1. Plant Behaviour
Chapter 11 (Behavioural Adaptions) page. 362.

3. What do plants need to survive?

4. Tropisms
 External factors that influence plant growth
and development: Light, gravity, touch, water.
 Tropism: The growth of a plant in response to
the direction of a stimulus, such as light,
gravity, or touch.
 Positive tropism: Plant grows towards a stimulus
 Negative tropism: Plant grows away from a
stimulus

5. Phototropism: direction of light as a stimulus
 When a plant moves or
grows in response to
light.
 Movement towards light
= Positive or negative
phototropism?

7. Phototropism: direction of light as a stimulus
 Charles Darwin (1809-1882) found that he could
prevent phototropism by covering the growing tips
of seedlings (coleoptiles).
 Conclusion:
The tip of the
coleoptile
influences the
bending that
occurs in
phototropism.

8. Phototropism: direction of light as a stimulus
We now know:
 The plant hormone auxin is produced in the tip of a
coleoptile that causes growth of cells. The tip is the
site of reception of the light stimulus.
 Auxin moves away from the light source to the
darker side of the tip. Increased concentration of
auxin in cells of that region increase the growth of
the cells (greater elongation). The uneven growth of
cells results in bending of the coleoptile.
 Draw diagram.

10. Geotropism: direction of gravity as a stimulus
 Gravity causes most of the auxin produced to
accumulate in the lower sides of the horizontal
shoot and root.
Auxin
 Stimulates growth of cells in shoots.
 Inhibits the rate of growth of cells in roots.
 How do you think the shoots and roots would grow?
 Draw diagram.

13. Thigmotropism: direction of touch as a
stimulus
 E.g. Grapevines- Tendrils and twining parts of the climbing
plant wrap around support structures or other plants.
 The twisting is caused by uneven growth of cells along the
tendril as it comes into contact with an object.
 What do you think happens to the distribution of auxin
when a growing plant comes into contact with another
object?
 Draw diagram.

15. Heliotropism/Solar
Tracking
direction of the sun as a stimulus (also time)
 Also a rhythmic behaviour because
direction of the sun depends on the
time.
 Leaves and flowers of many plants
are able to move during the day so
that they are oriented either
perpendicular (at right angle to) or
parallel to the sun’s direct rays.
 How do you think this would assist in
survival?

17. Rhythmic Behaviour
Plants also have a biological clock!

18. Rhythmic Behaviour (Time as a stimulus)
 Circadian rhythm (or circadian cycle) = An activity that
follows a 24-hour cycle.
 Include the opening and closing of flowers, and nectar
and perfume production.
 This kind of movement is
called nastic movement:
Does not depend on the
direction of the stimulus.

19. Plants that use a circadian
rhythm

21.  Photoperiodism: The response of plants to
particular periods of light and dark.
 Photoperiod: The relative of day and night length
 Exposure to “darkness/night-time” is important for
the flowering of some plants (can affect the timing of
flowering).
 Short-day plants and long-day plants.
Rhythmic Behaviour (time as a stimulus)
Nastic Movement: Timing of flowering

22. Short-day plants
 Flower only after being exposed to day lengths that
are shorter than a certain critical length.
 E.g. Chrysanthemums, poinsettia, datura stramonisum.

23. Long-day plants
 Flower only after being exposed to day lengths that
are longer than a certain critical length.
 E.g. Hibiscus, carnations.

24. Day-neutral plants
 Flowering is not influenced by length of exposure to
light/darkness.
 Example: tomatoes, maize, dandelions.

25. Key ideas
 Factors external to a plant influence growth and
development of the plant.
 The direction of response of a plant is sometimes
influenced by the direction of the external stimulus.
 Day-night lengths are critical for some plant
activities.