Topic: AN INVESTIGATIVE STUDY OF PLANT MOVEMENTS
Source: A seminar presented in Central Illinois Saturday Science Education Seminars for Elementary Teachers
Presenter: Joseph E. Armstrong Department of Biological Sciences Illinois State University Normal, IL 61790
Funded by: Scientific Literacy Center Illinois State Board of Education 1990-1993
Abstract: Does our educational system constrain free and creative thinking
by rewarding us for responding only when we already know the answers?
You already know the answer. This lesson was constructed to both teach you
about science and to teach you how to instruct via investigation. Once you
learn this technique, you will be able to apply it to new subjects, limited
only by your own imagination and knowledge. Investigative instruction
is easier on the classroom instructor than traditional teaching,
but the preparations are more demanding. Observations, explanations,
and predictions of plant growth movements provide the basis for
inquiry-based instruction. The activities can be adapted to several
levels depending on how they are used. Most will work well in grades
5 and 6 and can be easily modified for use with younger students.
PLANT MOVEMENTS (Teacher Copy)
INTRODUCTION
Instruction Through Investigation
This series of investigations employs manipulations (experiments), observations, explanations (hypotheses), and the testing of ideas (predictions). The process is the essence of science--one that allows us to find out about our world. If the idea of doing science worries you or scares you in any way, consider that the human mind unconsciously uses this exact method to solve any problem encountered. The only difference between regular, day-to-day problem solving and scientific problem-solving is that you must be consciously aware of the steps in science. Thus this series of lessons takes an approach that asks lots of questions. When students are told answers and facts, they can only learn by memorization. However, if students observe, reason and prove to themselves the answers, they not only learn some information, but they learn a process used to get answers. Knowing how the information was obtained will make the facts more meaningful and easier to remember, and with practice, students will be able to use the same process to learn new things. In other words, instruction through investigation teaches students problem-solving thinking, a skill more important than any subject matter. In my experience, getting grade school students to guess at answers to any question asked is not as big a problem as getting their teachers to do the same thing. Does our educational system constrain free and creative thinking by rewarding us for responding only when we already know the answers? You already know the answer. Given that there are many more questions than there are answers, if you can only answer a question when you are sure of the answer, there will be a great many questions to which you cannot respond. Science is a process that allows us to decide which of several probable answers is most likely to be correct by eliminating the impossible ones. The result is we will be left with the most probable answer.
Science and biology make an excellent subject to adopt an inquiry-based instructional approach. This lesson was constructed to both teach you about science and to teach you how to instruct via investigation. Once you learn this technique, you will be able to apply it to new subjects, limited only by your own imagination and knowledge. Investigative instruction is easier on the classroom instructor than traditional teaching, but the preparations are more demanding. Investigative teaching takes more time, so less material is covered, but it is quality knowledge.
Introduction to Plant Growth Movements
Plants move! Yes this comes as a surprise to many people, but
plants move in response to their environment. How plants move to orient themselves in their environment is the subject of this investigation. Animals use contractions of muscles to exert a force resulting in a change in their body's position. Like rubber bands, muscles can contract and relax over and over again. But plants do not have muscles or anything like them, so how do they move? Plants change positions by growth movements, which involve two different types of changes, water or turgor movements and cell elongations.
INTRODUCTION Continued
Turgor
An Observation: When plants do not get enough water, they wilt.
An Explanation: Plants constantly lose water from their leaves. If this is not replaced continually by water absorbed by the roots, and moved through the plant to the leaves, the plant wilts. Wilting eventually can result in death, but if the soil is watered, the plant's leaves and stems will resume their former positions.
Every plant cell is surrounded by a flexible cell wall made out of cellulose. Inside the cell wall, every plant cell is surrounded by a cell membrane, a type of fat/protein film that makes up the boundary between the inside and outside of the cell. The living cell surrounded by its membrane pushes outward upon the cell wall, like a balloon inside a box. The uptake of water, called osmosis (pronounced "oz-moe-sis"), causes an outward pressure against the cell wall, called turgor. When cells lose more water than they can absorb, the turgor pressure decreases and the cell gets limp. The combined results of numerous limp cells is wilting.
