Jan 18, 2016
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