Phototropism
Tropism
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Tropism = a growth movement. Plants cannot
move (walk, run, crawl) towards or away from
stimuli so they must grow to change their
orientation with respect to the stimulus.
Geotropism – response to gravity
Hydrotropism
Phototropism
How Science Works
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The study of phototropism is a classic example
of how the scientific process works.
It took over a century of experiments to
unravel phototropism.
Various scientists were involved, each testing
and building on work done by their
predecessors.
What causes plants to grow towards
the light?
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What hypotheses can you come up with to
explain this tropism?
Write down as many different possibilities as
you can.
Darwin and Peter Boysen Jensen
Arpad Paal
Three theories
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Light inhibits IAA production on the
illuminated side
IAA is produced equally on all sides but
transported to the shaded side
IAA is produced in equal amounts on both
sides but is destroyed by light on the
illuminated side.
Winslow Briggs et al
IAA and Geotropism (gravitropism)
Plant growth factors
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1.
2.
IAA is a plant growth factor, sometimes referred to
as plant hormones
Plant growth factors are different from animal
hormones in several important ways:
They are produced by cells distributed throughout
the plant rather than in special organs
Many have their effect on the tissues that produces
them rather than being transported away from their
site of production.
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Like hormones though they have widespread
effects
Their effects are long lasting