Tropisms refer to directional growth responses in plants. These include phototropism, the growth response to light, and geotropism, the growth response to gravity. In phototropism, shoots grow towards light (positive phototropism) while roots grow away from light (negative phototropism). In geotropism, shoots grow away from gravity (negative geotropism) while roots grow towards gravity (positive geotropism). These directional growth responses are regulated by the plant hormone auxin.

2. TROPISMS
 the movement is always
a growth movement
are responses by part of a plant
towards or away from a stimulus
coming from one direction

3. Tropic responses can be:
Negative –
if growth is away from
the stimulus
Positive –
if growth is towards
the stimulus
Gravity
Growth
Growth

4. Phototropism
is a growth
response to light
Light
Geotropism
is a growth response
to gravity

5. A shoot is:
POSITIVELY
PHOTOTROPIC
NEGATIVELY
GEOTROPIC

6. Advantages of a shoot growing:
 Towards light:
 To trap more light for photosynthesis
 Away from gravity:
 shoot of a germinating seed grows out of
the soil
 more chances for pollination & seed
dispersal

7. A root is:
 POSITIVELY GEOTROPIC
 NEGATIVELY PHOTOTROPIC

8. I am growing
towards gravity,
but where is the
water????

9. Advantages of a root growing towards
gravity:
 To anchor the plant
 To search for water
 To ensure that the root of a
germinating seed always grows
downwards, whatever, the position
of the seed in the soil

10. Study the pictures below and then complete the
table by putting a plus (+) if the shoot or root
grows towards the stimulus and a minus (-) if it
grows away from it.
Stimulus
Light Gravity
Shoot
Root

11. Study the pictures below and then complete the
table by putting a plus (+) if the shoot or root
grows towards the stimulus and a minus (-) if it
grows away from it.
Stimulus
Light Gravity
Shoot + -
Root - +

12. ANSWER THE FOLLOWING

13. Study the diagrams. What would you
expect to happen and why.
The shoot with the covered tip grows (straight
up / towards light) because it (can sense light /
cannot sense light).

14. Study the diagrams. What would you
expect to happen and why.
The shoot with the covered tip grows (straight
up / towards light) because it (can sense light /
cannot sense light).

15. The shoot with the exposed tip grows (straight
up / towards light) because it (can sense light /
cannot sense light).

16. The shoot with the exposed tip grows (straight
up / towards light) because it (can sense light /
cannot sense light).

17. Look at the diagrams below. What does this
experiment tell you about the tip of the shoot
and the part it plays in growth?
Tip produces chemicals for growth.

18. Look at the diagrams below. How would you
explain the results of this experiment?
Bending resulted due to unequal amount of
chemical . Light affects distribution of chemical.

19. AUXINS

20. Auxins
• are plant hormones, or growth regulating
chemicals produced in the root and shoot
tips that affect growth
What is the effect of auxins on plant cells?
Cells absorb water and elongate.

21. Three regions of a root tip
Cell division region
Cell
differentiation
region
Cell elongation
region
[Cells become specialised]

22. Which region does auxin affect?
Cell division region
(root tip that makes auxin)
Cell differentiation region
(cells change their shape;
no effect by auxin)
Cell elongation region
(auxin makes cells absorb water)

23. Explain why:
markings have
moved away from
each other ONLY in
the lower region.
Cell elongation
region

24. A shoot growing in even illumination grows
straight up. Why?
auxin
Cells on both sides of the
stem elongate equally.

25. A shoot growing in one-sided illumination.

26. A shoot growing in one-sided illumination
grows towards light. How is it possible?
Cells on the two
sides of the stem
GROW UNEQUALLY!!

27. Remember: UNEQUAL growth of stem
sides results in BENDING of stem
Lighted side:
normal size of cells
Shaded side of stem:
cells elongate
Light

28. Describe how a shoot bends towards
one-sided illumination.
Auxins are produced at the ___________ tip and
diffuse down the stem.
More auxin collects on the ____________ side of
the stem.
Auxins cause the cells to absorb water and so
elongate in the cell elongation region.
Thus the ___________ side grows more than the
lighted one.
This results in bending of the stem towards light.
shoot
shaded
shaded

29. Auxins sometimes stop growth
Auxin passes down the stem
So the plant grows tall and straight.
1.
Apical bud produces auxin.
2.
Lateral buds do not grow into
side branches.
3.

30. If the top is cut off from the
plant:
 flow of auxin stops
 side branches will grow
out

31. Sometimes gardeners cut the tops off plants to
make them more bushy

32. QUESTION: MAY, 2009
Give a biological explanation for each of the
following statements:
cutting off the tip of a shoot will stop the shoot
from getting taller but it will increase
bushiness; (3)
Cutting off the tip of a
shoot removes the
source of auxin.
Lack of auxin stimulates
lateral buds to develop
and side branches grow.

33. EXPERIMENTS TO SHOW:
1. Geotropism in a shoot
2. Geotropism in pea radicles
3. Phototropism in oat coleoptiles
4. Phototropism in a shoot
All make
use of an
apparatus
called:
clinostat
A motor that rotates
slowly – 4 times per hour

34. A clinostat
 makes factors uniform (evenly distributed)
 for control experiments
 contains a cork disc which can be set to
rotate in a vertical or horizontal plane by a
motor

35. AIM: To show geotropism in a plant shoot.
APPARATUS:
CONTROL
Give a precaution for this experiment.
Even illumination.
The apparatus was
left for 3 days.

