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Travis Lick
Capstone Overview
The Chemical Push for the Sun: Theories of Phototropism
The term tropism, describes a plant’s directional growth in response to an external
stimulus. The focus of this capstone is to explain the different types of plant tropisms
that exist, focusing on phototropism, while describing the two main theories about the
processes involved in these growth responses. First, the Cholodny-Went theory
developed in the late 1920’s which has withstood the test of time and scientific scrutiny
to remain as the leading candidate for describing phototropic response through a plant’s
use of a growth hormone called auxin. Second is the growth-inhibitor hypothesis. This
hypothesis was actually created prior to the C-W theory by a man named A.H. Blaauw in
1919 and is still considered a possibility based on his and current research.
In addition to the content-based portion of this capstone, a unit plan has also been
developed as a way to introduce and through inquiry based lessons, give students an
enduring understanding of plant tropisms. The unit plan contains a pre-assessment as
well as a post-assessment that will help to determine the level of student understanding.
As part of the laboratory experiments in this unit, several data sheets have been
developed as an additional form of on-going assessment. A short, internet webquest
developed by a Pennsylvania teacher is included that leads students through several
online questions. These questions are based on previous understanding as well as a
student’s ability to watch and interpret plant motions streamed on online videos.
Additional educational resources can also be found and used to supplement classroom
lessons and activities.
This capstone project was created as a way to show students and teachers the
exciting world of plants and what they are capable of. This capstone can be used as a
resource that provides a thorough background on the historical thinking of plant tropisms,
an explanation of different tropisms, the two leading theories about how these growth
movements occur, and lessons that can be used to demonstrate them.
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Travis Lick
The Chemical Push for the Sun: Theories of Phototropism
Introduction:
The idea that plants sense environmental stimuli has been a subject of debate
since plants were first depicted swooning over Apollo, the sun god, in early Roman and
Greek mythology. Since the time of Plato and Aristotle, a major classification barrier
between plants and animals, was a plant’s perceived inability to respond to the stimuli of
the surrounding environment. Early Aristotelian beliefs and the lack of experimental
data, supressed the thoughts and ideas of generations of botany savvy scientists. Despite
what seemed to be an easily observable phenomena - plants growing or bending towards
a light source - without experimentation the idea that plants were insensitive to the world
around them lingered (Hangarter & Whippo, 2006). Plants react to much of the same
environmental changes that humans and other animals respond to, such as light, pressure,
temperature, water, contact and even the change of day into night. The term used to
describe directional movements of a plant in response to external stimuli is called
tropism. Evidence supports the idea that plants respond to the world around them, but
exactly how they “move” has been strongly contested for hundreds of years. In the
following pages, I will discuss briefly the types of tropisms that plants exhibit and in
detail examine the two conflicting theories about how phototropism is chemically carried
out in plants; the Cholodny-Went theory and the A.H. Blaauw and the growth inhibitor
hypothesis.
Types of Tropisms
Gravitropism:
In order for a plant to successfully grow from a seed, the roots and the shoots
need to differentiate between up and down. The reaction of the pre-emergent stems and
roots is the result of gravitropism, or the directional growth caused by the gravitational
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pull on the seed. Just as the shoot and the root have opposite reactions to light, they also
have opposite reactions to gravity’s pull. Roots exhibit positive gravitropism and
therefore grow in the downward direction with gravity while the shoot has negative
gravitropism and grows against gravity. This detection of gravity is undertaken by cells
at the tip of a root, in a tissue called the root cap. Inside the cells of the root cap, there
are sensors called statocytes that contain starch granules. These statocytes settle on the
bottom-most side of cells in the root cap and indicate to the root cells the direction they
need to grow.
Hydrotropism:
In addition to a plant’s initial growth response to light and gravity, it responds to
differing levels of moisture in the soil by directing root growth towards these areas. This
positive growth reaction is called hydrotropism. “The hydrotrophic response has been
shown in lab tests where a root system can respond to a gradient in water potential as
small as 0.5 MPa by growing toward the higher water potential.” (Hirasawa, Suge.
Takano, Takahashi, 1995)
Thigmotropism:
Plants also exhibit the ability to change growth direction based on physical
contact with a stimulus. This phenomenon is called thigmotropism. Thigmotropism in
the root system occurs when the root cap contacts an object that is cannot penetrate, such
as a rock layer or other subsurface obstruction. In these instances, the plant utilizes outer
epidermal cells of the root cap as touch sensors. When these papillae contact an object,
the cells become deformed and growth ceases. This continued process of touch,
deformation, touch, deformation, causes the root to grow around the object obstructing it
(Vartarian, 1997). When the papillae sensors no longer contact the object, they continue
to grow normally. This reaction by the root is called negative thigmotropism.
Conversely, the stem system of plants exhibits positive thigmotropism and grows towards
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objects that its papillae contact. This can be seen in plants that are viney like the sweet
pea, cucumber, hops and kudzu.
Phototropism:
Plants are extremely sensitive to light and respond to it in a positive and a
negative way. The stems and leaves of a plant have positive tropism which means they
grow in the direction of light, while roots exhibit negative tropism in that they grow in
the opposite direction of light.
These observed changes in the directional growth of plants to different types of
environmental stimuli have been observed and discussed for centuries. People from the
era of Aristotle and even before have noticed that plants moved in conjunction with the
positioning of the light sources around them. These early observers believed this to be a
passive response that could not possibly be controlled by the plants themselves. The
thought that plants merely existed was a long held belief due to early scientists lack of an
informed evaluation of what they were observing. The reason for this was the lack of
experimental evidence to support ideas. The scientific method, which involves the
testing of hypotheses by experimentation, rather than by deducing ideas based on
traditional thought, is largely attributed to Roger Bacon, a 13th century English
philosopher. Because of his and others that followed, scientists now carry out in depth
procedures based on hypotheses and conclusions drawn from observations rather than
intuitive thinking. This scientific method has helped to both create and discredit ideas of
how plants are affected by their environment.
