1.3 STIMULI AND RESPONSES IN PLANTS

1.3 STIMULI AND RESPONSES IN
PLANTS

Prepared by : CINDY LING XIN JIE

➔ CONTENT STANDARD : 1.3 THE STIMULI AND
RESPONSES IN PLANTS

➔ LEARNING STANDARD:
- 1.3.1 Describe the parts of the plant that are sensitive
towards stimuli.
- 1.3.2 Justify how responses in plants ensure their
sustainability and survival.
- 1.3.3 Carry out experiments to study responses in
plants towards various stimuli.

So, let’s check out what are the
stimuli that plants will
respond to.

LOOK AT THE PLANT !

The plant is bending towards the light !

The light is stimulate
the plant to bend
towards it.

Yes, light is one the stimulus for
plants.

Can you think of others ?

What are the stimuli for plants ?

The stimuli for plants are :

● Light
● Water
● Gravity
● Touch

The parts of plants that respond towards stimuli are the shoots or the roots of
the plant.

Shoots

Roots

Plants response is Tropism Positive tropism- the part
divided into two Nastic Movement of plants grow towards

groups. the stimulus

Negative tropism- the part
of plants stay away from

stimulus.

WHAT IS TROPISM?

Tropism is plant-directed response to stimulus. The action is slow.

There are 4 types of tropism:

❏ Phototropism- the response to light
❏ Hydrotropism- the response to water
❏ Geotropism- the response to gravity
❏ Thigmotropism- the response to touch

WHAT IS PHOTOTROPISM ?

Phototropism is where plants respond towards light .

A The shoot grows As we observe the pictures that
shown. The shoot in diagram A
towards the light.

and B shows positive phototropism
because the shoot is growing
towards the light.

B The shoot bends

towards the light.

Does the root of plants grow towards sunlight ?
No. The root of the plant does not respond to light.
Therefore, we say that the root shows negative
phototropism.

SUMMARY

PHOTOTROPISM
Positive phototropism
★ The shoots grow towards the light.
Negative phototropism
★ The roots grow away from the light.

WHAT IS HYDROTROPISM

Hydrotropism is where the plants respond towards water.

❖ Base on what we observed from the diagram A and B, the root in diagram A grows straight
in the moist soil. The root of the plant in diagram B is bending towards the moist soil.

❖ The root of the plant show positive hydrotropism because it growing towards to water.
❖ However, the shoot of the plant is growing upwards. The shoot of the plant shows negative

hydrotropism.

WHAT IS GEOTROPISM

Geotropism is where the plants respond towards gravity.

The root of The shoot of the plant is
plant is growing upwards, and
growing away from the gravity.
towards
gravity. When the vase of the plant is placing vertically,we can see the root of
the plant is growing towards the gravity. The root shows positive
geotropism.

The shoot of the plant is growing away from gravity. The shoot shows
negative geotropism.

WHAT IS THIGMOTROPISM ?

Thigmotropism is where the plants respond towards touch.

The stem of plants is attached to any object or plant in contact with it.

❖ Positive thigmotropism
- The stems of the plants attached to the support such as fence, wooden sticks
or plants to grow upwards.

❖ Negative thigmotropism
- The roots is not showing any respond towards the stimulus.

Soft stem of morning glory is
twinning around to get support

from surrounding to grow
upwards.

Tendrils from the stem of
cucumber plant is twinning
around on wooden stick to get

support.

The importance of tropism of the plants

● Phototropism
- To help the plants to get light for photosynthesis
● Geotropism
- To help the roots penetrate the soil to get water and the roots can anchor on

the ground strongly.
● Thigmotropism
- To help plants grow upward to get light for photosynthesis
- To help the plant to get support.
● Hydrotropism
- To help the roots get water and mineral salts from the soil.

WHAT IS NASTIC MOVEMENT ?

Nastic movement is the reactions of the plants do not
depend on the direction of the stimulus. The action is fast.

The stimulus for nastic movement can be light, heat or
touch.

Example of nastic movement

Mimosa pudica
plants are closed
when touched.
The leaves of the Venus flytrap
close when the insects settle on
it.

The importance of nastic movement of the plants

1. To protect plants from injury
2. To helps the plant to get food.