Download Angle of the Dangle - Wisconsin Fast Plants

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Lesson1
Angle of the Dangle
Simulating the effect of gravity on plants
Overview
Plants respond to the force of gravity. However, because gravity is constant and can't be manipulated, it is difficult to experiment with gravity in
the classroom. But one can create a centrifugal force in the classroom
using a standard record turntable. A centrifugal force can simulate the
effects of gravity on plants.
Through this activity, students will examine the effect of gravity and
centrifugal force on plant stems. Students will form hypotheses and test
those ideas using Wisconsin Fast Plant seedlings, film canisters and a
rotating turntable.
Biological
Plant growth and development
Gravitropism
and
agricultural Centrifugal force
concepts
Gravitropism I
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The
teachable
moment
Background
Teacher material
Agriculture and biology teachers can use this activity to illustrate a
plant science unit.
Plants know the way to grow. But what controls them? Why do
shoots grow up and roots grow down? What causes a house plant to
bend toward the window, wheat to grow upwards after being flattened by a strong storm, or a pea tendril to curve around and cling to
any object in its reach?
When environmental stimuli cause a plant to bend in a specific direction the response is termed a tropism. Response to gravity is called
gravitropism (or geotropism), response to light is called
phototropism, and response to physical contact is called thigmotropism.
A plant's cells elongate at a faster rate on one side of the stem than the
other in response to the environmental stimuli.
Exactly what triggers the increased rate of elongation or how it is
mediated remains, despite centuries of intense interest and experimentation on the subject, a virtual mystery. Because tropic responses are
so important to basic plant growth and development, agriculture,
space exploration and many other fields, they are worth examining.
Gravitropism is one of the most important aspects of plant growth.
Gravity impacts both root and shoot growing tips, but in opposite
directions. Roots grow toward gravity (positive gravitropism) and
shoots grow away from gravity (negative gravitropism). A plant's
perception as to the direction of gravity appears to depend on the
settling of dense particles in organelles, called amyloplasts, located in
specialized cells. When the plant is turned, the amyloplasts sink
quickly toward the source of gravity, to the side of the cell that is
currently "down."
Curving or bending of the roots or stem results from asymmetric
growth as the plant elongates. Picture two layers of cells sandwiched
together. If only one layer elongates, the two layers will bend.
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Lesson 1
Movement will be away from the side where elongation occurs.
Elongation of plant cells is controlled by a plant growth hormone
called auxin. The amount of auxin present appears to correlate with
the distribution of the amyloplasts and gravity induces an auxin gradient in the root or shoot tip. It is not clear if the cell elongation is due to
a redistribution of auxin within the plant, an increased sensitivity to
auxin, or to some other mechanism.
If the root and shoot responded in the same way to auxin concentra-
tions, plants would grow in circles. Fortunately, auxin stimulates cell
elongation in shoots, but inhibits it in roots.
Consider gravitropism as it relates to conditions found in space. In
order for humans to stay in space for prolonged periods of time, they
must be able to grow their own food. Will this be possible? How will
astronauts create an artificial"gravity" that initiates root and stem
growth? Since space travel occurs in a microgravity environment,
astronauts will need a gravitational-like force that simulates gravity.
Could centrifugal force be used as a substitute for gravity in space?
Centrifugal force is an outward force that acts upon a mass rotating
about an axis. Centrifugal force is generally expressed in multiples of
the force of gravity, or G's. Therefore, the force of gravity on the surface of the earth is, by definition, one G. For this experiment it is
useful to measure the magnitude of the centrifugal force on the plant.
The magnitude of the centrifugal force exerted on an object depends on
its angular velocity and distance from the center of rotation. Relative
centrifugal force (FR) is represented by the following equation.
FR = 0.204v2 /D
where
v =peripheral speed of the film can in meters/sec
D = diameter of the circle of rotation in meters
Students will need to measure the diameter of the circle of rotation
(and divide by two to determine the radius), calculate the circumference (2m) and record revolutions per minute (rpm) of the turning
table.
To calculate v (in meters/sec), use the following equation:
v =(_meters/rev)(.___ rev /min)(l min/60 sec)
Gravitropism I
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Teacher material
In this activity, the student will investigate how the effects of centrifugal force can be used to elicit a gravitational-like response in young
Fast Plant seedlings (radish will work, too). The control in the experiment, which examines only the force of gravity on the plants, should
be examined and discussed thoroughly before examining the results
from the centrifugal force experiment. Results from the control can
help the students refine their predictions about the effects of the centrifugal force.
Teacher
management
Preparation
As an extension, the students can change the magnitude of the centrifugal force by placing the canister closer to the center of the tumtable (changing "D") or change the rpm of the turntable (changing "n").
They can then correlate the amount of bending with the amount of
force.
