Mimosa leaves rapidly closing after touch → nastic response

This distinction helps prevent the common mistake of calling every plant movement a tropism.

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Hormones Coordinate Plant Responses

Plants do not have:

• brains
• nerves
• muscles

like animals.

Instead, many plant responses are coordinated using:

chemical signals

called plant hormones or plant growth regulators.

Important plant hormones include:

• auxins
• gibberellins
• cytokinins
• abscisic acid
• ethylene

For tropisms, the most important hormone to understand is:

auxin

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Hormones Work at Low Concentrations

Plant hormones are effective in:

very small amounts

They can influence cells in different tissues and regulate:

• growth
• development
• germination
• fruit ripening
• leaf loss
• responses to environmental conditions

A plant's growth is therefore controlled by interactions between:

genes + hormones + environmental signals

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Auxin and Cell Elongation

In young shoots, auxin can stimulate cells to:

elongate

If auxin is distributed evenly:

both sides grow similarly

and the shoot remains relatively straight.

If auxin becomes unevenly distributed:

one side grows faster

and the shoot:

bends

This is the central mechanism behind many tropic responses.

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Differential Growth

Differential growth means different parts of an organ grow at:

different rates

Consider two sides of a shoot:

Side A grows 2 mm

Side B grows 5 mm

The shoot cannot remain straight.

It curves toward the:

shorter side

This simple idea explains much of plant tropism.

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A Useful Model

Think of a flexible strip made of two connected layers.

If both layers increase in length equally:

the strip stays straight

If one layer becomes longer:

the strip bends

A plant organ behaves similarly when cells on one side elongate more than cells on the other.

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Predicting Phototropism

Suppose a seedling receives light only from the:

right

What will happen?

The left side is more shaded.

Auxin accumulates more strongly on the shaded side.

Cells there elongate more.

Therefore:

the shoot bends right, toward the light

This is:

positive phototropism

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Predicting a Change in Light Direction

Suppose the plant has already bent toward a window.

You rotate the pot 180°.

The light is now coming from the opposite side.

What happens?

New growth begins responding to the new light direction.

Over time:

the shoot curves back toward the window

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Predicting Gravitropism

A germinating seed is placed sideways.

Predict the response.

Root:

curves downward.

Shoot:

curves upward.

Therefore:

root = positive gravitropism

shoot = negative gravitropism

This occurs even if the seed itself is positioned horizontally.

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Predicting Hydrotropism

Imagine a root growing through soil.

The soil on the left is:

dry

The soil on the right is:

moist

If other factors are suitable, root growth may curve:

toward the right

This demonstrates:

positive hydrotropism

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Predicting Thigmotropism

A climbing plant grows beside a thin pole.

A tendril touches the pole.

What happens?

The tendril begins:

curving and coiling around the pole

This anchors the plant and allows it to:

climb

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When Stimuli Conflict

Real environments are complicated.

Imagine a root experiencing:

• gravity pulling the normal growth direction downward
• greater water availability toward one side

The plant receives both:

gravitropic and hydrotropic signals

The resulting growth may reflect the interaction between these responses.

Plants constantly integrate:

multiple environmental signals

rather than responding to only one factor.

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Tropisms and Resource Acquisition

Tropisms help plants position their structures where resources are available.

Phototropism

helps shoots obtain light.

Gravitropism

helps roots enter soil and shoots grow upward.

Hydrotropism

helps roots locate water.

Thigmotropism

helps climbing plants reach light using external support.

Therefore, tropisms improve:

resource acquisition

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Tropisms and Survival

A plant that cannot orient its growth effectively may struggle to survive.

Imagine a seedling whose roots grew upward.

It would have difficulty obtaining:

water and mineral ions

Imagine a shoot that consistently grew away from available light.

It would have difficulty performing:

photosynthesis

Directional growth therefore provides important survival advantages.

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Tropisms and Reproduction

Tropisms can also indirectly increase:

reproductive success

A plant that acquires more light, water, and nutrients can potentially produce more:

• flowers
• pollen
• fruits
• seeds

Therefore:

better resource acquisition → improved growth → greater opportunity for reproduction

Tropisms ultimately contribute to the continuation of the species.

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Roots and Shoots Have Different Jobs

The opposite responses of roots and shoots make sense when we consider their functions.

Roots need:

• water
• minerals
• anchorage

Therefore they usually grow:

into soil

Shoots need:

• light
• access to air

Therefore they usually grow:

above the soil

Different tropisms coordinate these contrasting needs.

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A Germinating Seed Underground

Imagine a seed germinating several centimetres beneath the soil.

The plant initially cannot see where the surface is.

Yet:

root grows downward

and:

shoot grows upward

Gravitropism allows the young plant to orient itself before light becomes a major directional cue.

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Once the Shoot Reaches Light

After the shoot emerges above the soil:

phototropism

becomes especially useful.

The shoot can adjust its direction toward:

stronger light

Therefore, different tropisms can become important at different stages of plant development.

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Investigating Phototropism

A simple investigation can compare seedlings under different light conditions.

For example:

Group A: light from above

Group B: light from one side

Group C: complete darkness

Keep constant:

• plant species
• age
• water
• temperature
• soil
• investigation time

Measure:

direction and angle of shoot growth

This allows the effect of light direction to be investigated scientifically.

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Investigating Gravitropism

Germinating seeds can be placed:

horizontally

Their root and shoot directions can then be recorded over several days.

A student could measure:

• root angle
• shoot angle
• time
• growth length

The predicted result is:

roots curve downward while shoots curve upward

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Investigating Hydrotropism

A moisture gradient can be created so that one region around the root is:

wetter

than another.

Root growth direction can then be observed.