Part I: The Cholodny-Went Theory
The Cholodny-Went theory of plant tropisms has been the foundation of
understanding plant movements since the late 1920’s. Went suggests that “unilateral light
induces a later redistribution of endogenous auxin near the apex of the organ” (McDonald,
2003). Simply put, the group of hormones, called auxins (more specifically indole-3-
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acetic acid or IAA) are responsible for the elongation of plant cells that cause the overall
curvature of the plant in the direction of a light source. The auxin in plants is responsible
for the cell elongation, root, stem and leaf growth as well as apical dominance and
reproduction. The change in the directional growth of a plant is caused by auxin that
resides in the coleoptile of a maturing plant shoot. A coleoptile is “the protective sheath
that surrounds the young shoot tip and embryonic leaves of a plant during its passage
through the soil to the surface (American Heritage Dictionary, 2000). Figure 1 below
shows how this phenomenon occurs.
In the figure, you can see the auxin (purple) gathering on the left side of the
coloptile. As the auxin moves down the shoot, the redistribution stops when it has evenly
distributed itself along the shaded portion. Once this happens, the auxin stimulates cell
growth and cell division only in the region opposite the light source. The plant cells react
to the high auxin levels by transporting Hydrogen ions into their cell walls and lower the
pH. Due to this, the plant elongates more rapidly on the auxin rich side causing the
overall result, the extension of the plant in the direction of the light source. At the
completion of the experiment, a lateral difference in auxin concentration can be measured
in conjunction with the curvature of the plant.
Experimental Support:
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Corn seeds were raised for 4 days in a plant-culture vessel, soaked under running
water for 2 days and then planted with the embryo facing upward on paper towel. These
seeds were then soaked with de-ionized water (to stunt growth) and remained under red
light for 2 days. After two days, specific plants were chosen based on straightness and
length. They were placed parallel to each other in the dark for an additional day and
reselected according to likeness. (Furyura, Nick, and Schafer 1992)
These seeds were then allowed to grow under normal circumstances with all
receiving equal amounts of life sustaining support. Prior to the development of
embryonic leaves, the tip of the coleoptile was removed. The failure of the shoot to
respond to the directional change of the light showed the importance of the coleoptile tip
in the plants ability to respond. However, when the coleoptile was placed in an agar
block and then reattached to the plant (figure2), the plant resumed normal growth as
shown.
In figure 3, the coleoptile was removed and placed on the edge of the remaining
plant shoot . Given time to respond, the results showed that the response of the plant was
to bend away from the reattached tip.
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In figure 4, the tip of the coleoptile was removed and placed on an agar block.
The agar block was then attached to the remaining plant shoot as done in figure 2. The
result of this experiment showed that the coleoptile again had a curvature in the direction
opposite the placement of the agar block. The auxin diffused into the coleoptile and
allowed the growth response to take place. The idea of a diffusible substance into the
agar block was an enormous indicator that there was a chemical or hormone (auxin)
responsible for growth stimulation.
The final conclusion discussed in the Cholodny-Went model is the idea that the
auxin is laterally transported to the coleoptile of the shoot before it is polarly diffused
down the elongation zone of the shoot. Without the occurrence of diffusion, the
elongation of the cells would not occur and the photrophic response would not be seen.
Figure 5 shows the red mica barrier oriented perpendicular to the direction of light,
impeding the ability of the auxin to diffuse and consequently the even distribution of
auxin. When the mica barrier is limited to a distance of 1mm from the tip of the
coleoptile, the auxin diffusion takes place and the ability of the plant to show photrophic
response is intact.
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Part II: The Growth Inhibitor Hypothesis
The main opposition facing the Cholodny-Went theory was introduced by A.H.
Blaauw in 1919 which proposed that phototropism is a secondary consequence of
differential growth inhibition associated with photomorphogenesis (Blaauw, 1919).
Blaauw also observed the elongation of the shaded portion of plants through the process
of cell growth but instead of attributing this to the increased levels of auxin on the shaded
half, he believed that it was caused by a growth inhibition of the lighted side as opposed
to a growth stimulation (Galston & Sharkey). In a continuation of the Blaauw idea, J.
Van Overbeek observed that the auxin gradient did in fact increase with the shaded side
of the hypocotyls but that nearly “half of the differential growth associate with
phototropism can be attributed to light-mediated growth inhibition” (Overbeek, 1932).
Experimental Support:
Extractible and Diffusible Auxin:
After the light perception by chryptochrome, the chain of events leading
eventually to the phototrophic curvature of a plant organ is wholly unknown until the
final process, cell elongation (Bruinsma & Hasegawa, 1990). Blaauw’s hypothesis is still
debated today nearly ninety years after its proposal, not because of overwhelming
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evidence to support it, but because of the lack of evidence to refute it. The reason for
this, the often used bioassay cannot distinguish between an increase in the auxin level on
the shaded side of a plant as opposed to an increase in growth inhibition on the lighted
side. This indirect evidence leaves the door ajar for Blaauw’s model even today.
Extractable and diffusible auxin levels were tested in the “longitudinal, illuminated and
shaded halves,” of plants that were showing a phototrophic response to sunlight and
when measured, contained evenly distributed amounts of auxin (Bruinsma & Hasegawa,
1990). Furthermore, etiolated oat coleoptiles were separated and attached to agar blocks,
in the same manner as was mentioned earlier in the Went experiment, and reattached.
