Teacher Notes

Plant Tropisms

Student Laboratory Kit

Materials Included In Kit

Corn seeds, 60
Filter paper, 15
Modeling clay, 1 lb
Petri dishes, 15
Pots, 60
Potting soil, 8 lbs
Radish seeds, 600
Watering trays, 4

Additional Materials Required

Water, tap, 8–15 mL
Dark closet or lightproof box
Greenhouse or other lighted area
Paper towel, 2
Pipet
Tape, clear, several pieces
Wax pencil or marker

Prelab Preparation

Assemble the planting supplies and other non-kit materials prior to the start of the lab. Locate a work area where spilled potting soil will be the easiest to clean up and will do the least possible damage. A greenhouse or a plant growing area is ideal for this activity.

Prepare the corn seeds approximately 24 hours in advance of the exercise. Soak the corn seeds in tap water for 1–2 hours. After soaking, pour off the water and wrap the seeds in layers of wet paper toweling. Be sure all the seeds are in contact with the wet paper toweling. This can be done by placing them in layers in a flat container. Cover the container with plastic wrap to create a totally moist germination chamber. Keep the corn seeds in the wet towels until classroom use.

Approximately two hours before class time prepare the radish seeds. Place the seeds into a container and cover them with tap water. Let them soak for two hours prior to use. Students can retrieve the seeds from the soaking container or you can place them on wet paper towels until planting.

Disposal

Corn and radish seedlings can be grown in a greenhouse or garden upon completion of this activity or they can be disposed of following Flinn Suggested Disposal Method #26a.

Teacher Tips

  • Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. The activity will require one class period for the experimental setup. Small amounts of time will be required to water the plants daily during the experiment. One class period will be required to collect data, complete the worksheet and discuss results.

  • The time required for the plants to grow sufficiently to illustrate the tropic patterns will vary depending upon the growth conditions of your laboratory setup. Monitor the plant growth and direct students to collect final data when the plants have grown sufficiently to illustrate the tropisms.
  • A lighted growth chamber, plant stand or greenhouse is ideal for this laboratory activity. If these are not available, it will be critical to locate a well-lit area where this experiment can be conducted. Dark areas can be equally challenging to locate depending upon your laboratory setup. Completely dark closets that can be left undisturbed are ideal. In the absence of dark rooms, inverted cardboard boxes can be used for dark chambers. Shoeboxes placed over the plants work great. Enlist the help of your janitor to assist in constructing a temporary lighting apparatus or in locating a dark, unused area of the school.
  • This may be the first opportunity some students have to grow plants from seeds or to view roots and stems growing. Some may feel bad if they don’t have a “green thumb” and if all the seeds do not germinate. Assure students that the germination rate will not be 100 percent. Ten seeds are planted in Part II in the hope that at least some will germinate and grow. Students should be encouraged to examine other students’ results and determine trends from the various experimental treatments.
  • The simplicity of this experimental setup and the ease of growing radish seedlings provides a great opportunity for this laboratory to be extended. Encourage students to design further experiments with varying setups of light combinations or other variables to illustrate tropisms.

Correlation to Next Generation Science Standards (NGSS)

Science & Engineering Practices

Constructing explanations and designing solutions
Engaging in argument from evidence

Disciplinary Core Ideas

MS-LS1.B: Growth and Development of Organisms
HS-LS1.B: Growth and Development of Organisms

Crosscutting Concepts

Cause and effect

Performance Expectations

MS-LS1-4. Use argument based on empirical evidence and scientific reasoning to support an explanation for how characteristic animal behaviors and specialized plant structures affect the probability of successful reproduction of animals and plants respectively
MS-LS1-5. Construct a scientific explanation based on evidence for how environmental and genetic factors influence the growth of organisms.

Sample Data

Part I.

  1. Draw a sketch of the corn seedlings at the end of the germination period.
{10326_Data_Figure_2}

Part II.