PLANT MOVEMENTS (Teacher Copy)
Table of Contents
PART I: PLANT TURGOR MOVEMENTS
Lesson 1 Plant Cell Wilting...............................5
Lesson 2 Plant Sleep Movements............................6
Lesson 3 Rapid Plant Turgor Movements.....................7
PART II: WATER UPTAKE AND WATER LOSS FROM PLANT CELLS
Lesson 4 Measuring Water Loss.............................8
PART III: DIFFUSION AND MEMBRANES
Lesson 5 Diffusion in the Air............................11
Lesson 6 Diffusion in Water..............................13
Lesson 7 Diffusion in a Pseudo-Cell......................15
Lesson 8 Diffusion of Water from Plant Cells.............17
Lesson 9 Plant Wilting and the Effect of Salt............19
Lesson 10 Root Pressure of Plants........................20
PART IV: PLANT GROWTH MOVEMENTS
Lesson 11 Orientation of Seedlings.......................23
Lesson 12 Shoot Elongation in Response to Light..........25
Lesson 13 Plant Growth and Phototropisms.................26
SUMMARY........................................................28
PLANT MOVEMENTS (Teacher Copy)
LESSON 1 PLANT CELL WILTING: MODELS WITH HIGH AND LOW TURGOR
Materials construction paper scissors small balloons tape paper clips
Procedure
1. Construct two identical paper cells (boxes) that measure 4"-5" on a side. Cut a hole about the diameter of you finger in one side. Use tape to secure the edges. Insert a balloon in one cell, which will play the role of the cell membrane, and leave the nozzle hanging out through a hole about the diameter of your finger. Secure with a piece of tape or paper clip. Inflate the balloon until it begins to push out against the sides of the box, then use a paper clip to close off the balloon nozzle.
Predictions: Which box seems the strongest? Could they both hold the same amount of weight? Can you decide any way to determine this precisely? Is there a difference in the way the walls of the cell bend?
2. Inflate another balloon to about the size of the one in the cell. This could represent a cell without a cell wall. How much weight can the balloon alone support?
Hypothesis: Generate an explanation of how live plant cells get their strength and what happens when a plant wilts.
Plant cells have structural strength from the interaction between the cell wall and the membrane-bound cell within. It works best like this demonstration, but plant cells are filled with water not air. Neither the cell wall or the plant cell are as strong as they are together. This demonstrates plant cells in leaves, young stems, and other soft parts of plants. Plants have hard, woody parts where the cells have very thick, inflexible walls, and these cell walls are very strong and do not need support from a cell within.
When a plant cell looses water, it is just like the balloon deflating. The inflated balloon produces high turgor pressure in the cell, and deflating the balloon decreases the turgor pressure. The balloon is inflated by air pressure produced by your chest muscles. In subsequent investigations the process of osmosis, which inflates cells with water, will be explored.
PLANT MOVEMENTS (Teacher Copy)
LESSON 2 PLANT SLEEP MOVEMENTS
Background Information
Non-wilting Turgor Movements - Some plants have specialized cells that can alter their turgor pressure, either slowly or quickly, resulting in a leaf movement that is rather like a controlled wilting. Like wilting, this movement can be reversed, but unlike wilting, the change in turgor only takes place in certain cells and the lack of water did not cause the change. Many plants will change positions throughout the day especially as the sun sets and it gets dark. The leaves droop and fold up as the turgor pressure decreases producing so called sleep movements. Since only certain cells are involved, there is no general wilting.
Materials shamrock, bean, or pea plants one box large enough to cover the plant
Procedure
Observations: Place a plant in the dark and observe its leaf positions. (Note: all of these plants have leaves that bear three to many leaflets on a single central stalk.) Compare the three to many leaflets on a single central stalk.) Compare the leaf and leaflet positions of a "sleeping" plant to an "awake" plant. Sketch the awake and asleep positions of a plant leaf. The use of comparative observations is a common method in science and from the similarities and differences, we can sometimes guess how things work. Try answering the following questions.
Where are the cells located that can undergo a turgor change and produce sleep movements?
Do you guess that the turgor pressure in these cells is increased or decreased to create this movement?
PLANT MOVEMENTS (Teacher Copy)
LESSON 3 RAPID PLANT TURGOR MOVEMENTS
Materials Mimosa or sensitive plant Venus fly trap plants
Procedure
Observations: Touch the underside of a leaf of Mimosa. What takes place following your touch? Can you locate the places where the plant actually moves? Where are the cells located that change turgor pressure? Leave the plant undisturbed and see how long it takes to return to its previous position. What function might such a movement have, i.e., what could be its advantage to the plant? How is this similar or different from the sleep movements?
