1. 18. Growth responses
of green plants

2. 18 Growth responses of green plants
18.1 Do plants respond to external stimuli?
18.2 What are tropisms?
18.3 The growth response of shoots to light
18.4 The growth response of roots and shoots to gravity
18.5 The growth response of roots to water
18.6 What are the substances for controlling growth
movements in plants?
18.7 More about auxins
18.8 The role of auxins in phototropic and geotropic responses
Mind Map

3. Do Plants Respond to External Stimuli?

4. ? YES! Plants Will it happen Stimulus Response
18.1 Do plants respond to external stimuli? ?
Will it happen
Stimulus
YES!
Plants
Response

5. 18.2 What are tropisms?
What are Tropism?

6. growth 18.2 What are tropisms? Tropism
stimulus
growth
Tropism
directional growth movement made by a plant
Response
in response to a unilateral stimulus
plant

7. 18.2 What are tropisms? Direction of growth towards stimulus
away from stimuli
negative tropism
positive tropism
stimulus
stimulus
plant
plant

8. 18.2 What are tropisms? light water gravity Phototropism Geotropism
Hydrotropism
H2O
water
gravity

9. The Growth response of Shoots to Light

10. 18.3 The growth response of shoots to light
Phototropism
light
shoot
root

11. Positive phototropism Negative phototropism
18.3 The growth response of shoots to light
Phototropism
light
Leaves can gain maximum amount of light for photosynthesis
Importance:
Positive phototropism
shoot
Negative phototropism
root

12. 18.3 The growth response of shoots to light
Clinostat

13. 18.3 The growth response of shoots to light
Clinostat
cancel out the effects of unilateral light & gravity by rotating the disc
direction of rotation
set up as a control for investigation of growth responses of roots & shoots to unilateral light & gravity

14. Experiment 18.1 18.3 The growth response of shoots to light
To investigate the phototropic response of shoots
Select three pots of young seedlings of similar size and label them as pots A, B and C respectively.
2. Put pot A in a light-proof box with a small window on one side on that light reaches the shoot from one direction only. This is unilateral light.
unilateral light
light-proof box
set-up A

15. Experiment 18.1 18.3 The growth response of shoots to light
To investigate the phototropic response of shoots
Set up pot B in a similar way except that it is put on a clinostat. Switch on the clinostat.
Put pot C in complete darkness. Both pots B and C are controls.
Observe the plants after 1 or 2 days.
set-up B
unilateral light
clinostat
in complete darkness
set-up C
light-proof box

16. Experiment 18.1 18.3 The growth response of shoots to light
To investigate the phototropic response of shoots
Question 1:
What has happened to the shoots of plant A? Explain.
Answer:
Seedlings in pot A are exposed to unilateral light. The shoots respond by growing towards the light source.

17. Experiment 18.1 18.3 The growth response of shoots to light
To investigate the phototropic response of shoots
Question 2:
What has happened to the shoots of plant B? Explain.
Answer:
In pot B, as the effect of unilateral light is cancelled out by the revolution of the clinostat, the shoots do not show any curvature but grow vertically upwards.

18. Experiment 18.1 18.3 The growth response of shoots to light
To investigate the phototropic response of shoots
Question 3:
What has happened to the shoots of plant C? Explain.
Answer:
In pot C, in complete darkness, the shoots grow vertically upwards. The plant becomes taller with slender and longer internodes, and carries smaller leaves which are yellow in colour. This condition is called etiolation.

19. Experiment 18.1 18.3 The growth response of shoots to light
To investigate the phototropic response of shoots
Question 4:
What is the significance of phototropism to plants?
Answer:
The shoot tends to grow towards the light source. This phototropism ensures that a plant gets maximum light energy for photosynthesis.

20. The Growth response of Roots & Shoots to Gravity

21. ground Gravity 18.4 The growth response of roots & shoots to gravity
Geotropism
ground
shoot
root
Gravity

22. ground Gravity Negative Geotropism Positive Geotropism
18.4 The growth response of roots & shoots to gravity
Geotropism
Importance:
grow deep into soil to get a firm anchorage
Negative Geotropism
Positive Geotropism
Importance:
leaves can receive maximal sunlight for photosynthesis
roots can absorb maximum amount of water and mineral salts
ground
shoot
root
Gravity

23. Experiment 18.2 18.4 The growth response of roots & shoots to gravity
To study the geotropic response of roots and shoots
Add some moist wool in two Petri dishes.
2. Place three bean seedlings on the cotton wool in each Petri dish.
Seedling P – placed horizontally
Seedling Q – with radicle pointing upwards
Seedling R – with radicle pointing downwards

