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Chapter 25
25-2
Plant Responses
I. Tropisms
 A.
Tropism is a slow plant
movement, towards or away from a
stimuli, that is determined by the
direction of an environmental
stimulus.
 1. 2 Types
a.
Positive tropism –
movement towards a stimuli.
b. Negative tropism –
movement away from a
stimulus.
2. Phototropism

A. A plant movement in response to
light.
 1. Light causes hormone Auxin to
move to the shaded side of the
shoot, elongating those cells causing
the plant to lean towards the light.
a.
Solar tracking – leaves
tracking the sun’s light and
moving with the sun across the
sky.
3. Thigmotropism
A. A plant’s growth
response to touch or
touching a solid object.
Some plants coil when
they touch an object like
the growth of vines.
4. Gravitropism
A. A plants growth response to
gravity.
1. Positive gravitropism – roots
grow down with the
gravitational pull.
2. Negative gravitropism –
stem growth up and away from
the gravitational pull.
5. Nastic Movement
A.
A. A plants rapid movement
that occur in response to
environmental stimuli that
are independent of the
direction of the stimuli.
These movements are
regulated by changes in
the water pressure of
certain plants.
6. Thigmonastic
a. Venus Fly Trapa combination of
osmotic pressure
and cell wall
expansion causes
the leaf to snap
shut.
7. Nictinastic
A. Nictinastic –
plants movement in
response to the
daily cycle of
light and dark.
Turgur pressure is
more gradual.
8. Photoperiodism
a. Timing of seasonal
activity such as flowering
and growth.
Phytochrome, a plant
hormone is said to be
responsible for
photoperiodism.
1. Short-day plants – flower
when the days are short.
2. Long-day plants – flower
when the days are long.