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Tropisms
Stimulus
• Change in an organism’s surroundings that causes a response– Internal (chemicals and hormones)– External (light, touch, water, gravity)
• All living organisms, including plants, respond to stimuli
Response
• Reaction of a living thing to a stimulus
Stimulus/Response
Stimulus
Response
Internal Stimulus/Response
• Stimulus or response happens inside the organism
• Chemicals and hormones
External Stimulus/Response
• Stimulus or response occurs outside the organism
• Light, touch, water, gravity, electricity, temperature, darkness etc…
Tropism
• Automatic movement by an organism toward or away from a stimulus– Positive (toward)– Negative (away)
Phototropism• When a plant or animal moves toward or
away from light– Positive – toward light– Negative – away from light
Phototropism
Phototropism
Geotropism
• Also called gravitropism• Plants or animals response to gravity
– Positive - roots growing down– Negative - stems growing up
Geotropism
Geotropism
http://plantsinmotion.bio.indiana.edu/plantmotion/movements/tropism/tropisms.html
Hydrotropism
• Directional movement toward water– Usually occurs only in the roots
Hydrotropism
Thigmotropism
• When an organism responds to touch– Vines– Venus flytrap
Thigmotropism
Thigmotropism
Emergence of Seedlings
• Unbalanced forces allow seedlings to push their way through the soil and sometimes rock
Turgor Pressure
• Force in the form of pressure – helps some plants stand upright.– In animals, it also helps blood move around the
body
Turgor Pressure
Turgor Pressure
phototropism
turgor pressure
hydrotropismemergence of seedlings
thigmotropismgeotropism
organism’s response to
light
organism’s response to
gravity
organism’s response to
touch
seedling pushing up through the
ground
organism’s response to
water
pressure inside the plant that
allows it to stand up.