HOW DOES YOUR GARDEN GROW?

Lesson 4: Plant stimuli

Aim: How do plants respond to stimuli in their environment?

Do Now: Identify at least three characteristics of all living things. Answer: All living things are make up of cells, they contain similar chemicals, use energy, grow, develop, reproduce and die and respond to their surroundings.

Motivation: Display several plants that have been remarkably changed by stimulus. For example, a pink and a blue hydrangea, a wilted plant, plants bending toward the sun, or a vine twisted around a pole. Explain that different stimulus can affect the growth and development of plants. We will discuss each kind of stimulus as we move on with the lesson.

Discussion:

1.        There are several things living organisms need, oxygen, water, energy, temperature, light and living space.   All living things are make up of cells which work together to keep them alive. There are unicellular organisms such as one celled-bacteria and multicellular organisms, which are made up of trillions of cells (humans). Living things are composed of water, carbohydrates, fats, proteins and nucleic acids.  These chemicals provide energy, building materials for the cells and chemical instructions that direct the cells activities.  Organisms use energy to create new cells that grow and develop.  During reproduction, offspring are produced which are similar to their parents. Unicellular bacteria can produce other bacteria exactly like themselves. Stimulus is any change in environment that affects the growth of plants. Plants need water, light, proper temperatures, air and special chemicals to grow. Because these factors cause a response in plants, they are called stimuli.  

Note: This is a good time to review the cell theory with the students.

2.       Tropisms are growth movements toward or away from a stimulus.  Tropisms have different names assigned to them based on the stimulus that affects them.   Phototropism is demonstrated when plants bend toward light. Geotropism results from the pull of gravity.  Thigmotropism is the ability of a plant to coil around the objects they touch.   A vine growing around the surface of a pole is a good example. Elicit from the class “What does the word “Hydrotropism” mean?”  Ans. plant roots growing toward water.  There are both positive and negative tropisms.  An example of a positive geotropism is when the plant roots grow in a downward direction with the pull of gravity. When plant stems grow upward against gravity, they are demonstrating negative tropism.  Plants can respond to tropisms because they produce hormones called auxins.   A hormone is a chemical that speeds up the growth-regulating process in plant cells.  Auxins build up on the shaded side of a stem causing it to bend toward the sun.

3.        Energy provided for organisms comes mainly from the sun. Some plants use sunlight to make their own  food in a process called photosynthesis.  The manufacturing process takes place in the leaves (and sometimes the stems), of a plant. Carbon dioxide, from the air is combined   with water and minerals from the ground. Sunlight is the energy that is held by a green substance called chlorophyll, found in the chloroplasts of the plant. There are two stages in photosynthesis.  First the absorbed light energy is used to split each water molecule and hydrogen is given off.  Next, the hydrogen is combined with carbon dioxide to form glucose, which is used by the plant for energy.  Plants also make compounds such as complex sugars, fats and proteins. Animals get energy by eating these plants.

Demonstration: This lesson discusses the different kinds of plant stimuli and the tropisms resulting from exposure to the stimuli. Show the class plants that have been placed near a sunny window for several days prior to this lesson and are leaning toward the sunlight.  Ask the students “Why are these plants leaning toward the sunlight?” Answer: They are demonstrating phototropism, a positive stimulus which allows plants to move toward the sun to receive maximum sunlight.

There are other factors that affect the development of plants.  Some seeds are dormant in the winter.  When the temperature warms up in the spring, germination begins. In the fall, when the temperature is lower, plants change colors and die.  Plants respond to seasons and temperature. Flowers may form depending on temperature and hours of daylight.   The pH condition of the soil may determine the colors of a flower.

SCIENCEPLUS, “How’s it Growing?” is used to demonstrate to the class how pH can change the color of a flower.  Using the classroom computers, explain how this experiment is conducted in a virtual laboratory. A big screen monitor is helpful, since the children will have a better view. A local nursery has written a letter to the laboratory.   Hydrangeas usually produce blue flowers but some of the other pots are producing plants with pink flowers!  Acting as a detective, experiments are conducted using different variables for example, adding ladybugs, varying hours of sunlight, adding soil conditioners and fertilizers. It will be discovered that the soil conditioner was responsible for the color change.  Hydrangeas will develop pink flowers in basic soil (pH greater then 7.0) and blue flowers in acidic soil.  The results of the experiment are typed on a fax form and sent back to the nursery when the case is solved.   

The remainder to the class can be used to work on word searches, using the Internet.  While some of the students are making a list of words to use, others can work directly on line to develop their word searches.  If the Internet in not available, it may be possible to find a puzzle maker, software program. The puzzlers may be designed by hand if desired.  Word searches are exchanged and tested as the children try to solve each other’s puzzle.

Summary:  Plants often grow in response to stimuli.  Tropisms are named for the type of stimulus they create. In some plants, the ability to grow flowers is attributed to growth responses.  Polar Bears manage well in the cold but would not do well in a hot climate while orchids would die if exposed to extremely cold temperatures.

Evaluation: The children’s ability to use appropriate vocabulary words in their word searches will demonstrate their knowledge and understanding on this topic.

New York State Performance Standards: For reference see The Board of Education City of New York, Performance Standards in Science, first edition. Life Sciences Concepts: The students will develop skills in problem solving, questioning and thinking.  They will demonstrate scientific competence by completing non-experimental research using print and electronic information.

Homework: Find out more about plant tropism.  Use the Internet or other resources to gather information. Draw and label a model of a plant displaying one type of tropism.  Include at least two paragraphs about your model.

Additional Links:

 www.msn.com. Click on tropism first and then click on plant tropism. There are articles on tropism which will be useful for completing the homework assignment.  www.puzzlemaker.com.  This is a web site that has the template for word search puzzles. You determine how large, and how many words will be used, type in your list of words and the program makes a puzzle for you.

Software: SCIENCEPLUS Interactive Explorations CD-ROM, level red, for IBM and Macintosh.

Note:  My students were excited to use this software program.  There are other labs on the CD, and several children utilized this program during their lunch period!

Materials: Several varieties of potted plants directly affected by different stimuli.  If you are using Scienceplus, consider using two Hydrangea plants, one with blue flowers and one with pink flowers.

Tips: This would be a great time to visit The Botanical Gardens, Greenbelt, or Wetlands.  You can arrange for a representative from the local garden club to visit and speak to the class.