September 30, 2012

1. 7th Grade Science
Ms. DeFreese
September 24, 2012 Photo by: Andrey Armyagov

2. • Since plants must make their own food
to survive the ability to capture as much
of this light energy as possible is crucial
to its survival.
• Thus, plants have developed a chemical
response to light or the lack of it that
causes their stems to bend toward the
stronger light.

3. • Tropisms are directional movement
responses in plants that occur in
response to a directional stimulus.
• Tropisms are controlled by a plant
hormone called auxin.

4. A Tropism is a growth
movement
• Direction is determined by the
direction from which the stimulus
strikes the plant.
• Positive = the plant, or a part of
it, grows in the direction from which
the stimulus originates.
• Negative = growth away from the
stimulus.

5. Tropisms are
named
based upon
the stimulus.
• Gravitropism
–The way a plant
grows or bends
in response to
gravity or earth.

6. Phototropism =
Light Response
–Stems are
positively
phototropic.
–Roots are
negatively

8. Gravitopism =
Gravity Response
–Stems are
negatively
gravitropic
while
–roots are
positively

10. The adaptive value of
these tropisms is clear.
• Roots growing down and/or away
from light are more likely to find
the soil, water, and minerals they
need.
• Stems growing up and toward the
light will be able to expose their
leaves so that photosynthesis can

11. Roots growing down and/or away from
light are more likely to find the
soil, water, and minerals they need.
Stems growing up and toward the light
will be able to expose their leaves so
that photosynthesis can occur.
The adaptive value of these
tropisms is clear.

14. Use the folding Venn to compare
and contrast phototropism and
gravitropism.
Be prepared to share