Observations: What is the Venus fly trap famous for? Note that each leaf consists of a broad, flat leaf stalk and two leaflets that compose the trap. Look very closely at the inner surface of a trap. What do you observe?
The three hairs act as triggers activating the trap. A single hair must be touched twice in succession or two hairs must be touched in sequence, and then the trap snaps shut. Where are the cells located that change in turgor pressure? In this case high turgor pressure is needed to force the trap to stay open, and when this is released, like a spring on a mouse trap, the trap snaps shut.
Why do Venus fly traps trap flies? Does this plant really eat insects? Green plants make their own food from sunlight, water, and carbon dioxide, and this plant is green. But many insect capturing plants live in places where they do not get enough nitrogen, a common component of fertilizers. Plants need nitrogen to stay green. Insect-capturing plants use the insects as a source of nitrogen, not as a source of food.
PLANT MOVEMENTS (Teacher Copy)
LESSON 4 MEASURING WATER LOSS
Background Information
A potato is a stem with the special functions of storing food and water. The stored food is starch. You can demonstrate the presence of starch by putting a drop of iodine solution (available at pharmacies as tincture of iodine) on piece of potato. The resulting blue-black color indicates starch. Cellulose and starch are composed of the same subunits--glucose (the sugar that constitutes honey) but because the glucose is put together in different ways in starch and cellulose, these molecules are very different. You can digest starch, but you cannot digest cellulose. A drop of iodine on a paper towel will not turn color (and you don't want to eat it). A drop of iodine on notebook paper will turn blue because starch is used to make a smooth surface for writing.
Materials potatoes balance metric scale potato peeler
Procedure
Question: How does a potato get water?
Answer: Presently it doesn't, but it got water from the root system when the potato plant was actively growing. Now the green parts and roots are dead, but the potato remains alive storing its food and water until it again begins to grow (sprout).
Question: If a potato is not getting any water now, is it losing any water?
You may not know the answer to this question, which is yes. But that's OK. The proper scientific way to proceed is to assure that either situation is equally possible, i.e., potatoes may or may not be losing water.
Ask your class how would they find out if a potato loses water or not? If no one has any ideas, ask how they know they have grown. They can have their height measured and they can weigh themselves. So this can be done for a potato tool It can be measured and weighed.
Now that you have a procedure consider both possibilities and what you would expect if either is true. (Note: I'm going to label hypotheses, predictions, etc. as a scientist would just to make you aware of the process. I think grade school students can understand this process, particularly if presented as examples.)
The two logical alternatives can be presented as follows:
LESSON 4 Part II
Hypothesis 1: A potato does not loose water.
What would you predict if this is true? The potato's weight and size should remain constant (a prediction).
Hypothesis 2: A potato looses water.
Our prediction is that if a potato looses water it should get lighter and smaller.
Experiment
Using a balance and a metric scale you can now test which of these two hypotheses is true by determining which prediction is correct. Thus the weighing and measuring becomes an experiment to determine which of these two hypotheses is correct.
Let's reconsider the situation briefly. What about a potato might keep water in or cause it to loose water very slowly? What does a potato have in common with an apple, grape, peach, etc.? Does the skin of the potato help retain the water? This idea can be expressed in a second set of hypotheses.
Hypothesis 3: The potato skin does not allow any loss of water.
If this is true, the results should be the same as Hypotheses 1.
Hypothesis 4: The potato skin slows down the loss of water.
If this is true then if the skin is removed, the potato should loose water more rapidly.
Now design an experiment that will test all 4 hypotheses. How can this be done?
If the weight and size of a potato is monitored daily, it will either change or not, and either #1 or #2 will be correct. If you peel a potato, then measure and weigh it, and monitor its size and weight daily along with a regular potato, we should be able to determine the function of the potato skin.
Have your class construct a data table and decide how to measure and weigh the potatoes. You might have to mark them to accurately re-measure their length with a scale. Or can you think of another approach? Maybe a tape measure of string could be used to measure their girth.
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LESSON 4 PART II Continued
Results
With a typical classroom beam balance, there should be a recordable weight loss in the unpeeled potato, especially if it is left in a dry place. The peeled potato will dry out very fast. Can you determine from your data if the rate of water loss from the peeled and unpeeled potatoes was constant, increasing, or decreasing during this experiment? Do you know how to answer this question? How would you present the data to find out the answer?