Students can work individually or in groups. Four film can chambers
will be used to measure centrifugal force and one film can will be set
up as a control to measure the effects of gravity alone. Each team of
students will set up five film cans total.
Three days prior to this lab, sow enough Fast Plant seeds to ensure that
there will be at least 4 three-day-old seedlings per film can chamber
available for students. Plant 50 percent more seeds than you will need
(if you need 15 plants, plant 30 seeds). By lab day 1, secure the materials listed below.
Activity time
Ten to 15 minutes will be taken on day 1 for students to place four Fast
Plant stems on wicks of one germination chamber. These controls will
test the effects of bending due to gravity only. This can be done individually or in groups. Record necessary information on data sheet.
On day 2, 20 to 50 minutes will be needed. Students should predict
results for the control gravity chambers before looking inside. In
addition to making predictions, students can be assigned to create a
hypothesized mechanism to accompany their predictions. Students
will also examine their chambers, participate in pooling the class data
and discuss results.
Students will then create four experimental chambers. Each chamber
will have one wick and two plants. These chambers will then be taped
onto a turntable of a record player. Record necessary information on
data sheet.
Day 3 will require 30 to 50 minutes. Students should predict results
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Lesson 1
for the experimental chambers. They will then remove the chambers
from the turntable, observe the location of the stems, measure the
angle, record and discuss the results.
Materials
For students in groups of four:
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•
•
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12 three- to four-day-old Fast Plants
5 film cans
waterproof pen or pencil
l-inch piece of double-sided foam tape (if not available, masking
tape will do)
eye dropper or small pipette
5 paper towel wicks
25 ml of water
protractor
1 paper towel sheet (use kitchen-type, not the brown school-type)
Class as a whole will need:
• 1 turntable (with a 78 rpm setting)
• l-inch roll of labelling tape
Sources of
materials
Fast Plant seeds are available from Carolina Biological Supply Co.
You can purchase turntables from local second-hand stores at low cost.
Black plastic 35 mm film containers can be obtained from local film
processing or camera stores.
All other materials should be available from your school.
Tips and safety
Plant the seeds on a Friday. Experimental days 1 and 3 then fall on a
Monday and a Wednesday.
Make sure that film cans are securely fastened to the turntable.
The most difficult parts of this lab are:
• too much water in the film cans cause the plants to slide
around, while too little water causes the plants to dry out.
• accurately marking the initial and final positions of the
plants on the tape
• transferring the markings to the data sheets
• avoiding confusion about positive and negative angles and
control versus experimental containers.
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Teacher material
Optional: suspend a small fishing weight (1 \8 oz) tied to the end of a
string in a clear film can. Tape this can to the turntable along with the
four experimental chambers. When the turntable is rotating, the
weight will swing to the outside of the chamber, demonstrating to
students that a (centrifugal) force is acting on the plant stems when the
turntable is on.
Key terms
Amyloplasts: clusters of starch grains enclosed with a membrane;
located in the statocyte
Auxin: growth-promoting hormone found in some plant cells; causes
cells to elongate
Centrifugal force: fictitious or pseudo-outward force that acts upon a
mass rotating about an axis
Gravitropism: the physiological response of a plant to gravity
Gravity: the force of attraction between two objects, generally perceived as the force of attraction between the earth and bodies on or
near its surface
Shoot primordium: the growing tip in a stem; located at the point of
attachment of the cotyledons
Statocytes: gravity-sensitive cells of plant organs such as roots or
stems
References
"Centrifugal Force." McGraw -Hill Encyclopedia of Science and Technology, 7th edition. Vol. 3: 411. 1992.
"Gravitation." McGraw -Hill Encyclopedia of Science and Technology, 7th
edition. Vol. 8: 206-213. 1992.
"Gravitropism Revisited." Fast Plants Notes. Vol. 4, No.2. Spring 1991.
"Plants Know the Way to Grow." Fast Plants Notes. Vol. 1, No.2.
Evans, M.L., R. Moore, and K.H. Hasenstein. 1986. "How roots respond to gravity." Scientific American 255 (December 1986): 111-119.
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Iversen, Tor-Henning. The roles of statoliths, auxin transport, and auxin
metabolism in root geotropism. Univ. of Trondheim, The Royal Norwegian Society of Sciences and Letters, The Museum. 1974.
Suge, H. and Turkan, I. "Can plants normally produce seeds under
microgravity in space?" Japanese Journal of Crop Science 60(3): 427-433.
1991.
Wilkins, Malcolm. "Guidance Systems," Chapter 7 in Plant Watching.
Facts on File Publications: New York. pg. 64-77. 1988.
Extensions
1. How does the magnitude of the centrifugal force affect the degree of
bending? How can you change the magnitude of the centrifugal
force? Consider graphing the amount of bending versus the
magnitude of the force.
