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1. CHEMICAL COORDINATION IN
PLANTS
Chapter 12
Class: 9th O’ level
Edexcel IGCSE
Coordination is a way in which an organism responds
to a stimulus in his internal or external
environment……………………………………………
By Ziad Ahmed
Biology Teacher @ BFIS Doha Qatar.
Email: ahmadziad99@gmail.com
Phone: +974 555 96985
Written & composed by: Ziad ahmed
Phone number: +974-55596985
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2. Learning Objectives
• Understand that plants respond to stimuli.
• Describe the phototropic response of roots and stems
• Describe the geotropic responses of roots and stems.
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Phone number: +974-55596985
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3. Plants respond to stimuli:
Do you know that plants respond to stimuli?
Like animals, some species of plants can respond rapidly to the stimulus.
For example: Venus flytrap.
This plant has some adaptations for this features such as:
• It has modified leaves which close quickly around the prey and trapping it.
• It secrete enzymes and digest its prey.
• All these movements are brought about by rapid changes in turgor of
specialized cells at the base of the leaves of Venus flytrap.
Q. Are such plants are autotrophs? If yes, then why they prey on insects?
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4. TROPISM
• Most plants do not responds to stimulus as quickly as the Venus flytrap.
• When a plant respond to a stimulus, it change its growth rate, and as we know
that the growth rate in different parts is different, it is maximum near the tip of
roots and shoot, so the response to stimulus at roots or shoot will be maximum
• For example:
• Imagine a plant growing normally in a pot, usually most light will be falling on
the plant from above, if you turn the plant on its side and leave it for a day,
you will see that its shoot starts to grow upwards, because the growth rate is
maximum in shoot.
• How this growth is controlled????
• There are 2 stimuli acting on plant:
a. Direction of the light
b. Gravity
Both of these stimuli are acting in specific
Direction. Thus the growth response of plant in
to a directional stimulus is called Tropism.
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5. TROPISM
• The growth response of plant to a directional stimulus is called Tropism.
Types of Tropism:
Plants need of photosynthesis:
The shoot of most plants need sunlight for photosynthesis, this means that in
most plants the positive phototropism is the strongest tropic response of the
shoot.
Tropisms are given different names dependent on the stimuli.
A tropism where the stimuli is light is called phototropism.
The shoot of a plant grows towards the light to maximize photosynthesis.
This is called positive phototropism.
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Positive Tropism
If the growth response is towards
the direction of the stimulus
Negative Tropism
If the growth response is away
from the direction of the stimulus

6. GEOTROPISM:
• If there is no light under the soil, how does a germinating seed know which
way to grow?
• The movement /growth of plant toward earth is called geotropism. For
example, The roots are positively geotropic. So will grow down to find mineral
ions and water.
• Because the shoot is also negatively geotropic.
• So will detect gravity and grow in the opposite direction.
TERMS TO KNOW:
• Phototropism are growth responses to light from one direction.
• Geotropism are growth responses to the direction of gravity.
• Light from direction is called Unidirectional.
• If light shine on plant evenly from all directions, it is called Uniform light.
• If light shines on the plant evenly from all directions, this is called Uniform light
• The growth of roots in response to water is called hydrotropism.
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Phone number: +974-55596985
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7. GEOTROPISM:
• The table below summarize the main tropism:
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8. Detecting the Light stimulus: Plant Hormones
• Plants have No sense organ or Nervous system as found in animals, but they
can respond to certain stimulus such as: Light, Gravity, etc….
• Have you ever noticed:
• How plants bend toward light?
• How are they able to tell which direction the light is coming from, and how do
they bend toward it? They don’t have eyes or muscles, after all.
• Therefore they must have some system to detect the stimulus & coordinate
the response.
Experiments of Darwin:
Phototropism: The ability of plant to bend toward light is called phototropism.
• The detection system of phototropism was first investigated by the great
English biologist Charles Darwin in 19th century.
• He used cereal coleoptiles as these were easy to grow and use in experiment.
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9. • He used cereal coleoptiles as these were easy to grow and use in experiment
• Coleoptile: A coleoptile is a sheath protecting a young shoot tip in a grass or
cereal.
a) Darwin showed that the unidirectional light when fall on the tip of
coleoptile, the light was detected by the tip of coleoptile, and transmitted to
growth zone and thus the coleoptile shows growth and curvature toward
light.
b) When the tip of coleoptile was removed, no growth or bending was found.
c) When the end of coleoptile was covered by an opaque cap, it show growth
but no bending was detected.
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Darwin’s Experiment

