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Stimulus & Responses
StimulusA change in organism’s
environmental conditions which can affect the organism in either a positive or negative way.
Types of Stimuli▪ A stimulus can either be external or internal.
▪ External stimuli- come from factors found in the environment.Examples- temperature, predators, presence
of water or food, etc▪ Internal stimuli- come from factors within an
organismExamples- germs, dehydration, lack of
energy, pain
Stimulus
Is this stimulus external or internal?
ResponseAn organisms reaction to change.
Response
HomeostasisHomeo- sameStasis- state of balance
The purpose of response to stimuli in organisms is to maintain homeostasis, a balance within it’s internal systems.
Examples
▪ Stimulus = Is this internal or external?▪ Response =
Lack of water
Wilting
Tropisms▪ Tropism = plant’s growth response
toward or away from a stimulus
Tropisms▪ Tropism = plant’s growth response
toward or away from a stimulus
▪ Positive tropism
▪ Negative tropism
Tropisms▪ Tropism = plant’s growth response
toward or away from a stimulus
▪ Positive tropism = plant grows toward a stimulus
▪Negative tropism
Tropisms▪ Tropism = plant’s growth response
toward or away from a stimulus
▪ Positive tropism = plant grows toward a stimulus
▪Negative tropism = plant grows away from a stimulus
Hydrotropism
Hydrotropism = a plant’s roots grow toward water
Is this positive or negative?
PhototropismPlants grow toward a source of light
Examples
ThigmotropismA plant grows toward or away from pressure
Positive turns toward pressure
Negative turns away from pressure
Geotropism▪ A plant grows with or against gravity▪ Also called Gravitropism
Negative- A plant’s stems or trunk grow up, against the pull of gravity.
Positive- A plant’s roots grow down, with the pull of gravity
Regardless of the slope of the land, the tree grows up against gravity.
Regardless of the position a seed is planted in, the roots grow down with gravity and the stem up against gravity.
Dormancy▪ When plants can go through a period of
inactivity. During this time, the seeds wait to germinate until conditions are favorable for growth and development of a new plant. ▪ For example: The seeds of a longleaf
pine tree must be exposed to fire (heat) before they will germinate and grow.
Working Together
▪ Plant tropisms work together to maintain homeostasis within the plant
- geotropism
+ phototropism
+ geotropism
+ hydrotropism