A directional growth movement response to a
directional stimulus.
It is the involuntary response of plants
involving orientation towards ( Positive ) or
away from (negative) an external stimuli.
Types of Tropism
stimulus
gravity
Directional light
water
tropism
geotropism
phototropism
hydrotropism
shoot
negative
positive
no response
root
positive
negative
positive
response
Importance of Geotropism
provide firm anchorage for roots
 ensure the plant can get
adequate supply of water &
mineral salts

To show Geotropism
in Seedlings
in complete darkness
radicle
plumule
after 2 days
P
Q
in complete darkness
R
(a) Beginning of experiment
P
Q
R
(b) End of experiment
What has happened to the radicles and plumules ?
Ans: The radicles grow downwards, while the plumules
grow upwards.
in complete darkness
radicle
plumule
after 2 days
P
Q
in complete darkness
R
(a) Beginning of experiment
P
Q
R
(b) End of experiment
Why is it necessary to leave the apparatus in darkness ?
Ans: To avoid any stimulus from light.
in complete darkness
radicle
plumule
after 2 days
P
Q
in complete darkness
R
(a) Beginning of experiment
P
Q
R
(b) End of experiment
Do the radicles show positive geotropism or negative
geotropism ?
Ans: Positive geotropism.
in complete darkness
radicle
plumule
after 2 days
P
Q
in complete darkness
R
(a) Beginning of experiment
P
Q
R
(b) End of experiment
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.
Clinostat
 for control experiments
 contains a cork disc which can be set to rotate
in a vertical or horizontal plane by a motor
 to make factors uniform (evenly distributed)
To show the effect of
Gravity by using a Clinostat
plastic cover lined with
moist blotting paper
seedling in X
X
in complete
darkness
seedling in Y
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
clinostats X and Y 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
seedling in Y
Y
Does the direction of growth of the seedlings in
clinostats X and Y change ?
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.
Gravitropism Movies
Corn Root Gravitropism
Shoot Gravitropism
Importance of
Phototropism

enables leaves to be in a position
to receive as much sunlight as
possible for photosynthesis
To show Phototropism
in Shoots
A
unilateral
light
B
C
unilateral
light
light-proof
box
clinostat
in complete darkness
What has happened to the shoots of plants A, B and C ?
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 shoots of plants A, B and C ?
Ans: the shoots in pot C grow vertically upwards with
slender and longer nodes, and carries smaller leaves
which are yellow in colour.
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 (DIRECTIONAL) light on the shoot of plant B.
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.
Movies
Tomato Plant Phototropism
Sun Flowers Tracking
Phototropism
Importance of
Hydrotropism
 ensures that a plant gets enough
water as a raw material for
photosynthesis & enough mineral
salts to maintain normal growth
Auxins
 commonest plant hormones
 regulate growth & other physiological
processes in plants
 produced by root tip / shoot tip
 they affect the elongation region of the
tip
Effect on Plant Parts
positive
growth
shoot
Growth of
plant
negative
growth
low
conc.
plant growth
auxin
concentration
high
conc.
root
auxin concentration
high conc.
low conc.
shoot
positive
no effect
root
negative
positive
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 root tip produces auxins which diffuse to the
elongation region
 The auxins diffuse to the lower side due to gravity

High auxin concentration inhibits growth in root

The lower side grows slower than the upper side
 The root bends downward
Thigmotropism


Thigmotropism is the directional response of a
plant organ to touch or physical contact with a
solid object
Ethylene gas
Thigmotropism
Movies
Morning Glory “Hunting”
Nastic Movement
Venus Flytrap movement

Plant movement that is
caused by an external
stimulus, such as light or
temperature, but is
directionally independent
of its source, unlike
tropisms
Sun Flower Nutation. ( Not in response to
Light direction)
Nastic Movies
Sensitive Plant
Nyctinasty

Sleep movements, where the leaves or flowers of some
plants adopt a different position at night, are described
as nyctinasty
Circadian Responses Plants, like other living things, have biological
clocks that allow them to respond to changes in time. Among the
more obvious clock responses are "sleep movements" such as the closing of
flowers and changes in leaf position that many plants display at night
~END~