Conclusion
Even plant organs designed to store water will loose water. Removal of the thick skin increased the rate of water loss.
PLANT MOVEMENTS (Teacher Copy)
LESSON 5 DIFFUSION IN THE AIR
Observation: Does anyone smell anything out of the ordinary? Everyone who can smell something raise your hand. Can you identify the smell? Can anyone discover where it is coming from?
This demonstration can be done with simple things like ground coffee, perfume, peppermint extract, or perfume.
Why do the smells of things cooking, e.g., brewing coffee, compared to room temperature ground coffee, seem stronger and move farther and faster than other smells?
Given the information that everything is composed of molecules and smells are simply molecules that we can sense with detectors in our nose, what kind of explanation can account for these types of observations? To help you reach the right kind of explanation, ask yourself where are there the most molecules of the substance producing the odor? Your tentative explanation is a hypothesis.
Any tentative explanation is composed by generalization, sort of an educated guess. This type of generalized guessing uses inductive logic.
Hypothesis: Molecules will move from places where they are more crowded together (high concentrations) to places where they are less crowded together (low concentrations). This is a process called diffusion.
How long will diffusion continue? Will the molecules just keep spreading out? What happens if there is some barrier to their continued movement?
Consider this example. Suppose we crowd everyone in the class into a corner, packed so tightly no one can move. Then you were given instructions to begin moving apart (diffusing) from each other, step by step, but you were not allowed to leave the classroom. With the first step, only those people on the outside edge could move. People back in the pack could not move. Since molecules never stop moving and bounce off each other and other obstacles, to mimic molecules, any student that reaches a desk, bookcase, or other student should turn around and take their next step in the opposite direction. Whenever two student molecules get within arms' length of each other, they must turn around and move in opposite directions , just like at a wall.
Where will everyone be if the teacher yells "STOP" after 20-30 steps? After several STARTS and STOPS the students will be pretty evenly dispersed around the room. Would you ever get back to the starting position, clustered together in a corner? Are you continuing to get further apart? Now what would happen if we
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LESSON 5 Continued
did this outside with the same instructions to keep moving further apart, but with no boundaries? What happens to the odor molecules outside?
This thought experiment should give you some idea of what is happening to molecules that can move, even though we do not know what or why they are moving. That's OK because there has to be something left for universities to teach.
If molecules behave like the rules of our movement game they should: 1. Move from areas of high concentration to low concentration until there is on the average an even distribution of the molecules;
2. Bounce away from collisions with obstacles and each other, constantly moving.
Complete evenness of distribution never really occurs because molecules keep moving. In an open system the molecules just keep dispersing, spreading out and diluting. This accounts for things completely evaporating and "disappearing".
Odors move around because the scent molecules are highly concentrated at their source, so they diffuse outward. When they reach a concentration our nose can detect, we smell them and sometimes we can follow the increasing concentration of molecules to its source. The additional energy of the heat causes the molecules to move faster, and this is one definition of heat. Adding heat to the system is just like asking the students to play the same movement game, but by running instead of going step-by-step. (They will want to try it this way, but I recommend against it.)
Use this hypothesis to explain what you're doing when you make tea. Why do you use hot water? Can you use cold water? What is the time difference between the two methods?
PLANT MOVEMENTS (Teacher Copy)
LESSON 6 DIFFUSION IN WATER
After considering the results of the first experiment, ask your class to predict what will happen if you gently added a drop of food coloring to the top of a test tube of water. The dye should be added without any force, as if you were adding a layer on the top.
What would happen if you set up two test tubes and one was filled with cold water and one was filled with hot water? Record your predictions (basically just and intuitive, educated guess) and reasons for thinking that based on the previous information. To make predictions, think in terms of IF this is true, THEN this should happen, should be true also, should cause, etc. IF-THEN statements represent DEDUCTIVE LOGIC, which is used to make all predictions or guesses.
Here is a new situation where you can apply your hypothesis, your explanation, to a new situation to determine if it continues to explain everything observed. This is how scientists test their hypotheses.
Note for a larger classroom demonstration, you could place a small bottle or vial of food coloring in the bottom of a large bowl or jar of water. Use care to not disturb the water.
Observation: Set up two test tube or small jars, one with hot water and one with cold. Add a couple of drops of dye at the surface as if you were trying to add a layer at the top. Don't squirt the dye into the water.