24. Experiment 18.2 18.4 The growth response of roots & shoots to gravity
To study the geotropic response of roots and shoots
Cover the lid and place the whole Petri dish A with its edge on a support.
Attach Petri dish B onto the cork disc of a clinostat.
Set-up A
Set-up B
support
in complete darkness
Petri dishes
plumule
radicle P1 P2 Q1 R1 Q2 R2

25. Experiment 18.2 18.4 The growth response of roots & shoots to gravity
To study the geotropic response of roots and shoots
Put both set-ups into a light-proof box.
Switch on the clinostat in set-up B.
Observe the seedlings after 1 or 2 days.
Set-up A
Set-up B
support
in complete darkness
Petri dishes
plumule
radicle P1 P2 Q1 R1 Q2 R2

26. Experiment 18.2 18.4 The growth response of roots & shoots to gravity
To study the geotropic response of roots and shoots
Question 1:
What has happened to the radicles and plumules in set-up A?
Draw a diagram to show the appearance of the seedlings. P1 Q1 R1
Answer:
The radicles grow downwards, while the plumules grow upwards.

27. Experiment 18.2 18.4 The growth response of roots & shoots to gravity
To study the geotropic response of roots and shoots
Question 2:
What has happened to the radicles and plumules in set-up B?
Draw a diagram to show the appearance of the seedlings.
Answer:
All the radicles and plumules grow straight because the effect of gravity is cancelled out by the revolution of the clinostat. P2 Q2 R2

28. Experiment 18.2 18.4 The growth response of roots & shoots to gravity
To study the geotropic response of roots and shoots
Question 3:
Why is it necessary to leave the apparatus in darkness?
Question 4:
Do the radicles and plumules show positive geotropism or negative geotropism?
Answer:
The apparatus is left in darkness to avoid the stimulus of light.
Answer:
In set-up A, the radicles grow downwards and are positively geotropic, while the plumules grow upwards and are negatively geotropic.

29. Experiment 18.2 18.4 The growth response of roots & shoots to gravity
To study the geotropic response of roots and shoots
Question 5:
What advantage do the seedlings have by showing such geotropism?
Answer:
Being positively geotropic, roots grow downwards deep into the soil to provide a firm anchorage underground and absorb maximum water and mineral salts in the soil. Being negatively geotropic, shoots grow upwards to ensure that a plant gets enough light energy for photosynthesis.

30. The Growth response of Roots to Water

31. Shoot Root Wet Dry 18.5 The growth response of roots to water
Hydrotropism
Dry
Wet
Shoot
Root

32. Positive Hydrotropism
18.5 The growth response of roots to water
Hydrotropism
Dry
Wet
No response
Importance:
Shoot
get maximum water as a raw material for photosynthesis
Root
Positive Hydrotropism
get maximum mineral salts to maintain normal growth

33. Experiment 18.3 18.5 The growth response of roots to water
To investigate the relative effects of gravity and water on the growth response of green plants
Prepare a 20cm X 20cm gauze square. Bend it to form a shallow tray that is 2cm deep.
Place an even 1-cm deep layer of damp vermiculite.
Sprinkle about 20 pea seedlings onto the vermiculite. Cover the seedlings with damp vermiculite.

34. Experiment 18.3 18.5 The growth response of roots to water
To investigate the relative effects of gravity and water on the growth response of green plants
Set up the apparatus in darkness as shown in the diagram.
Observe the results after 2 days.
water

35. Experiment 18.3 18.5 The growth response of roots to water Question:
To investigate the relative effects of gravity and water on the growth response of green plants
Question:
Which response is stronger, hydrotropic or geotropic? Explain.
Answer:
Some pea seedlings reverse their normal geotropic response and grow upwards towards the damp vermiculite. This shows that hydrotropic response is stronger than geotropic response.
water

36. What are the substances for controlling
18.6 What are the substances for controlling growth movements in plants?
What are the substances for controlling
Growth Movements
in Plants?

37. 18.6 What are the substances for controlling growth movements in plants?
coleoptile
a protective sheath surrounding the young shoot of the embryo in plants of the grass family
first leaf
used in the experiments for finding out the factors controlling the growth movements in plants
grass seed

38. Charles Darwin (1880) Experiment Result Conclusion / Explanation
18.6 What are the substances for controlling growth movements in plants?
Charles Darwin (1880)
Conclusion / Explanation
Experiment
Result
light
The coleoptile grew towards the light.
coleoptile

39. Charles Darwin (1880) Experiment Result Conclusion / Explanation
18.6 What are the substances for controlling growth movements in plants?
Charles Darwin (1880)
Conclusion / Explanation
Experiment
Result
decapitated coleoptile
light
The shoot tip was sensitive to light.

