 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

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