Record the changes every few minutes. Where did the dye diffuse fastest? Did the results agree of disagree with our explanation? After the coloration is equal throughout the water, do any areas ever get darker or lighter? or does it always stay just the same?
To tell if there is a difference, is it better to run two experiments separately or simultaneously? (To tell which of two students is the fastest runner, would it be easier to have each one run past you separately, and then judge who went past faster, or would it be easier to have them both run by together?)
Explanation/Hypothesis: The dye molecules are crowded together in the drop at the top of the tube. So they start to move from a high concentration of dye molecules to a low concentration, the water. There is also a reciprocal movement of water. The concentration of water molecules are obviously higher in pure water than in the drop of dye. So there is actually a two-way diffusion of dye molecules and water molecules. The movement of the dye and water molecules will continue until the water is evenly colored, and there no longer is any difference in the
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LESSON 6 Continued
number of dye molecules at different places (an equilibrium). In warmer water, the dye molecules simply move faster. The dye and water continue to move, abut they are so small that we cannot see any differences in coloration after the initial diffusion.
The concept of equilibrium is rather like balancing. So long as no new dye or water is added, there will be no movement of the molecules. This could also be explained by a seesaw analogy. If two equal sized kids sit down on opposite sides of a seesaw, it will balance and will be in equilibrium (although in a different sense). If two kids sit on one end and only one kid sits on the other end, there is no equilibrium and the see saw moves.
PLANT MOVEMENTS (Teacher Copy)
LESSON 7 DIFFUSION IN A PSEUDO-CELL
Background Information
Now imagine the dye and water example, except instead of the bottle you have a balloon made of some material that will allow water to move in and out, but not the dye. Imagine that this happens because the water molecules are small and the dye molecules are big, so if our balloon has medium-sized pores, water can move in and out, but the dye is trapped. In this case, and based on the results of our prior experiments, what would we predict?
Why are we interested in membranes any way? This is a good question. All cells are surrounded by a membrane and this membrane influences how things diffuse in and out of cells. Cell membranes work a lot like our imaginary balloon. Water can move freely in and out of cells, but other things cannot.
If diffusion can only happen one way, then what do you predict will happen?
The water should move into the balloon and it should swell up.
This prediction can be tested two ways. One using an artificial cell constructed out of a cellulose membrane and the other using real plant cells.
Materials cellulose tubing molasses (or sugar syrup with some food coloring) string water jars or bowls scissors piece of glass tubing crayon balance
Cellulose tubing will function as an artificial membrane. A few feet will allow you to do this experiment many times.
Procedure
1. Cut a piece about 8" long and soak in water. To make an artificial cell, fold up 1 to 1 1/2" from the bottom and tie securely with string. Put about 1/4 cup of molasses in the tube and tightly tie the top end. This should make a limp sort of cell filled with brown liquid. Weigh the molasses-filled balloon on the scale and record the value. Place the balloon in a jar or bowl of water.
Prediction of results: What do you predict will happen? The concentration of sugar molecules in the balloon is very high, but because of their relatively large size, they cannot diffuse
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LESSON 7 Continued
outward. However water can diffuse inward from the high concentration (pure water) to the low concentration (the molasses). How could you determine if sugar is diffusing into the water? (Hint: how did you know scent molecules were diffusing into the room?) Both visual observation and weighing will confirm that water has moved into the balloon.
When will it stop? If the balloon membrane is strong enough, it will generate enough pressure to stop the osmosis, otherwise it will burst, and the sugar molecules will just diffuse into the surrounding water.
2. Diffusion across a membrane can be measured another way. Make a tubing cell filled with molasses as above, but then put a 3' glass tube in other end. Use rubber bands or string to tighten the end of the cell around the tube. Set the cell in a jar of water with the tube standing straight up. Mark the level of the liquid in the tube.
Prediction of results: What do you predict will happen to the liquid level in the tube given the results of the previous experiment?
Explanation: The diffusion of water across the pseudo-cell membrane into the cell (osmosis) generates a turgor pressure which can inflate or even burst the balloon. This is generally prevented by the cell wall pushing back. The turgor pressure can also push water up a tube. This helps explain how water moves into a plant. This phenomenon will be investigated further below.
Osmosis is essentially a one-way diffusion caused by a membrane that prevents certain molecules from moving the other direction.