After inspection, “practically the same amounts of IAA had diffused from the shaded and
lighted sides and from the dark control, but also that these amounts were much higher
than indicated by the bioassay. The IAA content in the blocks from the control and the
shaded side was 2.5 times as high as indicated by their auxin activity, and from the
lighted sides even 7 times higher (Bruisma & Hasegawa, 1990).” What this indicated
was that at the same time the auxin was diffusing laterally through the coleoptile, there
was also an inhibitor diffusing throughout, but most importantly in higher concentrations
on the illuminated side. These results are shown in figure 6 below.
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In Conclusion:
While the Cholodny-Went theory has withstood the test of time as a way of
explaining the way a plant uses auxin in photropic responses, given new experimentation
as well early experiments by Blaauw, it is too simplified and not a complete explanation.
In addition to lateral auxin diffusion in the coleoptile of plants causing cell elongation
and therefore an overall curvature in growth, the fact that growth inhibitors are also
present clouds the exact cause. While the curved growth response, the final step in
phototropism is accepted, there are still many things that can not be proven without
further experimentation. The study of phototropism is a microcosm of why the scientific
method plays such an important role in the advancement of knowledge.
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Works Cited Aristotle, (1891).Science. American Association for the Advancement of Science. 17. Blaauw, A.H. (1919). Licht und wachstum III. Meded. Landbouwhogeschool Wageningen vol.15, 89-204. Brown, A (1993).Circumnutations: From Darwin to Space Flight. Plant Physiol. 101, 345-348. Bruinsma, J, & Hasegawa, K (1990). A New Theory of Phototropism - its regulation by a light induced gradient of auxin-inhibiting substances. Physiologia Plantarum. 79, 700-704. Furyura, M, Nick, P, & Schafer, E (1992). Auxin Redistribution during the First Positive Tropism in Corn Coleoptiles. Plant Physiol. 99, 1302-1308. Galston, A, & Sharkey, T (unknown). Frits Warmolt Went. Biographical Memoirs. Hangarter, R, & Whippo, C (2006). Phototropism: Bending Towards Enlightenment. The Plant Cell. 18, 1110-1119. Hirasawa, T., Suge, H., Takahashi, H., & Takano, M. (1995). Hydrotropism in roots: sensing of a gradient in water potential by the root cap. Planta, 197, 410-413. Laudan, L (1978). Progress and Its Problems: Toward a Theory of Scientific Growth. California: Press. McDonald, M (2003). Photobiology of Higher Plants. Wiley. Overbeek, J.V. (1932). An analysis of phototropism in dicotyledons. Proc. K. Ned. Akad. Wet. vol.35, 1325-1335. Vartarian, Steffan (1997). Thigmotropism in Tendrils. Retrieved August 1, 2006, Web site: http://biology.kenyon.edu/edwards/project/steffan/b45sv.htm Figure 1: http://resources.ed.gov.hk/biology/english/images/environment/coleoptile.jpg Figure 2: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Tropisms.html Figures 3,4,5: http://www.bio.indiana.edu/~hangarterlab/courses/b373/lecturenotes/tropisms/phototrop.html
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Figure 6: Bruinsma, J, & Hasegawa, K (1990). A New Theory of Phototropism - its regulation by a light induced gradient of auxin-inhibiting substances. Physiologia Plantarum. 79, 704.
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Travis Lick
Pedagogy
Plant Tropisms
Unit Introduction/Overview: This unit plan is designed for middle school and early high school level students
with a background in plant biology. The focus of this unit plan is for students to
implement hands-on activities that will help to explain plant tropisms. The unit
plan includes a short pre and post assessment as well as laboratory experiment
data sheets that can be used to measure student understanding.
Enduring Understandings: 1. Plant tropisms can be positive or negative, which means the growth is towards
or away from a stimulus.
In order for a plant to successfully grow from a seed, the roots and the shoots
need to differentiate between up and down. The reaction of the pre-emergent
stems and roots is the result of gravitropism, or the directional growth caused by
the gravitational pull on the seed. Just as the shoot and the root have opposite
reactions to light, they also have opposite reactions to gravity’s pull. Roots
exhibit positive gravitropism and therefore grow in the downward direction with
gravity while the shoot has negative gravitropism and grows against gravity.
2. Plant tropisms are the directional growth of a plant in response to an
environmental stimulus.
Plants are extremely sensitive to light and respond to it in a positive and a
negative way. The stems and leaves of a plant have positive tropism which means
they grow in the direction of light, while roots exhibit negative tropism in that
they grow in the opposite direction of light.
Student Misconceptions: Students often have difficulty understanding most of the processes that plants
undergo, from photosynthesis and respiration to water uptake and tropisms. At the
elementary level, students often have the misconception that plants absorb “food” and
water with their roots. Hershey says, “There is “a recognized tendency, even for the
knowledgeable biologist, to overlook, underemphasize or neglect plants when teaching
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introductory biological courses” (Wandersee, 1999). Too often, biology is “botany
taught by a zoologist,” leaving students with “the popular delusion that biology is the
study of animals” (Nicoles, 1919). This results in widespread ignorance about under-
appreciation of plants” (Bozniak, 1994).”
Though misconceptions specifically about students thoughts on tropisms isn’t
readily available, having done some experimental work in this area with children before,
they often fail to correctly predict tropic responses by both the root and stem systems.
The lessons that were developed for this unit plan were created to simulate
student interest in plants through the use of exciting, hands-on, inquiry based lessons.
Carter says, “New teachers coming out of our universities and colleges are very poorly
trained in basic botany.” This is something that can be avoided if activity based
enrichment is used in early education and continued throughout.
Essential Questions: 1. What is the purpose of a plants ability to have stems grow upward and roots to
grow downward?
