1. Control Mechanisms
Section 3.5
Unit C

2. Objectives define phototropism and gravitropism
relate concepts of phototropism and gravitropism as control systems to natural phenomena
list other control mechanisms found in plants, giving examples

3. Introduction
Stimuli  change in the environment that causes a change in the organism
Way that react depends on type and direction of stimulus
Examples?
Plants respond to stimuli as well as to get what they need
What do plants need to survive?

4. Stimuli in Plants
Plants may respond by turning or bending toward or away from an external stimulus - tropism

5. Stimuli in plants
Look at these two pictures: What stimuli might be affecting the direction of plant growth?
gravity
light

6. Phototropism Phototropism
Growth movement in response to light stimulus
Positive phototropism  stems; toward light
Negative phototropism  roots (weak); away from light
Time Lapse Videos:
(Phototropism in Tomatoes -Time Lapse)
Also: sm/tropisms.html

7. Phototropism
(how time flies and sunflowers catch the sun HD)

8. Plant Growth
The shoot of a plant grows out of the lengthening outer sheath of seed
Sheath will stop growing, allowing the stem and leaves to break through and keep growing
The stem and leaves will show phototropism if the light comes from one direction only

9. Gravitropism Gravitropism
Growth movement response to gravitational force (also known as geotropism)
Positive gravitropism  roots; with gravitational force
Negative gravitropism  stems; against gravitational force

10. Gravitropism
The bean seed can be oriented any way and the root still always eventually grows down (positive gravitropism) – note effect of root tip removal

11. Mechanism of Gravitropism
Can orient plant any way and roots will still grow down
If plant tips, starch grains shift and settle in new location
Movement detected and growth response results

12. Analogy
If I turn this jar of jellybeans on its side, the jellybeans will shift and settle on the new bottom

13. Summary Plant Part Stimulus Tropism Stem Light
Weak negative phototropism
stem
Negative gravitropism
root
gravity
Positive phototropism
root
gravity
Positive gravitropism

14. Plant Growth: Darwin’s Experiment
Read the different treatments below carried out on oat plant seedling
What do you think would be the effect on direction of plant growth? Why?
1) Control (no treatment)
2) Tip of plant removed
3) Cover tip with opaque cap
4) Cover tip with transparent cap
5) Cover base of plant with opaque shield

15. Results What did you hypothesize as the result of this experiment?
Results: p. 327 Fig C3.25
What does this tell us?
The tip is somehow involved in detection of light stimulus (communicating with cells in area of bending  area of elongation)

16. Boysen-Jensen’s Experiment
Snipped off the tip and covered with 1) gelatin and 2) mineral mica
Saw phototropism in treatment with gelatin but not with mineral mica
Substance that was communicating info could diffuse through gelatin
From tip to area of elongation

17. Auxins Group of plant hormones Isolated by F.W. Went
In shoot tips – auxins stimulate elongation
In root tips – auxins retard (slow down) elongation
Moves from tip to area of elongation to accomplish this
Uneven distribution of auxins results in tropisms

18. Auxins and phototropism
Sunlight eradicates auxin, meaning that the part of the shoot tip of the plant which is receiving direct sunlight will have the least amount of auxin.
More auxin is present on the shaded side promotes more cell division and elongation, causing the plant to bend towards the sunlight after this lop-sided growth.

19. Auxins and phototropism

20. Auxins and hydrotropism
Note that in roots, auxins move towards moist side, slowing down elongation on that side, resulting in root bending to water

21. More hormones
Plants have several other groups of hormones, including gibberillins (which control germination and seed development) and cytokinins
For more info see: hormones.info/index.htm

22. Other Control Mechanisms (“Tropisms”)
Plants can also respond to:
Temperature  how?
Chemicals
Water (hydrotropism)
Light/dark periods  ex. need so much darkness or light to flower
All these responses to stimuli are controlled by chemical messengers
How do you think climate change will affect blooming?

23. Homework p. 328 # 1-6 Review p. 330 # 1-24
Unit Review p (Knowledge and Application Section)