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Plant Responses
Part 4
Stimulus and Response
• Stimulus – causes a response
• Response – reaction to a stimulus
• Ex: clapping in someone’s face
– Stimulus – clap
– Response – flinching
Tropisms
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Plant’s response to an external stimulus
Not reversible, but can change direction
Positive tropism – grows towards stimulus
Negative tropism – grows away from stimulus
Types of Tropisms
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Phototropism – response to light
Gravitropism – response to gravity
Thigmotropism – response to touch
Nastic responses – response to stimulus, but
doesn’t have a direction
Phototropism
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Positive – plants grow towards light
Negative – plants grow away from light
Stems are positive for phototropism
Roots are negative for phototropism
Gravitropism
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Positive – plants grow towards (down) gravity
Negative – plants grow away (up) from gravity
Stems are negative for gravitropism
Roots are positive for gravitropism
Thigmotropism
• Growth response to touch
• Some plants grow towards something to touch
• Ex: vines growing up a wall, tomato plants
growing up a stick, etc.
Nastic Response
• Doesn’t deal with plant growth
• More immediate response
• Some plants will close up when close to heat
or fire
• Venus fly traps snap shut when they are
touched by insects