Care must be taken because:

gravity also affects root growth

A good investigation therefore needs careful experimental design.

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Investigating Thigmotropism

Climbing plants can be grown with:

supports

and compared with plants grown without suitable supports.

Students can observe:

• tendril contact
• beginning of curvature
• coiling
• climbing direction

This provides visible evidence of a plant growth response to:

touch

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Measuring Tropisms

Instead of simply saying:

"the plant bent"

scientists can collect quantitative data.

For example:

angle of shoot curvature = 35°

Measurements might include:

• curvature angle
• growth length
• time to respond
• root direction
• number of tendrils attached

Quantitative data make comparisons more precise.

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Tropisms in Agriculture

Understanding plant responses has practical applications.

Farmers and horticulturists can manipulate:

• light direction
• plant spacing
• support structures
• irrigation
• plant hormones

to influence plant growth.

For example, climbing crops may be provided with:

trellises

that take advantage of thigmotropism.

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Tropisms in Space

Plant tropisms become especially interesting in:

space

On Earth, gravity provides a strong directional signal.

In microgravity, this signal is greatly reduced.

Scientists study plants in space to understand:

• how roots orient
• how shoots grow
• how light influences growth
• how plants might be cultivated during long space missions

Plant responses therefore have applications beyond Earth.

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Common Misconception: Plants Move Toward Light

It is more accurate to say:

plants grow toward light

Phototropism is a:

growth response

The plant is not walking or moving its whole body toward the light.

Unequal cell elongation causes the shoot to bend.

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Common Misconception: Auxin Moves Toward the Light

During shoot phototropism, auxin becomes concentrated more strongly on the:

shaded side

That side elongates faster.

The shoot therefore bends:

toward the light

Remember:

more auxin on shaded side → more elongation → bending toward light

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Common Misconception: Positive Means Beneficial

In tropisms:

positive = toward

negative = away

A negative tropism is not necessarily harmful.

For example:

negative gravitropism in shoots

is extremely useful because it helps shoots grow upward.

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Common Misconception: Roots Only Respond to Gravity

Roots can respond to several stimuli, including:

• gravity
• water
• chemicals
• touch

Their final growth direction can reflect:

several interacting signals

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Common Misconception: All Plant Movements Are Tropisms

A tropism must be a:

directional growth response

Rapid movements such as the closing of Mimosa leaves are not tropisms.

Likewise, the opening and closing of flowers in response to day-night cycles may involve other types of plant responses.

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Common Misconception: Plants Do Not Sense Their Environment

Plants lack animal-like sense organs, but they can detect many environmental signals.

They can respond to:

• light direction
• gravity
• water
• touch
• temperature
• chemicals

Their responses are coordinated through:

cellular signalling and plant hormones

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Check Your Understanding

1. Define a tropism.

2. Explain the difference between positive and negative tropisms.

3. A shoot grows toward a window. Name the tropism and state whether it is positive or negative.

4. Why do roots normally show positive gravitropism?

5. Explain how auxin causes a shoot to bend toward light.

6. A germinating seed is placed horizontally. Predict the direction of root and shoot growth.

7. A root encounters wetter soil on one side. Which tropism may affect its growth?

8. Explain how thigmotropism helps a climbing plant compete for light.

9. Why is the rapid closing of Mimosa leaves not considered a tropism?

10. Explain how tropisms can ultimately increase a plant's reproductive success.

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Key Terms

• Stimulus: Detectable change in the environment that causes a response.
• Response: Change in an organism resulting from a stimulus.
• Tropism: Directional growth response to a stimulus.
• Positive tropism: Growth toward a stimulus.
• Negative tropism: Growth away from a stimulus.
• Phototropism: Directional growth response to light.
• Gravitropism: Directional growth response to gravity.
• Geotropism: Alternative term for gravitropism.
• Hydrotropism: Directional growth response to water or moisture.
• Thigmotropism: Directional growth response to touch or physical contact.
• Auxin: Plant growth regulator involved in cell elongation and many tropic responses.
• Plant hormone: Chemical signal that influences plant growth, development, or responses.
• Differential growth: Unequal growth rates in different parts of a plant organ.
• Cell elongation: Increase in the length of a cell.
• Tendril: Specialized structure that helps some plants climb.
• Statolith: Dense cellular structure involved in gravity detection.
• Nastic response: Plant response whose direction does not primarily depend on the direction of the stimulus.
• Resource acquisition: Obtaining resources such as light, water, and mineral ions needed for growth and survival.

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Key Takeaways

• Plants detect and respond to environmental stimuli.
• A tropism is a directional growth response to a stimulus.
• Positive tropisms involve growth toward a stimulus.
• Negative tropisms involve growth away from a stimulus.
• Phototropism is a response to light.
• Young shoots normally show positive phototropism.
• Gravitropism is a response to gravity.
• Roots normally show positive gravitropism, while shoots show negative gravitropism.
• Hydrotropism is growth in response to water or moisture.
• Roots commonly show positive hydrotropism.
• Thigmotropism is growth in response to physical contact.
• Tendrils use thigmotropism to wrap around supports.
• Auxin is an important plant growth regulator involved in tropic responses.
• Unequal auxin distribution can produce differential cell elongation.
• Differential growth causes plant organs to bend.
• In shoot phototropism, greater elongation on the shaded side bends the shoot toward light.
• Roots and shoots can respond differently to the same hormone signal.
• Plants often respond to several environmental stimuli at the same time.
• Tropisms help plants obtain light, water, minerals, and physical support.
• Not every plant movement is a tropism; tropisms specifically involve directional growth.
• Tropisms improve survival and can ultimately increase reproductive success.
• The central idea is stimulus → detection/signalling → hormone redistribution → differential growth → directional response.