2. How does the length of time exposed to centrifugal forces affect
the degree of bending?
3. Does temperature affect the degree of bending? Does it affect the
bending differently for gravitationally-caused bending versus
centrifugally-caused bending?
4. Which is stronger, the bending due to gravitational force,
centrifugal force or light?
5. Does age of seedling affect degree of bending?
6. Do different plants respond to forces differently?
Ideas for
discussion
1. Describe how animals orient themselves to gravity. They have
multicellular organs ("ears") with fluid-filled cavities. Particles in
the cavity sink, stimulating sensory hairs that line the cavity.
Electrical impulses are carried from the hairs to the brain, which
returns signals to the organism's limbs to correct its orientation.
2. Describe the following experiment: Immature crustaceans such as
crayfish accumulate fine sand grains in their ears (statocysts).
Substitute iron filings for sand in the aquarium. When the
crustacean matures, place a magnet over the top of the statocysts.
The filing will rise to the top of the statocysts, causing the crustacean
to flip upside down.
Gravitropism I
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Student material
Lesson 1
Angle of the Dangle
Simulating the effect of gravity on plants
Introduction
You probably know that plants require light, water, carbon dioxide
and nutrients to grow. Did you also know that plants need gravity (or
a gravitational-like force) to grow and develop? And how is it that a
plant's shoots grow up and roots grow down?
When a seed germinates, it sends out an immature stem. The ideal
position for that stem is vertical because the plant will need to position
its leaves to receive maximum light and to begin making food through
the process of photosynthesis. If a plant is moved from its vertical
position, say it is blown to one side by the wind, it can correct its
orientation. This response to gravity is called gravitropism.
Plants respond to gravity by elongating the cells on one side of the
stem faster than on the other. This causes the stem to bend. We don't
know exactly what triggers the increased rate of elongation or what
controls this response. Because tropic responses are an important
aspect to basic plant growth and development, they impact agriculture, space exploration and many other fields. Though for centuries
researchers have studied and experimented with tropic responses,
basic questions still remain.
What we do know is that plant stems respond to gravity by secreting a
hormone called auxin. Auxin causes cells in the stem to elongate. If a
plant's stem is vertically oriented, then the hormone is evenly distributed to the cells. If a stem is not vertically oriented, then hormone
levels are increased to the cells that are closest to the gravitational
force. The increased level of auxin causes the cells to elongate, which
corrects the position of the stem back to vertical. Picture two layers of
cells sandwiched together. If only one layer elongates, it forces the two
layers to bend.
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Student material
Lesson 1
How does an individual stem cell react to gravity? In 1900 it was
discovered that the perception of the direction of gravity appears to
depend on the settling of dense particles (organelles called amyloplasts)
in specialized cells called statocytes. When the plant is turned, amyloplasts sink quickly toward the source of gravity, to the side of the cell
that is currently "down." Current research indicates that perhaps
amyloplasts possess an electrical charge that alters membrane permeability, therefore regulating the movement of auxin. The exact connection between auxins and amyloplasts, both of which are involved in
gravitropic responses, remains a mystery.
Can we simulate a gravity-like force? Many researchers use centrifugal
force as a substitute for gravity in the lab. You will create a centrifugal
force using a record player turntable.
Centrifugal force is an outward force that acts upon a mass that rotates
about an axis. For example, if you rode on a merry-go-round and did
not hold on, what would happen? You would be thrown off!
Centrifugal force is generally expressed in multiples of the force of
gravity, or G's. Hence, the force of gravity on the surface of the earth is,
by definition, one G. For this experiment it is useful to measure the
magnitude of the centrifugal force on the plant. The magnitude of the
centrifugal force exerted on an object depends on its angular velocity
and distance from the center of rotation. Relative centrifugal force (FR)
is represented by the following equation.
FR = 0.204v2 /D
where
v =peripheral speed of the film can in meters/sec
D = diameter of the circle of rotation in meters
You will need to measure the diameter of the circle of rotation (and
divide by two to determine the radius), calculate the circumference
(27tr) and record revolutions per minute (rpm) of the turning table.
To calculate v (in meters/sec), use the following equation:
v = ( _ meters/rev)(___ rev/min)(l min/60 sec)
Consider gravitropism as it relates to conditions found in space. In
order for humans to stay in space for prolonged periods of time, they
Gravitropism
I
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Student material
Lesson 1
must be able to grow their own food. Will this be possible? How will
astronauts create an artificial "gravity'' that initiates root and stem
growth? Since space travel occurs in a microgravity environment,
astronauts will need a gravitational-like force that simulates gravity.
Could centrifugal force be used as a substitute for gravity in space?
