THE TYPES OF MOVEMENT OBSERVED IN PLANTS

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A directional growth movement made by a part of a stationary plant response to unilateral stimulus. The dictionary defines tropism as an orientation of an organism to an external stimuli.

There are numerous types of tropisms :• Hydrotropism • Phototropism• Geotropism• Chemotropism• Thigmotropism• Heliotropism• Thermotropism• And many more………..

(These are the 3 main types of tropism and these would broadly discussed accordingly.)

Stimulus

Tropism

Response

WaterGravity Unilateral light

Shoot

Root

Hydrotropism

No response

Positive

Geotropism

Negative

Positive

Phototropism

Positive

Negative

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GEOTROPISM Geotropism is the growth of a living organism in response to gravity.

There are two types of geotropisms. They are:1. Positive geotropism2. Negative geotropism Positive Geotropism

It is the growth of an organism (i.e. plants) towards the centre of the

earth.

Negative Geotropism It is the growth of an organism

away from the centre of the earth.

An example of geotropism is given in the picture. NEXT

Positive Geotropism

Negative Geotropism

IMPORTANCE OF GEOTROPISMProvides firm anchorage for roots. Ensure the plant can get adequate supply of water & mineral salts.

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PHOTOTROPISMThe growth response of a living organism on response to light direction is called phototropism. Like geotropism even phototropism is of 2 types: 1. Positive

2. Negative

In positive phototropism living organism grow towards the light. For example- Stems are positively phototrophic.

In negative phototropism living organisms grow away from the light. For example- Roots are negatively phototrophic. NEXT

Positive Phototropism

Negative Phototropism

IMPORTANCE OF PHOTOTROPISM

Phototropism enables leaves to be in a position to receive as much sunlight as possible for photosynthesis.

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HYDROTROPISM

Hydrotropism is the growth of a living organism in response to water. Hydrotropism is the directed growth of the root in relation to the gradient in moisture. It begins in the root cap with the sensing of moisture.

Both positive and negative hydrotropism exist in living organisms and its direction of growth depends upon a stimulus or gradient in water concentration.

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Growing towards water (Positive Hydrotropism)

Growing away from water (Negative Hydrotropism)

IMPORTANCE OF HYDROTROPISM

Ensures that a plant gets enough water as a raw material for photosynthesis & enough mineral salts to maintain normal growth

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Experiment

Number 1

Let’s start the experiment

To show Geotropism To show Geotropism in Seedlingsin Seedlings

Q

P

R

In complete darkness

Plumule Radicle

(a) Beginning of experiment

After 2 days

QP

R

(b) End of experiment

In complete darkness

Why is it necessary to leave the apparatus in darkness ?

Answer: To avoid any stimulus from light.

Q

P

R

In complete darkness

Plumule Radicle

(a) Beginning of experiment

After 2 days

QP

R

(b) End of experiment

In complete darkness

Do the radicles show positive geotropism or negative geotropism? Answer: Positive geotropism.

Q

P

R

In complete darkness

Plumule Radicle

(a) Beginning of experiment

After 2 days

QP

R

(b) End of experiment

In complete darkness

What advantage do the seedlings have by showing such geotropism ?

Answer: Roots grow downwards deep into the soil to provide a firm anchorage underground and absorb water and mineral salts in the soil.

EXPERIMENT NUMBER 2

Want to know what a clinostat is ? CLICK HERE

To show the effect of To show the effect of Gravity by using a Gravity by using a

ClinostatClinostat

ClinostatClinostat¥Used for controlling experiments¥Contains a cork disc which can be set to rotate in a vertical or horizontal plane by a motor¥Used to make factors uniform (evenly distributed).

Let’s go back to the experiment.

plastic cover lined with moist blotting paper

X

Y

in complete darkness

seedling in X

seedling in Y

What is the purpose of marking the radicles at

equal intervals ?

Answer: To find out which region of the radicle

grows faster.

plastic cover lined with moist blotting paper

X

Y

in complete darkness

seedling in X

seedling in Y

Does the direction of growth of the seedlings in clinostats X and Y change ?

Answer: The radicle of the seedlings in clinostat X bends downwards in response to the pull of gravity since every side of the radicle of the seedling in clinostat Y is acted on by gravity, the radicle will grow straight horizontally.

plastic cover lined with moist blotting paper

X

Y

in complete darkness

seedling in X

seedling in Y

What is the purpose of setting up clinostat Y ?Ans: Serve as a control for comparison.

To show To show Phototropism in Phototropism in

ShootsShoots

EXPERIMENT NUMBER 3

Let’s start the experiment

unilateral light

unilateral light

in complete darkness

A B C

light-proof box clinostat

What has happened to the shoots of plants A, B and C ?

Ans: The shoots in pot A respond by growing towards the light source. The shoot in pot B do not show any curvature but grow vertically upwards the shoots in pot C grow vertically upwards with slender and longer nodes, and carries smaller leaves which are yellow in colour.

unilateral light

unilateral light

in complete darkness

A B C

light-proof box clinostat

What is the effect of the clinostat on the shoot of plant B ?

Answer: The revolving clinostat cancel out the effect of unilaterallight on the shoot of

plant B.

unilateral light

unilateral light

in complete darkness

A B C

light-proof box clinostat

What is the significance of phototropism to plants ?Answer: This tropism ensures that a plant gets enough light energy for photosynthesis.

Phototropism in Phototropism in ShootShootThe shoot tip produces auxins which diffuse

downward.The auxins diffuse to the shaded side due to the presence of unilateral light.

High auxin concentration stimulates growth in shoot.

The shaded side grows faster than the lighted side

The shoot bends towards the light.

Geotropism in RootGeotropism in Root

The root tip produces auxins which diffuse to the elongation region.

The auxins diffuse to the lower side due to gravity.

High auxin concentration inhibits growth in root.

The lower side grows slower than the upper side.

The root bends downward

SOME OTHERTYPES OF

TROPISM

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THIGMOTROPISM

Thigmotropism is the growth of a living organism in response to a touch/contact. As in all tropisms there exist positive and negative thigmotropism.

Positive thigmotropism is where a living organism grows and clings to a wall or a fence.

Negative thigmotropism is the opposite. It is where a living organism grows away from a touching object.

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HELIOTROPISMHeliotropism is quite similar to phototropism. Now, as we know that phototropism is the response of a living organism in response to light but heliotropism is the diurnal motion of the plant flowers or leaves in response to the direction of the sun. The most famous examples of heliotropism is the sunflower. Heliotropism was first described by Leonardo Da Vinci.

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THERMOTROPISMThermotropism is the tendency of plants or other organisms to bend

toward or away from heat. s the movement of a plant or plant part in response to changes in temperature. A common example is the curling of Rhododendron leaves in response to cold temperatures.

Thermotropism is often called thermotropic movement .

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Chemotropism is movement caused by chemical stimulus in organisms such as bacteria and plants. An example of chemotropic movement can be seen during the growth of the pollen tube. This growth of the pollen tube is always towards the ovules.

Chemotropism

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