  1. Draw a sketch to show the general growth pattern of most of the plants in each pot.
{10326_Data_Figure_3}

Answers to Questions

Part I.

  1. Answer these questions:
  1. Describe the pattern of root growth for the corn seeds. (Use the word gravitropism in the description.)

    The roots all grow toward the force of gravity. It would seem, therefore, that corn roots are positively gravitropic.

  2. Describe the pattern of stem growth for the corn seeds. (Use the work gravitropism in the description.)

    All the shoots are growing away from the force of gravity. Corn stems are negatively gravitropic.

  3. Speculate about a possible mechanism to explain the patterns described above.

    Answers are likely to vary a great deal; encourage creativity and divergent ideas.

Part II.
  1. Answer these questions:
  1. What is the purpose of Pot 1 in this experiment?

    Pot 1 represents a “normal” set of growing conditions; a control for comparison with the other three treatments.

  2. Is there any evidence from this experiment for gravitropic or phototropic responses of radish stems? Explain.

    Since the stems in all the pots grew or turned up away from gravity it seems they are negatively gravitropic. Depending upon the position of the natural light, some plants might exhibit positive phototropism.

  3. Design an experiment using radish seeds that would show clearly a phototropic response to a directional light source.

    Student answers may vary. Many experiments might be designed. Plants could simply be placed under boxes with a directed light opening from a specific direction.

Student Pages

Plant Tropisms

Introduction

Plants are anchored into the ground and may seem incapable of responding to favorable or unfavorable conditions in their surroundings. How do plants become anchored? Why do roots grow downward into the soil? Can plants respond to changes in their surroundings?

Concepts

  • Tropism

  • Gravitropism
  • Phototropism

Background

Directed movements resulting from external stimuli are commonly referred to as tropisms in plants. The main shoots of most plants tend to develop vertically. If a box is placed over a plant growing vertically and a hole is cut to admit light from one side, the tip of the plant stem will begin to bend toward the light in a relatively short time. If the box is later removed, a compensatory bend develops, causing the tip to grow vertically again. Such a growth movement toward light is called a positive phototropism. The shoot tips of most plants are positively phototropic, while roots are either insensitive to light or negatively phototropic. Growth responses to the stimulus of gravity are called gravitropisms. The primary roots of plants are positively gravitropic while shoots forming the main axis of plants are negatively gravitropic.

Many interesting experiments have been performed trying to elucidate the internal mechanisms that cause these tropic responses. It is clearly a complex interaction of various plant hormones and other cellular components. We know that plant auxins are produced in the growing tip of the stem. If the growing tip of the stem is covered or removed, the tip will not bend toward the light. It is also known that auxin promotes the elongation of cells. For some time it was believed that stem tips bend toward the light because auxin is destroyed or inactivated on the sun-exposed side of the stem. This would leave more growth-promoting hormone on the side away from the light, causing the cells there to elongate more and produce a bend. Careful experiments, however, have shown that stem tips growing in non-directional light have the same total amount of auxin present as do stem tips from the same species receiving bright light from one side. Other experiments indicate that the auxin migrates away from the light, accumulating in greater amounts on the opposite side and promoting greater elongation of cells on the “dark” side. Apparently, an active transport system enables the auxin to migrate against a diffusion gradient.

Gravitropisms are equally as complex to explain. There is evidence that plant organs perceive gravity through the movement of large starch grains in special cells in the root cap. The starch grains, called statoliths, will, within a few minutes, begin to float or tumble down until they come to rest on the side of the cells closest to the gravity stimulus. In roots, the cells on the side opposite the stimulus begin elongating very quickly bringing about a downward bend of the root. The opposite reaction seems to occur in the stem. Some other researchers have suggested that mitochondria or Golgi bodies also respond to gravity. Precisely how the gravitropic mechanism is coordinated in and between cells is not yet entirely understood.

