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Session 74: 6.L.2.2 Explain how plants respond to external stimuli (including
dormancy and forms of tropism) to enhance survival in an environment.
● Changes in environmental conditions can affect the survival of individual organisms
and entire species.
● Dormancy is a period of inactivity in a mature seed prior to germination; seed
remains dormant until conditions are favorable for growth and development of the
new plant.
● Plants have mechanisms that enable them to respond to their environment.
● Plants grow, reproduce, and shift the position of their roots, stems and
leaves in response to environmental conditions such as gravity, sunlight,
temperature and day length.
● Tropism is a plant’s turning or bending movement toward or away from an external
stimulus such as light, heat or gravity.
● If the tropism is positive, the plant grows toward the stimulus.
●
If the tropism is negative, the plant grows away from the stimulus. This enhances
the survival rate for that plant in a given environment.
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Warm Up
1. What is this bat doing?
2. Notice the location of the stamens in this flower. How
does their location ensure the reproduction of this plant?
Changes in environmental conditions can affect the survival of
individual organisms and entire species.
● Dormancy is a period of inactivity in a mature seed prior to
germination; seed remains dormant until conditions are favorable for
growth and development of the new plant.
Dormancy can happen to mature plants as well;
when growth and activity are temporarily
stopped, usually due to environmental
conditions.
When do plants go dormant?
● Plants have mechanisms that enable them to respond to their
environment.
● Plants grow, reproduce, and shift the position of their roots, stems
and leaves in response to environmental conditions such as gravity,
sunlight, temperature and day length.
TROPISMTropism is a plant’s turning or bending movement of an organism toward or
away from an external stimulus such as light, heat or gravity.
(“tropo” – ”turn”)
.
There are many types of tropisms :
• Phototropism
• Geotropism
•Thigmotropism
• Hydrotropism
• Chemotropism
• Thermotropism
3 main types
● If the tropism is positive, the plant
grows toward the stimulus.
●
If the tropism is negative, the
plant grows away from the
stimulus.
● Is the plant above responding
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positively toward sunlight?
● What about gravity?
PHOTOTROPISM
(“Photo” – light)
The growth response of a plant in
response to light direction is
called phototropism.
Ex. Stems growing toward the window to get
to the light
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IMPORTANCE OF
PHOTOTROPISM
Enables leaves to be in the best
position possible to receive adequate
light for photosynthesis
http://www.darienps.org/teachers/otterspoor/botany/tropism
s/Gravitropismwlight.jpg
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GEOTROPISM
(“Geo” – Earth)
Geotropism is the growth
of a plant in response to
gravity.
Negative Geotropism
Positive Geotropism
It is the growth of a plant towards the
center of the earth-down with gravity.
Ex. roots growing down
Negative Geotropism
It is the growth of a plant away
from the center of the earthopposite from the pull of gravity
Ex. stems grow up
Positive Geotropism
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IMPORTANCE OF
GEOTROPISM
• Pulls roots down to anchor a plant
• Roots can get needed water and
minerals if they stay in the soil
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CORN TROPISM LAB
http://www.youtube.com/watch?v=iFCdAgeMGOA&feature=
related
THIGMOTROPISM
(“Thigmo” – “touch”)
Thigmotropism is the growth
of a plant in response to
touch/contact.
Photo by Christopher Meloche
Tendrils on a sweet pea
Discovery Ed Clip on
Plants Responsive to
Touch
Vines growing on a wall or fence
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● All of this enhances the survival rate for that
plant in a given environment.
Time for a StudyJam on Plant
Adaptations:
http://studyjams.scholastic.com/stu
dyjams/jams/science/plants/plantadaptations.htm
Written Response#1:
A plant is placed on a windowsill
facing the sun. After a week, the
plant is rotated away from the sun.
How will the plant most likely
respond?
SOME OTHER
TYPES OF
TROPISM
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THERMOTROPIS
Thermotropism is the
tendency of plants or other
M
organisms to bend toward or away from heat.
(“Therm” – “heat”)
Ex. curling of Rhododendron leaves in response to
cold temperatures.
HYDROTROPISM
(“hydro” – “water”)
Hydrotropism is the growth in
response to water. Ex. roots
growing toward moisture
Chemotropism
(“Chemo” – chemical”)
Chemotropism is movement caused by chemical stimuli.
Ex. Growth of a pollen tube is always towards the ovules
so that reproduction can occur
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