PLANT MOVEMENTS (Teacher Copy)
LESSON 8 DIFFUSION OF WATER FROM PLANT CELLS
Background Information
Plant cells work just like the molasses-filled pseudo-cells in the previous experiment. The membrane around each cell works like the cellulose membrane. The cell membrane prevents molecules inside the cell from moving out, but it will allow water to move in and out. How do you predict water will diffuse in when the cells are in pure water? How do you predict water will move when the cells are in a salt solution?
Materials potatoes water apple corer or cork borer balance 5% salt solution metric scale
Procedure
1. Potatoes make a good source of plant cells. A cork borer or an apple corer can make nice cores of potato cells. Make two potato cores and cut them to the same length. Weigh each one and measure their lengths to the nearest millimeter. Put one core in a bowl of the tap water. Put the second core in a bowl of 5% salt solution (see below to make salt solutions).
You can also use a rectangular (French fry) shape which can be cut with a knife.
2. Using the working hypothesis of diffusion, make the following predictions.
a. How will the length of the potato cores change in pure water and salt water?
b. How will the weight of the potato cores change in pure water and salt water?
3. After 1 hour, remove each core from the bowls and gently blot the surface with a paper towel to remove surface water. Re-weigh and measure each core again and record the new length. Calculate how much they changed in length.
4. Now gently bend the two potato cores. Do you observe any differences? Which one is the crisper, the limper? If you had limp carrot sticks, what would you do to make them crisp? Which core has the most turgor pressure? which one the least? can their conditions be changed by reversing the solutions?
Explanation/Hypothesis: Plant cells have both a membrane and a flexible cell wall. Plant cells are filled with molecules in
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LESSON 8 Continued
solution, so under ordinary conditions, water will diffuse into the cells. Just like the cellulose balloon cells, the cell swells slightly and the membrane pushes out against the cell wall. This makes the cells more rigid and this pressure helps plants retain their stature.
Stop and consider just how much this simple series of investigations has improved your understanding of physics and plant biology. Smells, membranes, and water in cells now can be understood much better. What other things can you now explain? Can you think of other day-to-day examples of diffusion?
PLANT MOVEMENTS (Teacher Copy)
LESSON 9 PLANT WILTING AND THE EFFECT OF SALT
Procedure
Water several similar plants then watch until they begin to wilt. This may take a couple of days. When the leaves have become limp and drooped, water one plant with pure water (0% salt solution), the next one with a 2% salt solution, the next one with a 5% salt solution, and the last one with a 10% salt solution. These are easy solutions to make. Simply add 1 gram of salt for each % you want to 100 ml of water. Most measuring cups these days have ml on one side.
% salt g of salt ml of water 0 0 100 2 2 100 ÄÄÄÄ¿ 5 5 100 ÃÄ or 98, 95, 90 10 10 100 ÄÄÄÄÙ (i.e. add water to make 100 ml)
This works out so nicely because 1 ml of water has 1 gram of mass (which is the same as saying weight as long as we are on the surface of the Earth). This is not a coincidence, but shows how carefully planned the metric system of measure was. Just try figuring out these solutions using cups and teaspoons.
Observe what happens. Which plants revive? Can you measure how much? Try to explain the results using information gained in the previous experiments. What problem would plants have that are growing near the ocean?
PLANT MOVEMENTS (Teacher Copy)
LESSON 10 ROOT PRESSURE OF PLANTS
Background Information
How does water move around in a plant? Animal circulation systems have a pump, the heart, that generates a pressure (blood pressure) to move blood through tubes (arteries, veins, and capillaries) around the body. Obviously plants lack such a circulatory system. They don't have a pulse, but they do have vascular tissue in which water moves from the roots to the aerial portions of a plant in very small tubes.
Materials plant stem (survivors from the previous experiment will be fine) water in a glass 1 to 3 inches of soft plastic or rubber tubing that fits snugly around the stem rubber band length of glass tube to fit inside the rubber or plastic tube ring stand or vertical support
Procedure
1. Use a razor blade to cut off a plant stem a couple of inches above the soil line. Immediately put a drop of water on the cut surface and place the cut end of the top piece with the leaves in a glass of water. Slip a 1 to 1-1/12" piece of flexible plastic or rubber tubing over the cut end of the stem and use a rubber band to make a snug fit around the stem. Fill the tubing with water. Insert a glass tube into the other end of the rubber tube and again make a snug fit with a rubber band or string. Use a ring stand or tape the vertical tube to some support. Mark the level of liquid in the tube. Keep the pot well watered.