Materials

Part I
Water, tap, 3–5 mL
Corn seeds, pre-soaked, 4
Filter paper
Modeling clay, small dab
Paper towels, 2
Petri dish
Pipet
Tape, clear
Wax pencil or marker

Part II
Water, tap, 5–10 mL
Dark closet or lightproof box
Greenhouse or other lighted area
Pipet
Pots, 4
Potting soil, to fill 4 pots
Radish seeds, pre-soaked, 40
Watering tray (shared)
Wax pencil or marker

Safety Precautions

Plant materials used during laboratory work should never be consumed. Seeds are often treated with mold inhibitors and other chemicals. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Part I.

{10326_Procedure_Figure_1_Experimental setup}
  1. Select four pre-soaked corn seeds and place them on the bottom of an empty Petri dish. Place the four seeds with their pointed ends toward the center of the Petri dish and evenly spaced so that each seed points toward the center from an imaginary north, south, east and west direction. See Figure 1 for a view of the setup.
  2. Soak a piece of filter paper in tap water and then carefully place it over the top of the corn seeds without disturbing their orientation or position.
  3. Pack the dish tightly with crumpled, damp paper towels so that when the cover is placed on the dish the seeds cannot move. When the dish is set on its edge, the corn seeds should stay in place and be visible through the bottom of the dish.
  4. When the seeds are in perfect position and will not move, place the cover on the dish, and seal it shut with clear tape.
  5. Stand the Petri dish on edge with one seed in the top position. Mark the dish with the word “top.” Use a small dab of modeling clay to fashion a foot to hold the dish in a vertical position. The setup should look like Figure 1 with one seed germinating in the “normal” position, two germinating in a horizontal position and one “upside down.”
  6. Place the dish in its vertical position in a safe place as directed by your instructor.
  7. Let the seeds germinate for three days and make observations as they grow.
  8. At the end of three days, note the direction in which the roots and stems have grown, and complete Part I of the Plant Tropisms Worksheet.

Part II.

  1. Fill four pots with potting soil to approximately ½" below the rim of the pots. Gently, but firmly, pack the potting soil in the pots. Water the soil thoroughly before doing the planting. This can be done over a sink or a watering tray.
  2. Use a wax pencil or marker to number the pots 1 through 4. Label the pots appropriately with other key information (e.g., name, class, date).
  3. Plant 10, pre-soaked radish seeds in each pot by gently pressing the seeds, one at a time, down into the potting soil. Do not press any deeper than 12 mm. Spread the seeds as evenly as possible over the entire top surface of the pot.
  4. Place the pots in a watering tray in a well-lit area (next to a sunny window, in a greenhouse, or under artificial growing lights).
  5. Monitor the germinating radish seeds over the next three days. Water the seedlings each day to keep the surface of the soil moist. Be sure there is approximately 1 cm of water in the bottom of the watering tray each day. Also make sure to check the moisture on the seedlings before any weekends during the experiment.
  6. Within 2–3 days of planting, the young germinating plants should be poking up through the surface of the potting soil.
  7. Five days after planting (or when the seedlings are at least one inch tall), treat the four pots as follows:

Pot 1: Continue to grow in light undisturbed—pot upright
Pot 2: Continue to grow in light—pot turned on its side
Pot 3: Place in complete darkness—pot upright
Pot 4: Place in complete darkness—pot turned on its side

Be sure that the plants placed in the dark receive absolutely no light!

  1. Each day for the next several days, check the growth pattern of the plants in each pot. For the plants in the dark, check them very quickly and then return them to their dark environment. Allow plants to remain in their specific treatment conditions for the amount of time specified by your instructor.
  2. Record your summary observations on Part II of the Plant Tropisms Worksheet when directed to do so by your instructor. Answer the questions on Part II of the worksheet.
  3. The seeds may be planted in a garden or greenhouse upon completion of the lab work or disposed of as directed by your instructor.

Student Worksheet PDF

10326_Student1.pdf

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