2. How does light affect the roots, stems and leaves of plants?
3. How does the color of light affect plant tropisms?
Content Standards and Strategies That Can Be Used to Address Them: National Science.5-8.1 Science as Inquiry:
- Abilities necessary to do scientific inquiry
- Understandings about scientific inquiry
Through inquiry based activities, students will see how roots and stems will grow
properly regardless of their orientation in the soil or the position of a light
source.
National Science.5-8.3 Life Science:
- Diversity and adaptations of organisms
Students will observe plant tropic responses and their usefulness in helping plants
adapt and survive.
National Science.5-8.7 History and Nature of Science:
- Science as a human endeavor
- Nature of science
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- History of science
Students will understand the history of phototropic and gravitropic
experimentation.
National Arts-Visual Arts.5-8.3 Choosing and Evaluating a Range of Subject
Matter, Symbols, and Ideas:
- Students use subjects, themes, and symbols that demonstrate knowledge of
contexts, values, and aesthetics that communicate intended meaning in artworks
Students will create visual representations of plant growth and response to
different environmental stimuli.
National Language-English.K-12.7 Evaluating Data:
- Students conduct research on issues and interests by generating ideas and
questions, and by posing problems. They gather, evaluate, and synthesize data
from a variety of sources (e.g., print and nonprint texts, artifacts, people) to
communicate their discoveries in ways that suit their purpose and audience
Students will construct procedures and experiments based on previous activities
to test their hypotheses (Phototropism Activity).
National Language-English.K-12.8 Developing Research Skills:
- Students use a variety of technological and information resources (e.g., libraries,
databases, computer networks, video) to gather and synthesize information and to
create and communicate knowledge.
At the conclusion of observational activities, students will perform a webquest
that furthers understanding and combines aspects of lecture and inquiry
knowledge.
National Language-English.K-12.12 Applying Language Skills:
- Students use spoken, written, and visual language to accomplish their own
purposes (e.g., for learning, enjoyment, persuasion, and the exchange of
information)
Students will create a presentation that will be delivered to the class. This will
explain the hypotheses developed and how they think the procedures will help
prove them. .
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National Technology.K-12.3 Technology Productivity Tools:
- Students use technology tools to enhance learning, increase productivity, and
promote creativity.
- Students use productivity tools to collaborate in constructing technology-
enhanced models, prepare publications, and produce other creative works.
At the conclusion of observational activities, students will perform a webquest
that furthers understanding and combines aspects of lecture and inquiry
knowledge.
Students will use computer graphing equipment to prepare graphs of data.
Background Information: When a germinated seed begins to grow, the seed coat cracks and an
embryonic root emerges from the seed and starts to gather nutrients and water. At
this point, the hypocotyl (stem) elongates and helps to push the cotyledons (seed
leaves) upward toward the surface where the cotyledons will expand and provide
a source of energy for the plant until the true leaves grow.
The orientation of seeds is not critical to proper root and stem growth due
to the root’s response to positive gravitropism and the shoot’s response to
negative gravitropism. The detection of gravity is undertaken by cells at the tip of
a root, in a tissue called the root cap, inside a pre-developed seed. Inside the cells
of the root cap, there are sensors called statocytes that contain starch granules.
These statocytes settle on the bottom-most side of cells in the root cap and
indicate to the root cells the direction they need to grow. The coleoptile (stem tip)
is responsible for detecting the force of gravity and in turn, growing away from it
towards the surface and more importantly, light!
Plants are extremely sensitive to light and respond to it in a positive and a
negative way. The stems and leaves of a plant have positive tropism which means
they grow in the direction of light. Auxin is a chemical that promotes the rapid
elongation of growth cells in the shoots of a plant. Auxin also helps the plant to
remember where it has branched off in the past and also in which direction it
needs to grow. In the figure, you can see the auxin gathering on the left side of
the coloptile, which is the covering of a plant shoot that enables it to grow. As
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auxin moves down the shoot, the redistribution stops when it has evenly
distributed itself along the shaded portion. Once this happens, the auxin
stimulates cell growth and cell division only in the region opposite the light
source. The plant cells react to the high auxin levels by transporting Hydrogen
ions into their cell walls and raising the pH. Due to this, the plant elongates more
rapidly on the auxin rich side causing the overall result, the extension of the plant
in the direction of the light source.
Classroom Activities: Enduring Understanding 1: Plant tropisms can be positive or negative, which
means the growth is towards or away from a stimulus.
Activity One1: Which Way Is Up?
Pre-Activity Thought Questions:
1. “How does a developing seed “know” which way is “up” and which way is
“down?”
2. How will the roots and stems of seeds oriented in different ways grow? Sketch
a drawing on your activity sheet (Appendix A).
Time Frame:
Construction of the seed germinator and placing the seeds properly will take one
50 minute class period. The observation portion of the activity will take place
over a three day period.
Learning Objectives:
- Students will determine whether their hypotheses about plant growth were
correct about seed orientation and root/stem growth.
- Students will understand that plant roots and stems reorient in the direction of
their growth to conform with the direction of gravitational force.
Materials:
- two soda bottle caps - forceps 1 Modified Lesson from Williams, 2007
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- plastic wrap - hand lens
- paper towel - elastic band
- plant seeds - hypothesis/data sheet
Procedure:
1. Cut two layers of paper towel into circles that will fit into the bottom of a soda
bottle cap.
2. Place the towel in the cap and moisten it with water.
3. Orient the four seeds in a north-south-east-west position on the moist towel
surface, making sure that the brown area of the seed is facing the center of the
cap.
4. Make a mark on the bottle cap that indicates the direction of north, or up.
5. Cover the open cap with plastic wrap and secure the wrap with an elastic
band.