10. • Since plants do not have any nervous system, then there was a question that
what caused the coleoptile to bend towards the light stimulus………….
• Biologist began to look for a chemical messenger or hormone, that might be
the cause of phototropism in coleoptiles.
• Between 1910-1926, different scientists investigated this problem, some of
their results are summarized:
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Darwin’s Experiment

11. • In 1st experiment:
• The stimulus for growth ( light ) was passed though a material “gelatin”
which absorb water soluble chemicals, and they found that the coleoptile
bended and showed growth.
• Then they passed the light through a material such as “mica” which is
impermeable to water, and there was no bending or growth found in the
coleoptile.
• So the biologist concluded from the 1st experiment that the stimulus was a
chemical that was soluble in water.
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Experiment No: 1

12. • In 2nd experiment:
• The tip of the coleoptile was removed (coleoptile was decapitated) and
placed on the one side of decapitated stalk.
• It was shown that the phototropic response could be brought about even
without unidirectional light.
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Experiment No: 2

13. • In 3rd experiment:
• The tip was removed from the coleoptile and it was placed on agar block jelly
(a water absorbing chemical).
• The hormone was collected in the agar jelly.
• Now that agar was placed on one side of decapitated coleoptile.
• Now having no cap, the decapitated coleoptile bending occurs in the absence
of unidirectional light.
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Experiment No: 3

14. • Scientists believed that this is the hormone which bend the coleoptile and
thus stimulate the growth on the side of coleoptile from the light.
• Means: Hormone is produced in the tip of shoot & is then diffused back
down the shoot.
• If the shoot is in dark OR if the light is all around the shoot, so it stimulate the
shoot equally on all sides.
• However if the shoot receive light from one direction, the hormone will move
away from the light as it diffused downward, thus the concentration of
hormone in dark side of shoot growth in the cells, making the shoot bend
toward the light.
• This hormone was named
Auxin. But as such substances
influence growth & development in
Plants, thus these substances are
Preferred by the scientists to call them
“Plant growth substances”.
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Scientists observations:

15. • Clinostat is an apparatus which consist of an electric motor turning a cork disc.
• Germinating seeds are attached to the disc.
• The motor turn the disk and seeds very slowly so that any kind of directional
stimulus cannot interfere and affect on the seeds.
Uses of Clinostat:
• Clinostat can be used to demonstrate tropism.
• For example geotropism in roots. Few beans are soaked in water over night and
then placed on wet cotton wool for two days until the first root of each seed
grows to a length of 2 cm.
• Wet cotton wool is attached to the cork disks of two clinostats.
• Four seeds are attached to each of the disc in a way that their radicals pointing
outward.
• Covers are placed over them to keep the moist air round them.
• One clinostat is switched on while the other is switched off and the cotton wool
is watered regularly to keep it damp for some days.
• The radicals of seed will grow downward showing geotropism which were on
control clinostat.
• Seeds on moving clinostat will grow straight because the continuous moving of
clinostat cancel the effect of gravitational stimulus and thus cancel the geotropic
response. 15
Using A Clinostat to show Tropism:

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17. TO BE CONTINUED…………..
• NEXT LECTURE…… INSHAALLAH
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Phone number: +974-55596985
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