36. Why did the shoot attached to the clinostat
grow horizontally ?
As shoot rotated, gravity acted on all sides.
No unilateral stimulus to respond to.

37. AIM: To show geotropism in pea radicles.
APPARATUS:
 The apparatus was left for 3 days.

38. QUESTION: MAY, 2005
A biology student wanted to test whether the
position in which seeds are placed in the soil
affects the direction of shoot and root growth.
Design a simple experiment which the student
can use to carry out this investigation.(5)

39.  Seeds of the same type are placed at
different orientations in a single container.
 A similar container is set up as a replicate.
 Left in darkness for a few days.
 Radicles are observed.

40. QUESTION: MAY, 2005
When growing plants from cuttings, farmers
usually dip the cutting into Rooting Hormone
powder before planting it in the soil. Briefly
explain how this increases the chances of
successful plant growth. (2)
Roots are stimulated to form.
Thus plant can absorb more
water and ions for growth due
to more root hairs present.

41. AIM: To show phototropism in oat coleoptiles.
APPARATUS:

42. METHOD:
1. Thirty oat grains were soaked in water for 1 day.
2. Ten grains were placed in each of three small pots of
compost and left to germinate.
3. When grown to a height of 1cm, they were treated as
follows:
 One set was covered with a large box, excluding
light;
 Another set was covered with a box with a slit at
the side, allowing one-sided illumination;
 The control was left in the light.
4. The pots were left at room temperature for a few
days.

43. RESULT:
 In the dark: coleoptiles were yellow, straight and
very long.
 In one-sided illumination: coleoptiles bent towards
the light.
 In the light: coleoptiles were green, straight and
shorter than those in the dark.
CONCLUSION:
 The coleoptiles responded to the stimulus of one-
sided illumination by growing towards it. Coleoptiles
are positively phototropic.
 Light is needed for chlorophyll to form.

44. AIM: To show phototropism in a shoot.
APPARATUS:
unilateral
light
unilateral
light
light-proof
box
clinostat
A B

45. unilateral
light
unilateral
light
light-proof
box
clinostat
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...
What has happened to the shoots of plants A and B ?
A B

46. unilateral
light
unilateral
light
light-proof
box
clinostat
A B
What is the effect of the clinostat on the shoot of plant B ?
The revolving clinostat cancel out the effect of
unilateral light on the shoot of plant B.

47. VIEW THE ACTUAL EXPERIMENT
IN THE NEXT
3 SLIDES

51. Question: MAY, 2006
5. The experiment shows the effect of a stimulus
on the growth of a root and a shoot. The seedlings
were kept in the dark throughout the experiment.

52. a) Continue the diagram to show the direction of growth
of the shoot and root in:
i) the stationary
ii) rotating clinostat. (4)

53. b) What type of stimulus response is exhibited by
the seedlings? (1)
Geotropism.
c) Why were the seedlings kept in the dark? (2)
Any changes observed would be due to gravity.
d) Name the substance produced by the plant which
brings about the growth changes. (1)
Auxin

54. e) This experiment was investigated in an orbiting
spacecraft where weightlessness is observed.
Predict the results obtained in this situation. (1)
Grow in all directions.

55. Under which condition (light/dark)
was each seedling of bean grown?
Grown in the
light
Grown in the
dark

56. What is the advantage of a
longer stem in seedlings
grown in the dark?
light
dark

57. Four differences between a seedling
growing in the light & another in the dark.
In the light In the dark
1. Short, thick stem long, thin stem
2. Green shoot yellow shoot

58. Four differences between a seedling
growing in the light & another in the dark.
In the light In the dark
3. Broad leaves small leaves
4. Short internode long internode

59. QUESTION: MAY, 2009
Give a biological explanation for each of the
following statements:
the internodes of a shoot growing in the dark
are longer than the internodes of a shoot
growing in the light; (3)

60. Differences in the response shown by:
A shoot in unilateral light A hand touching a hot
object

61. Differences in the response shown by:
A shoot in unilateral light A hand touching a hot
object
1. Response is positive. 1. Response is negative
2. Slow response. 2. Rapid response
3. Response involves
growth.
3. No growth involved
4. Response does not
involve muscles and
nerves.
4. Response involves
muscles and nerves.

62. EXPERIMENTS USING OAT
COLEOPTILES

63. A Coleoptile is a :
 hollow, cylindrical sheath that surrounds the
primary leaf of a germinating monocot seed
Coleoptile First leaf

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

65. unilateral
light
few days later
black covers
Conclusion :
the tip is the place responsible for bending
Experiments
on Oat Coleoptiles

66. Experiments
on Oat Coleoptiles
few days later
dark box
agar plate
Conclusion :
the head must have produced a diffusible
chemical substance which stimulates growth

67. Experiments
on Oat Coleoptiles
bending directions
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
the tip is put aside
mica
plate
Can the explanation still work ?
bending directions