6. Position the bottle cap germinator so the seeds are in a vertical orientation by
standing it in a second bottle cap.
7. Make a drawing of the seeds in the germinator, including the orientation of the
brown area toward the center of the circle. Mark this in circle 1 on your data
sheet.
8. When the roots begin to emerge, record the direction of the emerging root
from each seed with a drawing on circle 2. Use the hand lens for detail.
9. Be sure to keep the paper towel moist each day. After 48 hours, make a third
drawing showing the roots and hypocotyls and cotyledons.
10. Reorient the germinator 90 degrees and predict what the seedlings will look
like after 24 hours in circle 4.
11. 24 hours after reorientation, draw the seedlings on the data sheet in circle 5.
Concluding Group Discussion or Independent Activity:
- What changed from data circle 4 to data circle 5?
- Compare the outcome of the orientation with what you predicted would happen.
- In response to gravity, which direction will roots and stems grow?
Question for future lesson:
- What might be the possible influence of light in this experiment?
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Activity Two2: Gravitropism
Pre-Activity Thought Questions:
1. How responsive are germinating seedlings to Earth’s gravity?
Time Frame:
Construction of the gravitropism chamber and placement of the seeds within, will
take on 50 minute class period. Observations need to be conducted for three
consecutive days after construction.
Learning Objectives:
- Students will learn that seedling hypocotyls orient in the direction opposite the
force of gravity.
- Each student will construct a gravitropism chamber.
- Students will make predictions and confirm the accuracy of them based on
four different pieces of observational data (Appendix B).
Materials:
- 35 mm black film container with lid - two additional film can lids
- double stick tape - white masking tape
- mm graph paper (.5cm x 4cm) - permanent marker
- 4 strips of paper towel (4.5cm x 10cm) - plant seeds
- foam disc (size of canister) -forceps
- water bottle - permanent marker
Procedure:
Preparing the gravitropism chamber---
1. On each film can lid place a 3cm strip of double stick tape and then attach the
lids to the outside wall of the film can so that each lid is opposite the other.
- Mark the film can using a permanent marker to draw arrows on the film can
lid and one of the mounted lids to indicate “FRONT.”
2 Modified Lesson from Williams, 2007
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- With the front facing you, stick a white label on the right side of the chamber
and draw a compass label, marked with the angles of 00/3600, 900, 1800, and
2700, corresponding to north, south, east and west.
2. Place the foam disc in the bottom of the film can.
3. Use the water bottle to add enough water to saturate the foam.
4. Dip the end of a germination strip into the bottom of the can, touching the
water until it wicks up some of the free water.
- Align the germination strip vertically inside the film can, with the grid strip
against the inner wall and the wick strip overlapping it and adhering to the
wall.
5. Align the germination strip with the front orientation of the chamber. At this
stage you may let the strip extend above the rim of the chamber.
6. Repeat the procedure with the other germination strips, aligning them to create
four strips opposite each other aligned at 90 degree angles.
7. Now remove one strip pair from the chamber and with your fingers or using
seed forceps to pick up one seed, place it about 2cm down the strip.
8. With the forceps align the seed in a downward pointing direction.
9. Replace the strip with the bottom of the wick touching the wet foam disc and
the top of the strip just below the rim of the chamber.
10. Repeat steps 8-10 until all four seeds are on the wicks and in the chamber.
11. Gently place the film can lid on and seal the chamber.
12. Put a white tape label on the top of the lid and label your name, date, time on a
24 hour clock, and a 00 symbol indicating the initial orientation of the
chamber. Add the information from the chamber label into the first three
columns on the Data Sheet (Appendix B).
13. Place the chamber in the upright 00/3600 position, where a relatively uniform
temperature of between 22 and 30 degrees Celsius.
14. The chambers need to be viewed every 12 to 24 hours, noticing any
elongation of the hypocotyls.
15. Over the first 24 to 48 hours, the seedling hypocotyls will grow to between 1
and 2cm. Make a drawing of the vertical seedling on each strip in the
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appropriate box in the first row of the Data Sheet. Record the time, and total
hours from hour zero.
16. Rotate the chamber 900. Predict the possible outcomes of reorienting the four
seedlings.
- What will the seedling on each strip look like after 12 to 24 hours? Make a
drawing on the Data Sheet of the predictions.
17. Record the data on the Data Sheet of the first 900 rotation and each rotation
after.
18. After 3 to 12 hours or 24 hours, observe the chamber. Mark the date and time
on the Data Sheet next to the predicted drawing and make a drawing of the
observations.
- Rotate the chamber another 900 to the 1800 position. Make a drawing on your
Data Sheet, predicting the behavior after 3 to 12 hours. Record the date and
time.
19. Continue rotating, observing and drawing until the chambers have completed
a 3600 rotation.
20. When the rotations are finished, remove the seedlings and make a final
drawing.
- Stretch out the seedling to straighten it, then record the length of the
hypocotyls in millimeters on the Data Sheet.
Concluding Group Discussion or Independent Activity:
- Discuss the outcome of the experiment relative to the original hypothesis.
Was the hypothesis verified? How strong is your evidence? Were you able to
successfully predict how the seedlings would respond to successive
reorientation of the chamber?
- What is the average length of the hypocotyls in your chamber after X hours of
germinating in the dark?
- Is there a limit to how long it would grow? If so, what is it?
- When it is elongating, how is the hypocotyls actually growing longer? By
what mechanism?
- Is there a limit to how much bending a hypocotyls can undergo?
21
Enduring Understanding 2: Plant tropisms are the directional growth of a plant in
response to an environmental stimulus.
Activity One3: Phototropism
Pre-Activity Thought Questions:
- How much light is needed to bend a seedling?
