16.4 Tropisms in Plants

Question 1

What are plant tropisms?

Answer 1

Growth responses of a plant

to unidirectional stimuli

Question 2

What is the growth of plants in response to unidirectional light called?

Answer 2

phototropism

Question 3

What is the growth of plants in response to gravity called?

Answer 3

geotropism

Question 4

What type of phototropism do plant shoots show?

Answer 4

positive phototropism

Question 5

What type of phototropism do plant roots show?

Answer 5

negative phototropism

Question 6

Describe & explain how a plant responds to unidirectional light:

Answer 6

• Unidirectional light causes auxin to move to dark side of shoot
• Auxin stimulates elongation of cells in dark side of shoot
• ∴ shoot grows towards light, showing positive phototropism

Question 7

Describe & explain the results of this phototropism experiment:

Answer 7

• Shoot without division:
  
  • auxins diffuse away from light towards dark side
  • higher concentration of auxins on left side of shoot
• Shoot with division:
  
  • auxins cannot diffuse to dark side of shoot, despite unidirectional light, due to division
  • auxin concentrations remain fairly similar on both sides

Question 8

Outline a practical investigation into plant phototropisms:

Answer 8

• Germinate & grow seedlings in dark, all round & unidirectional light. Record directions of growth of shoots & roots.
• Germinate & grow seedlings in unilateral light with different colour filters to see which wavelengths trigger phototropic response.

Question 9

Describe & explain the results you would expect to see in these experiments:

Answer 9

1. auxins are able to diffuse away from light source, cause elongation of cells on dark side & positive phototropsim
2. no response. no elongation since auxins are produced by tip of shoot which has been removed.
3. no response, since light must be detected by the tip of shoot.
4. positive phototropism. Auxin acts on the dark side of shoot.
5. no response. Diffusion of auxin down shaded side is prevented.
6. Gelatin allows chemicals through but not electrical impulses. Shoot grows towards light since auxin is chemical hormone.

Question 10

16.4 Tropisms in Plants

Why may plants grow faster in the dark?

Answer 10

avoid competition with other seedlings

by being the first to grow out of the soil & sprout leaves

Question 11

What type of geotropism do plant shoots show?

Answer 11

negative geotropism

(grow away from gravitational pull)

Question 12

What type of geotropism do plant roots show?

Answer 12

positive geotropism

(grow alongside gravitational pull)

Question 13

Outline a practical investigation into plant geotropisms:

Answer 13

• Plants grown on clinostat, so gravitational pull is applied evenly.
• Seeds grown in petri dishes & rotated 90° periodically.