Plants grow and bend in response to external stimuli through a process called tropism. There are several types of tropism, including phototropism (response to light), geotropism (response to gravity), and hydrotropism (response to water). Geotropism causes roots to grow downward and stems to grow upward, while phototropism causes plant parts to grow toward or away from light. Tropisms allow plants to effectively absorb water, minerals and light via controlled growth. Hormones play a key role in tropisms by regulating plant cell growth.

1. Tropism
6th Grade Science

2. Have you ever wondered how
seedlings “know” which way to grow?
• The plant must send its roots down into the
soil for minerals and water.
• The plant must send its stems up into the
light and air so that it can manufacture
food.
• Seeds grow the correct way no matter
which way they are planted into the
ground.

3. Tropism?
• Tropism is the growing or bending of a
plant in response to an external source of
stimulation.
• Several different types of tropism:
– Phototropism (photo = light)
– Geotropism (geo = earth)
– Hydrotropism (hydro = water)

4. Geotropism
• In growing straight up and down, the plant is
responding to a very powerful force – gravity.
• Geotropism is the way a plant grows or bends in
response to the pull of gravity.
• The roots respond positively to gravity by
growing down into the ground.
• The stems and leaves respond negatively to
gravity by growing up into the light and air.
• Click here to see a video showing geotropism.
(The video is courtesy of Roger Hangarter's lab)

5. Phototropism
• Triggered by light.
• Phototropism is the way a plant grows
or bends in response to light.
• The roots respond negatively to light by
growing away from it.
• The stems and leaves respond
positively to light by growing towards it.
• Click here to view a video showing
phototropism.
The video is courtesy of Roger Hangarter's lab

6. Hydrotropism
• Triggered by water.
• The way a plant grows or bends in
response to water.
• More difficult to observe; plant’s roots
will grow towards or away from water.

7. Three important stimuli for plant
growth
• Touch- Some plants show
a response to touch
called thigmotropism.
The stems of vines, such
as grapes coil around
objects when they grow.
• Light- All plants exhibit a
response to light called
phototropism. The plants
will grow towards the
light, showing a positive
phototropism.
• Gravity-Plants also
respond to gravity. This is
called gravitropism.
Roots show positive
gravitropism—they grow
downward. Stems, show
negative gravitropism—
they grow upward.

8. Hormones and Tropisms
• Plants are able to
respond to touch, light,
and gravity because they
have hormones. A
hormone produced by a
plant is a chemical that
affects they way that they
grow and develop.
• One important hormone is
called auxin. Auxin
speeds up the rate at
which a plant’s cells grow.

9. Photoperiodism
• The amount of darkness a plant receives determines the
time of flowering in many plants. A plant’s response to
seasonal changes in length of night and day is called
photoperiodism.

10. Winter Dormancy
• As winter draws near,
many plants go into a
state of dormancy.
• Dormancy is the period
when an organism’s
growth or activity stops.
• Dormancy helps plants
survive during the winter
months. It helps them
survive freezing
temperatures and lack of
water.

11. Life Spans of Angiosperms
• Angiosperms are classified as annuals, biennials, and
perennials.
• Flowering plants that complete a life cycle in one growing
season are called annuals.
• Biennials will complete their life cycle in two years. In the
first year, they germinate and grow roots, with very short
stems and leaves. In the second year, they lengthen
their stems and produce flowers and seeds.
• Perennials will live more than two years. Most will flower
every year. They will die during the winter months, and
new ones will grow during the spring.
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