Growth Response of Plants
Prepared by Ms W.S.Kwan
Plants have no nervous system.
Can plants detect stimulus?
Can plants response?
A directional growth movement
made by a part of a stationary
plant in response to unilateral
stimulus
Types of Tropism
stimulus
gravity
unilateral light
water
tropism
geotropism
phototropism
hydrotropism
Geotropism
• Geotropism is the growth
movement (growth response)
shown by a plant part in
response to gravity
To show
Geotropism in
Seedlings
To show Geotropism in Seedlings
Why is it necessary to leave the apparatus in darkness ?
Ans: To avoid any stimulus from light.
To show Geotropism in Seedlings
What has happened to the radicles and plumules ?
Ans: The radicles grow downwards, while the plumules
grow upwards.
To show Geotropism in Seedlings
Do the radicles show positive geotropism (towards
gravity) or negative geotropism (away from gravity)?
Ans: Positive geotropism.
What advantage do the seedlings have by showing such
geotropism ?
Ans: Roots grow downwards deep into the soil to provide
a firm anchorage underground and absorb water and
mineral salts in the soil.
Importance of Geotropism
(Roots grow towards gravity)
provide firm anchorage for roots
 ensure the plant can get
adequate supply of water &
mineral salts

Clinostat
 for control experiments
 contains a cork disc which can be set to rotate
in a vertical or horizontal plane by a motor
 Cancel out the effect of unilateral stimulus
 to make factors uniform (evenly distributed)
To show the effect of
Gravity by using a Clinostat
plastic cover lined with
moist blotting paper
X
in complete
darkness
Y
What is the purpose of marking the radicles at equal
intervals ?
Ans: To find out which region of the radicle grows faster.
plastic cover lined with
moist blotting paper
seedling in X
X
in complete
darkness
seedling in Y
Y
Does the direction of growth of the seedlings in clinostat
X change ?
Ans: The radicle of the seedlings in clinostat X bends
downwards in response to the pull of gravity.
plastic cover lined with
moist blotting paper
seedling in X
X
in complete
darkness
Y
seedling in Y
Does the direction of growth of the seedlings in clinostat
Y change ? Why ?
Ans: Since every side of the radicle of the seedling in
clinostat Y is acted on by gravity, the radicle will
grow straight horizontally.
plastic cover lined with
moist blotting paper
seedling in X
X
in complete
darkness
seedling in Y
Y
What is the purpose of setting up clinostat Y ?
Ans: Serve as a control for comparison.
Shoots show negatively geotropic
Phototropism
• Phototropism is the growth
movement (growth response)
shown by a plant part in
response to light.
To show Phototropism
in Shoots
A
unilateral
light
B
C
unilateral
light
light-proof
box
clinostat
in complete darkness
What is the effect of the clinostat on the shoot of plant B ?
Ans: The revolving clinostat cancel out the effect of
unilateral stimulus (light) on the shoot of plant B.
A
unilateral
light
B
C
unilateral
light
light-proof
box
clinostat
in complete darkness
What has happened to the shoots of plants A and B ?
Ans: The shoots in pot A respond by growing towards the
light source.
The shoot in pot B do not show any curvature but
grow vertically upwards.
A
unilateral
light
B
C
unilateral
light
light-proof
box
clinostat
in complete darkness
What has happened to the shoot of plant C ?
Ans: the shoots in pot C grow vertically upwards with
slender and longer nodes(elongated), and carries
smaller leaves which are yellow in colour (etiolated).
A
unilateral
light
B
C
unilateral
light
light-proof
box
clinostat
in complete darkness
What is the significance of phototropism to plants ?
Ans: This tropism ensures that a plant gets enough light
energy for photosynthesis.
Importance of Phototropism
(Shoots grow towards light source)

enables leaves to be in a position
to receive maximum sunlight for
photosynthesis
Hydrotropism
• Hydrotropism is the growth
movement (growth response)
shown by a plant part in
response to water.
Importance of Hydrotropism
(Roots grow towards water supply)
ensures that a plant gets
 enough water as a raw material
for photosynthesis &
 enough mineral salts to maintain
normal growth
Types of Tropism
stimulus
gravity
unilateral light
water
tropism
geotropism
phototropism
hydrotropism
shoot
negative
positive
no response
root
positive
negative
positive
response
Mechanism
Darwin Experiments
Experiments
on Oat Coleoptiles
cut & replaced tip
dark box
few days later
decapitated
no growth
Conclusions :
The tip is responsible for the growth
grows
Experiments
on Oat Coleoptiles
unilateral
light
few days later
black covers
Conclusion :
The tips are sensitive to light.
(responsible for the detection of light)
Experiments
on Oat Coleoptiles
dark box
agar plate
few days later
Conclusions :
The tip must have produced a diffusible
chemical substance which stimulates growth
Experiments
on Oat Coleoptiles
bending directions
bending directions
mica
plate
Can the explanation still work ?
the tip is put aside
Explanation of results :
The side with the tip has a higher concentration of that particular
chemical, the growth rate is faster than the other side.
Auxins
 commonest plant hormones
 produced by shoot tip
 regulate growth & other physiological
processes in plants
 promote growth by inducing cell
elongation & cell division.
Effect of auxin on Plant Parts
positive
growth
shoot
Growth of
plant
negative
growth
low
conc.
plant growth
auxin
concentration
high
conc.
root
auxin concentration
high conc.
shoot
Positive (promote)
root
Negative (inhibit)
low conc.
no effect
Positive (promote)
Phototropism in Shoot

The shoot tip produces auxins which diffuse downward

The auxins diffuse to the shaded side due to the presence
of unilateral light
 High auxin concentration
stimulates growth in shoot
 The shaded side grows faster
than the lighted side

The shoot bends towards the light
Geotropism in Root
 The auxins diffuse to the lower side due to gravity

High auxin concentration inhibits growth in root

The lower side of root grows slower than the upper
side
 The root bends downward
~END~