Time Frame:
The phototropism chamber will take approximately one 50 minute class period to
construct. Observations will then take place for 4 consecutive days at specific
intervals.
Materials:
- Four 35mm black film containers with lids - Two additional film can lids
- tape - electrical tape
- graph paper (.5cm x 4cm) - permanent marker
- 4 strips of paper towel (4.5cm x 10cm) - plant seeds
- water bottle - pencil
- scissors - four 1.5 cm squares of foil
- hole punch - protractor
Procedure:
1. Make a single hole about 1.5cm from the rim of each of the four film
containers and cover it with clear tape to make a window.
2. Puncture the center of a piece of foil with a pencil, on another make a slightly
larger hole (2mm), the third a large hole (6mm) and the last piece no puncture.
3. Measure the size of each hole and record it on data sheet 1 (Appendix C).
4. Place each of the foil pieces over the windows of each film container.
5. Cover each of these with a small piece of electrical tape.
6. Cover the bottom of each chamber with a small amount of water and place
one seed on a germination strip.
7. Place the seed and the strip in the container opposite the window.
3 Modified Lesson from Williams, 2007
22
8. Place each chamber in a position where light can enter the window once the
electrical tape is removed.
9. Let the seeds germinate in the dark for 48 hours and then remove the window
covers.
10. Open the lid of each chamber and observe the orientation of each seedling.
Place the seedling on Data Sheet 2 (Appendix D) and draw a line indicating
the curvature of the seedling.
11. Use Data Sheet 2 and a protractor to measure the angle of the curvature of
each stem and note it on Data Sheet 1, under T1. Also record the amount of
time that has passed.
12. Repeat steps 10 and 11 after 6, 24 and 48 hours.
Concluding Group Discussion or Independent Activity:
- Students will create a graph using the data collected. The x-axis will be the
diameter of the hole in the window and the y-axis, the degree measure of the
stem curvature.
- A follow-up lesson to this is observing a plant’s phototrophic response to
different colors of light (Appendix E).
Assessment of Student Learning
In order for assessment to be effective, they must align to current standards and
curriculum as well as incorporating a variety of assessment techniques.
Assessments need to be on-going, provide snap-shots of student learning and be
formative and summative in design (Brough, 2005). Assessment needs to guide
instruction and be varied in scope. These guidelines were used in the
development of the classroom activities shown earlier. The students will be given
a pre-assessment prior to beginning the unit on tropisms, will be evaluated
formally and informally through teacher observation as well as written and verbal
lab question reports and finally will be given a post-assessment after the
completion of the unit.
23
Additional Resources: 1- Plants in Motion Webquest:
http://www.scasd.us/hs/science/Vitkauskas/Biology/Ch25PlantResponsesWebque
st.htm
This webquest asks students to use their knowledge of using the internet as a
research tool to help them answer a series of questions.
2 - Plant Processes self-quiz 1:
http://highered.mcgrawhill.com/sites/0078617022/student_view0/unit2/chapter11/
section_2_self-check_quiz-eng_.html
Web based quiz on plant processes that is student guided and self checked.
3 - Plant Processes self-quiz 2:
http://highered.mcgrawhill.com/sites/0078693896/student_view0/unit4/chapter13/
chapter_review_quiz.html
Web based quiz on plant processes that is student guided and self checked.
4 - Potato Maze Lab:
http://www.conservatoryofflowers.org/education/potato_maze.htm
In this experiment, students will watch as a potato plant winds its way through a
maze in its quest for sunlight.
5 - Germinating Seeds in Gelatin:
http://www.all-science-fair-
projects.com/science_fair_projects/50/625/4611fe1a62e1961d454c65c351aeabdd.
html
Students will germinate seeds free of mold on a simple gelatin culture media
using a modified sterile technique and observe the growth of plants under varying
conditions.
6 - Gravitropism Lab:
http://starryskies.com/try_this/plant_growth.html
Student observe a plant’s gravitropic response
24
7 - The Importance of Tropisms:
http://school.discovery.com/lessonplans/programs/tropisms/
Student will observe a plant’s response to stimuli during this experiment.
8 - Plants in Motion:
http://plantsinmotion.bio.indiana.edu/plantmotion/starthere.html
Videos that show plants moving in response to environmental stimuli
25
Appendix Appendix A: Data Sheet for Which Way Is Up?
Appendix B: Data Sheet for Gravitropism
Appendix C: Data Sheet 1 for Phototropism
Appendix D: Data Sheet 2 for Phototropism
Appendix E: Lesson testing a plant’s response to different colored light sources
Appendix F: Pre-Assessment Appendix G: Post-Assessment
26
References:
-Bozniak, E.C. 1994. Challenges facing plant biology teaching programs. Plant Science
Bulletin 40: 42–46.
-Carter, J. L. 2004. Developing a curriculum for the teaching of botany. Plant Science
Bulletin 50: 42–47. http://www.botany.org/bsa/psb/2004/psb50-2.pdf
-Hershey, D. R. 2005. Plant Content in the National Science. Action BisoScience.
http://www.actionbioscience.org/education/hershey2.html
-Nichols, G.E. 1919. The general biology course and the teaching of elementary botany
and zoology in American colleges and universities. Science 50: 509–517.
-Wandersee, J.H., and E.E. Schussler. 1999. Preventing plant blindness. American
Biology Teacher 61: 82,84,86. - Williams, P. 2007. Fast Plants. http://www.fastplants.org/activities.php
27
Launching the Seed Student Sketch SheetLaunching the Seed Student Sketch SheetLaunching the Seed Student Sketch SheetLaunching the Seed Student Sketch SheetLaunching the Seed Student Sketch Sheet
Circle 1:Circle 1:Circle 1:Circle 1:Circle 1:
Sketch
and
label your
bottle cap
seed germinator
at time of placement of seeds.
