Control
Mechanisms
Section 3.5
Unit C
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
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?
Stimuli in Plants
• Plants may respond by turning or bending
toward or away from an external stimulus tropism
Stimuli in plants
• Look at these two pictures: What stimuli might
be affecting the direction of plant growth?
gravity
light
Phototropism
• Phototropism
• Growth movement in response to light stimulus
• Positive phototropism  stems; toward light
• Negative phototropism  roots (weak); away
from light
Time Lapse Videos:
• http://www.youtube.com/watch?v=Ze8NV7cvW8k (Phototropism in
Tomatoes -Time Lapse)
• Also:
http://plantsinmotion.bio.indiana.edu/plantmotion/movements/tropi
sm/tropisms.html
Phototropism
• http://www.youtube.com/watch?v=g8mr0R3ibPU&feature=related
(how time flies and sunflowers catch the sun HD)
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
Gravitropism
•Gravitropism
• Growth movement response to
gravitational force (also known as
geotropism)
• Positive gravitropism  roots; with
gravitational force
• Negative gravitropism  stems;
against gravitational force
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
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
Analogy
If I turn this jar of
jellybeans on its side,
the jellybeans will
shift and settle on the
new bottom
Summary
Plant Part
Stimulus
Tropism
Stem
Light
root
Light
Positive
phototropism
Weak negative
phototropism
stem
gravity
root
gravity
Negative
gravitropism
Positive
gravitropism
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
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)
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
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
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.
Auxins and phototropism
Auxins and hydrotropism
• Note that in roots, auxins move towards moist
side, slowing down elongation on that side,
resulting in root bending to water
More hormones
• Plants have several other groups of hormones,
including gibberillins (which control
germination and seed development) and
cytokinins
• For more info see: http://www.planthormones.info/index.htm
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?
Homework
• p. 328 # 1-6
• Review p. 330 # 1-24
• Unit Review p. 334-336 (Knowledge and
Application Section)