Materials
•
•
•
•
•
•
•
•
•
12 three- to four-day-old Fast Plants
5 film cans
waterproof pen or pencil
l-inch piece of double-sided foam tape (if not available, masking
tape will do)
eye dropper or small pipette
5 paper towel wicks
25 ml of water
protractor
1 paper towel sheet
Class as a whole will need:
• 1 turntable (33, 45 or 78 rpm)
• 1 roll of labelling tape
Procedure
Dayl
1. Cut a piece of paper towel to produce small wick strips that are
approximately 4.5 em long and 1 em wide.
2. Pre-moisten four wicks with several drops of water and place them
along the inner sides of the film can so that there is one wick each on
the top, bottom and both sides of the can when it is placed on its
pedestal. Each wick can be slid in and out of the can by gently
pushing or pulling it with the sharp tip of a pen or pencil.
3. Cut eight three-day Fast Plants seedlings at soil level leaving the
small stem (hypocotyl) and seed leaves (cotyledons) intact. Stick
two of these seedlings onto each wick by placing the cotyledons
against the wick. The water on the wick should hold the seedlings
in place. If the plant is reluctant to stick, you may need to add an
additional drop of water on the wick.
4. Add a couple of drops of water to the bottom of the film can when
all of the plants are in place and put a lid on the can. Make sure the
ends of the wicks don't protrude out of the can. The extra drops of
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Lesson 1
water in the can should keep the air in the can moist. This will prevent the wicks from drying out.
Paper
Towel Wick
Double-Stick
Foam Tape
Film Can.:::=:::~~=~Top
5. Place the chamber in a warm (but not hot) location. A Fast Plants
light bank works well. After 2 to 4 hours, gently take the lid off and
observe the orientation of the hypocotyls. Continue your observations
for the next five to seven days. Keep your eye out for new growth of
your seedlings.
Day 1 Question. What do you think will happen to the stems? Given what
you know about plants, explain why you predict this.
Day2
1. Examine your chamber from yesterday. Which way did each if the
stems bend? What happened? With a protractor, measure the
angle of the stems. How many degrees did the stem bend?
Fill in the day 2 data sheet.
2. Label four film cans as El, E2, E3 or E4 (E is for experimental.)
3. Set up the four chambers
as you did in steps 1-4 on
Day 1, except put one
wick with two seedlings
on the lid of a can.
Seedlings
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Lesson 1
4. With a protractor, measure the angle of the stem of the seedlings.
5. Press the cans down over the lid, closing the chamber.
6. Attach the experimental chambers to the turntable with labelling
tape, so that the stems are perpendicular to the surface of the turn
table and the cotyledons are facing down. Mark an x on the side of
the chamber that faces the center of the turntable.
7. Set the turntable to 78 rpm and turn it on. Record the date and
time on the data sheet provided.
Day 2 Question or hypothesis from students: What do you think will happen
to the stems on the turntable? Explain your prediction.
Day3
1. Turn the turntable off. Record the date and time on the Student
Data Sheet.
2. Take the cans off the turntable and open them.
3. Using a protractor in the same orientation as you used for the
original measurement, measure the angle of the stems. Record the
angles.
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Data sheet
Lesson 1
Student name:
----------------------
Day 1: Directions: Record the information requested in the blanks provided.
Diameter of circular path = - - rpm setting on turntable =--------Relative force expressed in G's =-------------Beginning date of exposure to centrifugal force =_ _ _ _ _ _ __
Beginning time of exposure to centrifugal force = _ _ _ _ _ _ __
Ending date of exposure to centrifugal force = - - - - - - - - Ending time of exposure to centrifugal force =-------------Total duration of exposure to centrifugal force = _ _ _ _ _ _ __
Day 2: Results from control chamber
Directions: Draw in the stems showing how they bent or did not bend.
Seedling #
Degree of bending
1
2
3
4
Day 3: Results from turntable experiment
Angle of change in plant subjects
Container or plant
Degree of change
Average degree of change in the control plants: _ _ __
Average degree of change in the experimental plants: _ _ __
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Student material
Lesson 1
Results and
discussion
1. Compare and contrast the effects of gravity and centrifugal force
on the plants in the germination chambers.
2. Did the plants in the chambers respond to gravity? Explain.
3. Stems respond to light after they break the soil's surface. Is it
possible for roots, which grow in darkness, to respond to light?
How could root studies in space prove your answer?
4. How could you test whether certain plant cells have structures that
respond to gravitational-like forces?
5. Calculate the relative force at 78 rpm. How does it compare to
the relative force you calculated?
6. You are given the opportunity to do one experiment in space. This
is your once-in-a-lifetime opportunity to clear up questions about
gravitropism. You do not want to waste this opportunity. Design
an experiment. Include your hypothesis.
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