Circle 2:Circle 2:Circle 2:Circle 2:Circle 2:
Sketch
and
label your
bottle cap
seed germinator
as the roots emerge from the seeds.
Circle 3:Circle 3:Circle 3:Circle 3:Circle 3:
Sketch
and label
your bottle
cap seed
germinator 24 to
48 hours after placement of seed.
Circle 4:Circle 4:Circle 4:Circle 4:Circle 4:
Sketch
and label
your bottle
cap seed
germinator with
your prediction of the effects of
reorientation on your seedlings.
Circle 5:Circle 5:Circle 5:Circle 5:Circle 5:
Sketch
and label
your bottle
cap seed
germinator 12 or
24 hours after reorientation to
compare with sketch in Circle 4.
Write about what you have learned about germination and orientation.
28
Envir
onm
ent
Stu
den
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e 1
Tem
per
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ge:
˚C
Stu
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Ave
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ily T
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Germ
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Da
te
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te
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te
Tim
eTim
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eTim
eTota
lTota
lTota
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lH
ours
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mH
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n1111 1
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4444 4
(24
hr
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ck)
(24
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ck)
(24
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ck)
(24
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ck)
(24
hr
clo
ck)
Hours
Hours
Hours
Hours
Hours
La
st R
ota
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nLa
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ota
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nA
ngle
Angle
Angle
Angle
Angle
pre
dic
ted
observ
ed
pre
dic
ted
observ
ed
pre
dic
ted
observ
ed
pre
dic
ted
observ
ed
length o
f hyp
ocotyl
aft
er
hours
:m
mm
mm
mm
m
00 90
1
80
2
70
3
60
29
Appendix C
30
Phot
otro
pism
Dat
a S
heet
Phot
otro
pism
Dat
a S
heet
Phot
otro
pism
Dat
a S
heet
Phot
otro
pism
Dat
a S
heet
Phot
otro
pism
Dat
a S
heet
Ang
le (
Ang
le (
Ang
le (
Ang
le (
Ang
le ( OOOO O
) at
Hou
r of L
ight
Exp
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e) a
t H
our o
f Lig
ht E
xpos
ure
) at
Hou
r of L
ight
Exp
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e) a
t H
our o
f Lig
ht E
xpos
ure
) at
Hou
r of L
ight
Exp
osur
e
Cham
ber
Cham
ber
Cham
ber
Cham
ber
Cham
ber
Ape
rtur
eA
pert
ure
Ape
rtur
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ure
Ape
rtur
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rea
ofA
rea
ofA
rea
ofA
rea
ofA
rea
ofTTTT T 1111 1
TTTT T 2222 2TTTT T 3333 3
TTTT T 4444 4
Ape
rtur
e #
Ape
rtur
e #
Ape
rtur
e #
Ape
rtur
e #
Ape
rtur
e #
Diam
eter
(mm
)Di
amet
er (m
m)
Diam
eter
(mm
)Di
amet
er (m
m)
Diam
eter
(mm
)A
pert
ure
(mm
Ape
rtur
e (m
mA
pert
ure
(mm
Ape
rtur
e (m
mA
pert
ure
(mm
2222 2 )))) )hrhrhrhr hr
hrhrhrhr hrhrhrhrhr hr
hrhrhrhr hr
hilo
avg
10
0
2 3 4
ambi
ent
tem
pera
ture
dur
ing
tim
e pe
riod
(˚C):
hilo
avg
hilo
avg
hilo
avg
Enviro
nmen
t
Stu
den
t N
ame
1T
emper
atu
re R
an
ge:
˚C
Stu
den
t N
ame
2A
vera
ge D
aily
Tem
per
atu
re o
f Gro
win
g E
nviro
nm
ent:
˚
C
Stu
den
t N
ame
3
Stu
den
t N
ame
4
31
WFP061298
Phototropism:Do Plants Prefer the Blues?
Sample Hypothesis:Sample Hypothesis:Sample Hypothesis:Sample Hypothesis:Sample Hypothesis:My leaves are green,Could it be green?Or is it the red?I’ll guess blue,And test if it’s true.
Question: A Phototropic RiddleQuestion: A Phototropic RiddleQuestion: A Phototropic RiddleQuestion: A Phototropic RiddleQuestion: A Phototropic RiddleIf you were a plantOr a plant were you,Which hue would you chooseTo tie your shoe?Is it red, green or blue?
IntroductionIntroductionIntroductionIntroductionIntroductionThis activity will deal mainly with phototropism, illustrating how plants use various colors of light fordifferent tasks. Unlike the gravitropism activity in which light was excluded, experiments in theclassroom on Earth are done in the ever-present 1 g force. This fact can provide fascinating questionsand design challenges for students.
DesignDesignDesignDesignDesign• Give germinating seedlings a choice of red, green or blue light, each coming from a different
direction, and see if they bend toward one color more than toward the others.
Time FrameTime FrameTime FrameTime FrameTime FrameConstruction of the phototropism chamber will take approximately half of one 50 minute class period.The observational activities will take place over a period of 60 to 72 hours, with the actual time ofobservation and recording data requiring about 15 minutes at each interval.
Learning ObjectivesLearning ObjectivesLearning ObjectivesLearning ObjectivesLearning ObjectivesIn participating in the activity students will:• learn to construct their own experimental equipment from low-cost materials;• learn to set up a simple experiment, make a prediction and observe results; and• understand that blue wavelengths of visible light affect the bending of plants more than red or
green, demonstrating the partitioning of various energy levels of light to different growth functions.
