1. Growth Response of Plants
Prepared by Ms W.S.Kwan

2. Plants have no nervous system. Can plants detect stimulus
Plants have no nervous system. Can plants detect stimulus? Can plants response?

3. Tropism (Tropic Movement)
A directional growth movement made by a part of a stationary plant in response to unilateral stimulus

4. Types of Tropism stimulus gravity unilateral light water tropism
geotropism
phototropism
hydrotropism

5. Geotropism
Geotropism is the growth movement (growth response) shown by a plant part in response to gravity

6. To show Geotropism in Seedlings
Investigation 17.1
To show Geotropism in Seedlings

7. To show Geotropism in Seedlings
Why is it necessary to leave the apparatus in darkness ?
Ans: To avoid any stimulus from light.

8. To show Geotropism in Seedlings
What has happened to the radicles and plumules ?
Ans: The radicles grow downwards, while the plumules grow upwards.

9. To show Geotropism in Seedlings
Do the radicles show positive geotropism (towards gravity) or negative geotropism (away from gravity)?
Ans: Positive geotropism.

10. 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.

11. Importance of Geotropism (Roots grow towards gravity)
provide firm anchorage for roots
ensure the plant can get adequate supply of water & mineral salts

12. 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)

13. To show the effect of Gravity by using a Clinostat
Investigation 17.2
To show the effect of Gravity by using a Clinostat

14. What is the purpose of marking the radicles at equal intervals ?
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.

15. Does the direction of growth of the seedlings in clinostat X change ?
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.

16. 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 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.

17. What is the purpose of setting up clinostat Y ?
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.

18. Shoots show negatively geotropic

19. Phototropism
Phototropism is the growth movement (growth response) shown by a plant part in response to light.

20. To show Phototropism in Shoots
Investigation 17.3
To show Phototropism in Shoots

21. What is the effect of the clinostat on the shoot of plant B ?
unilateral light
in complete darkness A B C
light-proof box
clinostat
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.

22. What has happened to the shoots of plants A and B ?
unilateral light
in complete darkness A B C
light-proof box
clinostat
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.

23. What has happened to the shoot of plant C ?
unilateral light
in complete darkness A B C
light-proof box
clinostat
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).

24. What is the significance of phototropism to plants ?
unilateral light
in complete darkness A B C
light-proof box
clinostat
What is the significance of phototropism to plants ?
Ans: This tropism ensures that a plant gets enough light energy for photosynthesis.

25. Importance of Phototropism (Shoots grow towards light source)
enables leaves to be in a position to receive maximum sunlight for photosynthesis

26. Hydrotropism
Hydrotropism is the growth movement (growth response) shown by a plant part in response to water.

27. 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

28. Types of Tropism stimulus tropism response water gravity
unilateral light
geotropism
phototropism
hydrotropism
negative
no response
shoot
positive
root
positive
negative
positive

29. Mechanism

30. Darwin Experiments

31. Experiments on Oat Coleoptiles
cut & replaced tip
dark box
decapitated
few days later
no growth
grows
Conclusions :
The tip is responsible for the growth

32. Experiments on Oat Coleoptiles
unilateral
light
black covers
few days later
Conclusion :
The tips are sensitive to light.
(responsible for the detection of light)

33. Experiments on Oat Coleoptiles
dark box
agar plate
few days later
Conclusions :
The tip must have produced a diffusible chemical substance which stimulates growth

34. 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.

35. Auxins commonest plant hormones produced by shoot tip
regulate growth & other physiological processes in plants
promote growth by inducing cell elongation & cell division.

36. Effect of auxin on Plant Parts
shoot
positive
growth
root
Growth of
plant
low
conc.
auxin
concentration
high
conc.
negative
growth
auxin concentration
plant growth
high conc.
low conc.
shoot
root
Positive (promote)
no effect
Negative (inhibit)
Positive (promote)

37. 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

38. 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

39. ~ END ~