1. Gr.12 Life Sciences Paper 1 :Session 4C
Topic
Plant Responding to Environment
Video: time lapse montage

2. DEFINITION OF CONCEPTS
Hormones
Chemicals that allow a plant to respond to some stimulus in the environment
Phototropism
Growth of a plant stem towards light
Geotropism
Growth of a plant root downwards into the soil in response to gravity

3. PLANT GROWTH RESPONSES
Video: biology plant lessons
- Responding to Stimuli…

4. HORMONES IN GROWTH OF PLANTS video: plant Physiology
Growth and development in plants are controlled by hormones:
Auxins
Gibberellins
Abscisic acid

5. Plant growth negative Positive movement Tropic movement
Direction of growth is towards or away from the stimulus
Growth in response to external stimulus

6. THE ROLE OF AUXINS IN: GEOTROPISM AND PHOTOTROPISM
Stimulate:
Cell division
Cell elongation (growth in stem length)
The development of fruit
The abscission of leaves and fruit
The development of adventitious root in stem cuttings
Tropic movement in stem and roots
Apical dominance it suppress the growth of the lateral buds.

7. THE ROLE OF AUXINS IN: PHOTOTROPISM AND GEOTROPISM

8. THE ROLE OF AUXINS IN: PHOTOTROPISM AND GEOTROPISM

9. THE ROLE OF GIBBERILLINS IN PLANTS
Stimulate:
Stem elongation
Root growth
The germination of seeds
Promotes flowering
plump grapes in grocery stores have been treated with gibberellin hormones while on the vine

10. EXPERIMENT SHOWING PHOTOTROPISM
plant  
  clinostat
Control Experiment
The control has a clinostat that causes the pot to rotate, all sides of the stem are exposed to light, causing the plant to growth straight.
The experiment has no clinostat. The plant is exposed to light only on the one side. The stem grows towards the light and is therefore, phototropic.

11. EXPERIMENT SHOWING GEOTROPISM
Clinostat wound up Unwound clinostat
  damp cotton roots growing
wool down towards
gravity
cork discs
germinating
seeds
Control Experiment
The control has a clinostat that causes the cork disk to rotate the germinating bean seeds, causing the roots to grow in the direction in which they were placed.
With the experiment the clinostat is unwound and and does not rotate the germinating bean seeds. The roots grow downwards towards gravity.

12. REVISION QUESTIONS
A grade 12 learner performed an investigation to determine the effect of light on the growth of a plants
Shoots. The learner divided the plants that were used into three groups as follows:
Group A – The tips of the shoot was intact.
Group B – The tips of the shoot was removed.
Group C – The tips of the shoot was covered by a cap that does not allow light to pass through.
The diagram below shows each shoot at the start of the investigation and next to each the same shoot at the
the end of the investigation.

13. REVISION QUESTIONS plant hormone is being investigated?
Q -State TWO factors that must be kept constant in this investigation. The influence of which
plant hormone is being investigated?
A – Same environment/temperature which the shoots are placed
Same type of shoots
Q- Explain the results observed in investigations A and C, as
stated in the diagram above.
A-In A unilateral light from the right, caused auxins to move to the shaded side of the shoot
allowed for cell elongation and caused the stem to bend towards the right
In C light had no effect on the distribution of auxins therefore shoots grow upright.
Q-State TWO ways in which the learner could improve the reliability of
this investigation.
A - repeat the investigation
- increase the sample size of the shoots
Q-Name the dependent variable in this investigation
A- Growth of plant shoot

14. DEFENCE MECHANISMS
Acacia species have thorns to prevent excessive feeding by giraffes, kudu, impala
Some plants produce chemicals that act as poisons, repellent, or affect the herbivores digestion.