MaterialsMaterialsMaterialsMaterialsMaterials• 35 mm black film can with lid• one floral foam disc, 28 mm diameter and 2 to 4 mm thick
(Floral foam is available from most florist supply stores cut to deminsions noted)• three germination strips (See preparation of germination strips)• three Fast Plant seeds• water bottle• forceps to handle seed• hand-held hole punch• 2 cm wide clear adhesive tape• 2 cm wide black vinyl electrical tape• three 1.5 cm squares, 1 each of red, green and blue transparent plastic mylar (Roscolux® films red
#26, green #89 and blue #69, work well) or colored acetate from art stores or theatre departments
© 1998 Wisconsin Fast Plants, University of Wisconsin-Madison, College of Agricultural and Life SciencesDepartment of Plant Pathology, 1630 Linden Drive, Madison, WI 53706 1-800-462-7417 [email protected]
32
• Making germination strips:- Hold a wick strip with a grid strip aligned on top of it. Moisten
the wick strip.
- As the wick strip becomes moist through capillary action, thegrid strip will adhere to it through the adhesive forces of thewater. Together the wick and grid strip make a germinationstrip.
- The wet germination strip will adhere to the inner wall of the film can gravitropism chamber.
• Making wick strips:- Fold a square sheet of kitchen paper toweling to form an eightlayered rectangle.
- With scissors, trim end and folds to make a rectangle with thedimensions 4.5 cm x 10 to 12 cm.
- Cut wick strips from the rectangle by cutting 1 cm strips.
Preparation of Germination StripPreparation of Germination StripPreparation of Germination StripPreparation of Germination StripPreparation of Germination Strip• Making grid strips:
- Photocopy millimeter square graph paper onto an overhead transparency sheet.
- Cut the sheet along the lines to make strips with the dimensions 0.5 cm x4 cm.
- Grid strips can be reused after rinsing, soaking for 20 minutes in a20% bleach solution, then rinsing again and drying on paper toweling.
ProcedureProcedureProcedureProcedureProcedure1. With a hand-held hole punch, make three windows about 1.5 cm
from the rim of the black film can at approximately 120 degreeintervals.
2. Use a 10 cm strip of clear adhesive tape to cover each windowwith a red, green and blue square.
3. As with the gravitropism chamber, place a floral foam disc in thechamber and wet it with water.
4. Set up three germination strips. The germination strips shouldbe aligned vertically, each spaced between two windows (Figure1). Be sure that the germination strips are below the chamberrim and that there is sufficient, but not excess, water in the floralfoam disc.
33
5. Place a seed, oriented with micropyledown, 2 cm down on each strip.
6. Snap the lid tightly onto the film can andplace the phototropism chamber under alight bank where light will enter all threewindows.
7. Make a top view drawing of your chamber,predicting how the plants will appear after48 to 72 hours of germination.
8. After 48 to 72 hours, open the lid andindicate whether or not your prediction isto be accepted or rejected. As evidence,draw what you observe and compare itwith your prediction.
b
red window
blue window
seed ongerminationstrip
red window
greenwindow
Figure 1:Figure 1:Figure 1:Figure 1:Figure 1: Film can phototropism chamber,view from above.
clear tape(around film can)
Concluding Activities and QuestionsConcluding Activities and QuestionsConcluding Activities and QuestionsConcluding Activities and QuestionsConcluding Activities and QuestionsIn this activity students will have observed the effects of light in orienting the growth of seedlingsin the presence of gravity. Have students consider the following:
- Within the mix of colors making the white fluorescence of your plant lights, which color tells the plant which way is up? Is this the same for humans? Are you sure?
- What has been the influence of gravity on the phototropic response? How would the seedlingsrespond to light if this experiment were carried out in microgravity?
- What will happen to the seedlings if you darken the windows? What will happen if you darkenonly the blue window?
- Recently plant physiologists have isolated minute amounts of a yellow molecule calledflavochrome or cryptochrome that absorbs blue light and is active in the signal transductionpathway that transmits energy from the blue light to the bending response.
34
PRE-ASSESSMENT
Question 1
http://www.tea.state.tx.us/student.assessment/resources/online/2006/grade8/science/8science.htm
Figure 1 above shows a normally growing house plant. Figure 2 shows the same plant lying on its side. If plant 2 is left in this position for several weeks, explain in detail what changes, if any, the plant will show in growth. In your description, be sure to describe the affects of both the roots as well as the stem/leaves. Finally, draw what you believe the plant will look like at the end of this several week period.
35
Question 2
Below are three identical bean seeds growing in soil. Each of them is oriented in a different direction in the soil as can be judged by the emerging cotyledon shown. Draw the root structure and stem system for each seed as it will appear in several weeks time. Explain in detail what caused the roots and stems to develop in the way you have shown.
http://io.uwinnipeg.ca/~simmons/images/seed.gif
36
POST ASSESSMENT
http://www.tea.state.tx.us/student.assessment/resources/online/2006/grade8/science/8science.htm Figure 1 above shows a normally growing house plant. Figure 2 shows the same plant lying on its side. If plant 2 is left in this position for several weeks, explain in detail what changes, if any, the plant will show in growth. In your description, be sure to describe the affects of both the roots as well as the stem/leaves. Finally, draw what you believe the plant will look like at the end of this several week period.
37
Question 2: Below are three identical bean seeds growing in soil. Each of them is oriented in a different direction in the soil as can be judged by the emerging cotyledon shown. Draw the root structure and stem system for each seed as it will appear in several weeks time. Explain in detail what caused the roots and stems to develop in the way you have shown.
http://io.uwinnipeg.ca/~simmons/images/seed.gif
38
Question 3: What is the purpose of a plants ability to have stems grow upward and roots to grow
downward?
Question 4:
How does light affect the roots, stems and leaves of plants?
Question 5:
How does the color of light affect plant tropisms?
39
