Plant Tropisms
What are we talking
about?
Tropism - directional growth of a
plant, or part of a plant, in
response to an external stimulus
Stimulus
Stimuli – an action or condition
that causes a response
Stimuli can be either internal or
external
Types of Stimuli
External stimuli- come from factors found in the
environment.
Examples- temperature, presence of water or
nutrients, light etc
Internal stimuli- come from factors within an
organism
Examples- germs, dehydration, lack of
nutrients, etc.
Stimulus
Response
An organisms reaction to
change.
Response
What are we talking about?
Stimulus – an action or condition that
causes a response
Response – an action or condition that is a
reaction to a stimulus
 Negative response – when the growth of a
plant is away from the stimulus
 Positive response – when the growth of the
plant is towards the stimulus
Example
Stimulus = Lack of water.
Is this internal orExamples
external?
Response = wilting
Is it positive or negative?
Homeostasis
Homeo- same
Stasis- state of balance
The purpose of response to stimuli in
organisms is to maintain
homeostasis, a balance within it’s
internal systems.
What are we talking
about?
Tropism - directional growth of a
plant, or part of a plant, in
response to an external stimulus
Usually named for the stimulus
involved
Types of Tropism
Today we are going to be studying
the types tropisms:
Heliotropism
Hydrotropism
Stimulus: Water
Response: A plant’s roots grow towards
the source of water
 Is this a positive or negative response?
Thermotropism
Stimulus: Temperature
Response: a plant grows in the direction of/away from a
source of heat or cold
 Example: In the first picture, this tree has a positive response to
the cold.
 Example: A plant called a Rhododendron curls its leaves away
when it experiences cold weather.
Geotropism/Gravitropism
Stimulus: Gravity
Reaction:
 Positive – the roots
grow downward, with
gravity
 Negative – the stalk
grows upward, against
gravity
Thigmotropism
Stimulus: Touch
Response – the leaves
(or part of the plant that
is touched) move or coil
away from the touch, or
trigger some other action
 Example: The Venus Fly
Trap is a plant that shows
thigmotropism. It closes
when a bug or some other
object touches it.
Phototropism
Stimulus: Light
Response – the plant grows in the
direction of a unidirectional light (it only
comes from one direction)
 What type of response is this?
Heliotropism
Stimulus: Sunlight
Response – a plant grows or turns to face the direction of
the sun
 Example: Young Sunflowers face the sun all day long. As the sun
goes across the sky, the sunflower turns so that it is always
facing it.
 Is a Sunflower growing towards the sun an example of
Phototropism or Heliotropism?
Working Together
Plant tropisms work together to maintain
homeostasis within the plant




Heliotropism
+Geotropism
-Geotropism
Hydrotropism
References
Pictures:
 http://virtualastronaut.tietronix.com/textonly/act25/images/hydrotropism.
gif
 http://k53.pbase.com/u48/roving_rube/large/30489110.a.c.birch.jpg
 http://www2.brevard.edu/jefrick/rhod.htm
 http://www.xtec.cat/~mherna23/summer03/amspot/picture/gravitropism.
gif
 http://img1.photographersdirect.com/img/17231/wm/pd1198793.jpg
 http://wpcontent.answers.com/wikipedia/commons/9/9d/Dionaea_musci
pula_closing_trap_animation.gif
 http://img1.photographersdirect.com/img/17231/wm/pd1198796.jpg
 http://farm2.static.flickr.com/1423/1402179998_4e1712d584.jpg
Information:
 http://www.hasc.qld.edu.au/csnotes06/biowedGG/Notes%20for%20San
dra/Plant%20Tropisms.doc
 www.wikipedia.org