A change in an organism’s surroundings that causes the organism to react.

External Stimulus

Stimulus occurring on the outside

Example: sun causes you to squint your eyes

Internal Stimulus

Stimulus occurring on the inside

Example: you get goose bumps because you are cold

a reaction to a stimulus

A plant’s response to a stimulus

Three types of Tropisms are:

Geotropism

Phototropism

Thigmotropism

the way a plant grows or bend in response to the Earth (or gravity).

the way a plant grows or bend in response to light.

               

Thigmotropism

the way a plant grows or bend in response to touch

A plant’s response to water

Responses can be positive or negative.

A Positive response is when the plant moves toward the stimulus

A Negative response is when a plant moves away from the stimulus

Roots respond positively to gravity by growing down into the soil

Trunk and branches respond positively to light by growing toward the light

Trunk and branches respond negatively to gravity by growing up toward the sky

The pressure of the cell contents against the cell wall

A force is exerted on the cell wall that keeps the plant rigid

NO TURGOR PRESSURE TURGOR PRESSURE