2. Now put the cut end of the leafy top in a glass of food coloring (red or blue). This can also be done with a fresh, leafy stalk of celery, but a whole stalk of celery is just one leaf. Make a fresh cut on the bottom of the stalk and place in the dye.
What do you predict will happen? What is making the water move up the tube? Where is this pressure coming from?
The hypotheses developed from the potato and tubing balloon experiments should help you answer these questions. Diffusion of water into root cells fills their membranes and generates an osmotic pressure. This pushes water into and up the vascular tissue. With our cellulose-membrane pseudo-cell, which part is like the vascular tissue? Which part is like the root cells?
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LESSON 10 Continued
Where is the vascular tissue and how big are the conducting tubes? After a day or so the movement of the dye up the stem of the leafy tops should answer that question. Make a fresh cut a couple of inches above the bottom and look for the distribution of dye. In a stem the vascular tissue will be in a ring or circle of bundles. In the celery (and all leaf stalks) there will be an arch of vascular bundles.
PLANT MOVEMENTS (Teacher Copy)
INTRODUCTION TO PART IV: PLANT GROWTH MOVEMENTS
How does a plant know which way to grow? When you plant a seed, how does the shoot know how to grow up and the root down? Seeds don't come with any labels or arrows saying, "This side up." This investigation will show how plants respond to their environment using growth.
Plants increase in size by both adding new cells and then enlarging the cells. New cells are added at the tips of shoots and the tops of roots. Most of the increase in size is from an increase in cell length, elongation. Changes in cell size can be influenced by many factors in the plant's environment (gravity, light, water, touch, temperature, chemicals), but unlike turgor changes, cell elongations are slower and not reversible.
PLANT MOVEMENTS (Teacher Copy)
LESSON 11 ORIENTATION OF SEEDLINGS
Procedure
1. Soak some seed corn in water over night. Now carefully examine the seed. The plant embryo, called the germ in cereal grains, forms a light-colored oval patch on one side. If you make a careful cut with a razor blade through the germ dividing it into left and right halves, the embryonic plant can be easily seen with a magnifying glass or low power microscope. In corn there is an embryonic shoot at the top and an embryonic root at the bottom. The innermost organ of the embryo is the cotyledon, which will absorb food stored within the grain for the growth of the young plant.
What happens when you don't plant the seed with the root pointing downward and the shoot pointing upward?
2. To answer this question take three of the corn grains and decide how to plant them to orient them right side up, sideways, and upside down. Soak a small sponge in water and curl it so the sponge fits snugly within a glass or jar. Near the top edge of the sponge, push in corn grains oriented three ways (use duplicates if there is room). Keep the sponge wet and note how the shoot and root grow over the next few days.
What tells a plant which way is up? What tells you which way is up? What things might tell a plant seed which way is up or down?
Two of the primary factors that influence seedling growth are light and gravity, the force of attraction that produce s the sensation of having weight.
What do you predict the response will be of a plant shoot to light?
How do you predict a plant shoot will respond to the gravitational pull of the Earth?
How do you predict a plant root will respond to the same factors?
3. Now there is an experimental problem we must deal with. There are three seeds in our jar. Is there any way you can think of to separate the seedlings' response to gravity and light? Put another way, the seedlings in our jar are being influenced by both light and gravity. Can we separate these influences? Obviously we can't turn off gravity. But we can turn out the lights! Set up another jar identical to the first
Continued
LESSON 11 Continued
and place this one in a dark cupboard. Leave it there, but don't forget to add water when you water the other jar. When the seedlings exposed to light have developed roots and shoots, examine their orientation. Note where any of them turn or bend. Now compare the results to the seedlings germinated in the dark.
Do the results of this experiment suggest that recently germinated seedlings respond primarily to gravity or light? Why might this be the case? Stop and think where a seed might be when it begins to grow.
Plant growth in response to external factors is called a TROPISM. Plant growth in response to gravity would be GEOTROPISM. If the plant responds by growing toward the stimulus we say the tropism is positive. If the plant responds by growing away from the stimulus we say the tropism is negative.
Do germinating plant shoots show a positive or negative geotropism?
Do germinating plant roots show a positive or negative geotropism?
Does this experiment show you whether the roots and shoots demonstrate a PHOTOTROPISM (response to light)?