40. Charles Darwin (1880) Experiment Result Conclusion / Explanation
18.6 What are the substances for controlling growth movements in plants?
Charles Darwin (1880)
Conclusion / Explanation
Experiment
Result
opaque cap
light
This further confirmed that the tip was sensitive to light.

41. gelatin allows substances to diffuse down
18.6 What are the substances for controlling growth movements in plants?
Boysen-Jensen (1913)
Conclusion / Explanation
Experiment
Result
light
gelatin allows substances to diffuse down
A substance produced at the tip diffused through the gelatin down the shaded side.
light

42. it does not allow substances to diffuse down
18.6 What are the substances for controlling growth movements in plants?
Boysen-Jensen (1913)
Conclusion / Explanation
Experiment
Result
light
mica plate
Substance causing the bending movement moved down the shaded side of the shoot.
it does not allow substances to diffuse down

43. Paal (1919) Experiment Result Conclusion / Explanation
18.6 What are the substances for controlling growth movements in plants?
Paal (1919)
Conclusion / Explanation
Experiment
Result
A substance had diffused from the tip & stimulated growth.

44. Fritz Went (1928) Experiment Result Conclusion / Explanation
18.6 What are the substances for controlling growth movements in plants?
Fritz Went (1928)
Conclusion / Explanation
Experiment
Result
The substance passed from the coleoptile tip into the agar block and then down into the stump The stump bent away from the side with a source of that substance.
agar block
stump

45. Went named this substance auxin
18.6 What are the substances for controlling growth movements in plants?
Fritz Went (1928)
Result
Experiment
Result
Went named this substance auxin
agar block
stump

46. 18.7 More about auxins
More about
auxin

47. 18.7 More about auxins organic substances plant hormone auxin
can regulate growth & other physiological processes
auxin
auxin
auxin
auxin
act on parts of the plant other than the part that produces them

48. 18.7 More about auxins Site of production shoot tip produces auxins
auxins are transported downwards to the root
roots tip also produces a small amount of auxins

49. 18.7 More about auxins Site of production auxin exert their effect at2 3 4 5 6 7 8
auxin exert their effect at
shoot tip produces auxins
region of elongation 1 2 3 4 5 6 7 8
mark at 2 mm intervals

50. 18.7 More about auxins shoot root
Effect of auxins on different parts of a plant
Stimulate shoot growth
Stimulate root growth
No effect on shoot growth
Higher conc.
Inhibit root growth
% growth stimulation
Lower conc.
shoot
root
% growth inhibition
Auxin concentration / parts per million (p.p.m.)

51. Geotropic Phototropic The Role of Auxins in & responses
18.8 The role of auxins in phototropic & geotropic responses
The Role of Auxins in
&
Phototropic
Geotropic
responses

52. Phototropic response of shoots
18.8 The role of auxins in phototropic & geotropic responses
Phototropic response of shoots
auxins
unilateral light
Unilateral light causes auxins to accumulate on the shaded side of the shoot 1

53. 2 3 4 18.8 The role of auxins in phototropic & geotropic responses
Higher concentration of auxins stimulates shoot growth 3
unilateral light
The shaded side grows faster than the illuminated side
The shoot grows & bends towards the light 4

54. Geotropic responses of shoots and roots
18.8 The role of auxins in phototropic & geotropic responses
Geotropic responses of shoots and roots
gravity
Auxins diffuse down to the lower side due to gravity

55. 18.8 The role of auxins in phototropic & geotropic responses
high concentration of auxins inhibits root growth
high concentration of auxins stimulates shoot growth
lower side of the root grows more slowly
lower side of the shoot grows faster than the upper side
the root bends downwards
the shoot bends upwards

56. Mind Map Tropism growth movement phototropism geotropism hydrotropism
is caused by
growth movement
includes
phototropism
geotropism
hydrotropism
unilateral stimulus
auxins
nature of stimulus
nature of stimulus
nature of stimulus
which are
light
water
pull of gravity
plant
hormones
produced by
result
illuminated unilaterally on shoots
on shoots and roots
positive response by roots
shoot tips and root tips
auxins diffuse down to lower side
auxins move to shaded side
importance
stimulate growth at
so the shoots
so the roots
roots can absorb more water and mineral salts
so the shoots
region of elongation in shoots and roots
bending toward light source
bending upward
bending downward
roots can penetrate into soil for anchorage and to absorb more water and mineral salts
leaves gain maximum amount of light for photosynthesis
result
result
result
import-ance
positive response by shoots
negative response by shoots
positive response by roots
import-ance

57. ~End~