STIMULI AND RESPONSE IN PLANTS

• . Two types of responses: tropism and nastic movement

• Tropism - Growth response in a particular direction          - Occurs slowly, not noticeable

TROPISM

• Tropism is the response by plant to stimuli.

• 2 types of tropism:

• a. positive tropism -response by plants towards the stimuli

• b. negative tropism -response by plants away from the stimulus.

• Plants responds to light , gravity, and water also stimulus of touch.

Type of tropism

• The tropic movement are important to plants because these movements help the plants to get necessities like light, water and minerals. This enable plants to grow healthy.

Type of tropism

stimulus

phototropism light

geotropism gravity

Hydrotropism water

Thigmotropism touch

Nastic movement ( seismonasty)

touch

Responses of different parts of the plant

Response part of plant Direction of growth

advantage

Positive phototropism

Stem tip Growth towards light

To get maximum light for photosynthesis

Negative phototropism

Root tip Growth away from light

Less chance of drying out

Positive geotropism

Root tip Towards gravity More chance of finding moisture

Negative geotropism

Stem tip Away from gravity

More chance of finding light

Phototropism-light

- Response of plant towards light

- Shoot grow towards light (positive phototropism) while root grow away light (negative phototropism)

• Advantage: Enables plants to

obtain sunlight for

photosynthesis

Geotropism-gravity- Response of plant towards gravity - Roots grow downwards (positive

geotropism) and shoot grow upwards (negative geotropism)

-Advantage: Support the plants and enables them to obtain water and minerals

Hydrotropism-water• Response of plant towards

water - Root grow towards water (positive hydrotropism) and shoot grow away from water (negative hydrotropism)

• -Advantage: Enables plants to obtain water

Thigmotropism-touch or contact  - Plant response

towards touch or contact

- Root grow away from solid objects such as stone

- - Stem or tendrils grow towards the objects that come into contact such as cucumber and bitter gourd.

- Advantage: Support the plants

Nastic Movements

         - Response part of plant towards touch, light and heat

         - Does not depend on the direction of the stimulus         - Movement is reversible         - Example: Mimosa pudica, Venus fly trap

- Advantage : Acts as defence

Importance of plant responses

•     - To ensure plant get their basic need.    - Plant depend on phototropism to get

sunlight for photosynthesis.    - Geotropism and hydrotropism enables

the plant to obtain water and minerals salt

That’all• Thank you. See you in the next chapter 5