PLANT MOVEMENTS (Teacher Copy)
LESSON 12 SHOOT ELONGATION IN RESPONSE TO LIGHT
Background
In the previous experiment, do you notice any other differences between the dark-grown and light-grown seedlings? Which seedlings have the tallest shoots? Let's examine this phenomenon in some greater detail.
Procedure
1. Set up two jars with sponges as before, but this time let's use bean seeds which were soaked in water over night. One jar will be placed in the dark and the other will be left out in the light near a window. Take care when adding water to the jar in the cupboard, expose the seedling to as little light as possible.
2. When the light-grown seedlings have fully expanded their first pair of leaves and the next pair of leaves are visible, compare the two sets of seedling.
3. Which seedlings are the tallest? How can this be measured? Can you find two points on the seedling that will allow you to compare differences in height accurately?
4. Measuring between the point where the cotyledons are attached to the stem and the point where the first pair of true leaves are attached to the stem will solve the latter problem.
What does this experiment show? What stimulates seedlings to grow?
PLANT MOVEMENTS (Teacher Copy)
LESSON 13 PLANT GROWTH AND PHOTOTROPISMS
Background Information
Recall the difficulty in determining the role of light in the first experiment. This experiment will help determine if plant shoots are positively phototropic or just negatively geotropic. When the plant is growing straight up and down, positive phototropism and negative geotropism will produce exactly the same results, i.e., our predictions are identical, so we can't distinguish between them. Does this give you any ideas about how to design a new experiment? (Hint: note the underlined phrase above.)
Imagine what would happen if a plant growing straight up were turned 90 degrees onto its side. Now the shoot is oriented perpendicular to the force of gravity and sunlight. What do we expect to happen? We expect the shoot to turn and grow upward, but is this positive phototropism or negative geotropism?
Materials For this experiment you will need three growing plants and three cardboard boxed. Coleus work very well, but you could also use the light-grown beans from the previous experiment. It is also easy to grow new beans in peat pot plant starters.
Procedure
1. Put one plant on its side, cover it with one of the boxes, and leave it there over night. Examine the plant the next day. Continue for an additional day if necessary.
Is this response a positive phototropism or a negative geotropism?
2. Cut a 3-4" hole in one end of the other two boxes about half way up. Although not completely necessary, painting the inside of the box black or lining it with black construction paper will help.
3. In one box, lay a plant on its side at about the same level as the hole. Use a brick or blocks or a smaller box to hold the pot. You may have to use tape or string to hold it in place. Place the box over the plant such that the shoot is opposite the hole, but as far away from it as possible. Put a desk lamp or light bulb about 2-3' away from the hole and at about the same height.
4. In the other box, stand the plant straight up in the middle of the box. You man use the same light if you put this box opposite the other box.
Continued
LESSON 13 Continued
5. After a similar period of time, examine the orientation of the plant shoots.
Is this response a positive phototropism or a negative geotropism?
From this experiment what would you conclude about the reactive responses of shoots to the two tropisms? Which response is the strongest? Remember we can't turn gravity off.
PLANT MOVEMENTS (Teacher Copy)
SUMMARY
This series of exercises was designed to do several things.
1. Teaching by investigation.
By using explicit steps, of observing, generating a tentative explanation (hypothesis), making predictions about new situations, and then testing them to see if they happen or not, we are employing the scientific method. Normal problem-solving thinking uses the same approach, but not consciously. In science the use and cycling back and forth between inductive and deductive logic is conscious.
By learning through investigation, students develop concepts rather than just memorizing some information. Concepts can be applied to new situations. Concepts are understood better and retained longer than memorized information.
By learning through investigation, students learn a process for solving new problems. In other words, they are learning how to learn, and this is more important than any particular piece of information that you can teach.
2. Diffusion
Diffusion is a process that can be encountered in everyday life. By learning through observation and investigation, students are more likely to recognize diffusion when they next see it.
Students should now have a working (though still rather imperfect) concept of how things diffuse. This concept is good enough to make predictions about new situations.
3. Diffusion of water in a living organism
Diffusion in living organisms always involves membranes, and this limits the diffusion to water (and other small molecules). The diffusion of water across membranes plays a role in plant structure and in moving water around in plants. (However, there is another mechanism, but it was not investigated here.)
4. Tropisms
Plant growth movements in response to gravity, geotropisms, and light. Phototropisms were investigated to show that these responses can be positive or negative. Roots and shoots differ in their geotropisms. Geotropisms are strong in seedlings.