Teacher's Pages
ACTIVITY 8 - " Grass Plant Experiment"
ACTIVITY 8-STUDENT'S PAGe
(It may be helpful to view the student page alongside this teacher
page.)
Purpose:
This is a very simple experiment that can be used to
explore and challenge students'
conceptions that water, soil, and sunlight are possible sources
of food for plants. Grass plants are grown in the soil in the light and dark. The plants in the dark begin
to grow (using food from the cotyledon), but then die without sunlight. The plants in the dark die even though
they have water and soil. So the activity challenges the hypotheses about water and soil being food for
plants. It emphasizes the necessity of light. For many students this activity gets them wondering about whether
plant food is a combination of things. Ideally, students will finish this experiment saying something
like: "Well, I'm not sure about water anymore. It seems to be important but it is not enough. Plants need
sunlight, too. I'm not sure." When students get to this point of wondering about their hypotheses, they are ready
for the concept of photosynthesis to make sense to them!
Materials:
_____grass seeds _____small cups for planting _____potting soil
_____water source _____light source _____trays
_____utensils for filling the cups with soil _____masking tape
and markers for labels
Advance Preparation:
Think about how you will have students
"vote" in this activity. Will it be a public or private vote? A public vote is preferable if you think your
students can resist simply following the crowd or their friends.
A Possible Teacher Narrative
FRAME
TEACHER SAY AND DO AND STUDENT RESPONSES
Point to the class Hypothesis Chart and say: "I'm going to take a poll on how we are all thinking
about some of these hypotheses as of TODAY. As you think about our hypotheses, I want you to
remember the definition of food that we are using-- does this material provide ENERGY for the
plant to grow?"
"How many people think TODAY that water is food for plants?" Count and write down results."
What are your reasons? What is your evidence?
Most students will think that water is food for plants.
Students will point to the bean seed experiment --the beans only grew when they were given water.
Some may challenge: But remember we learned that water does not provide food energy for people
so I don't think it provides food energy for plants.
"How many people think TODAY that soil is food for plants?" "What are your reasons? What is your evidence?"
Students who began the unit thinking soil is food for plants will likely continue to think so at this point.
Others will challenge: In our bean seed experiment, the seeds grew without soil.
"How many people think TODAY that sunlight is food for plants?" "What are your reasons? What
is your evidence?"
Most will say that sunlight is food for plants. Some will point to the definition of food as providing
energy and say that the sun provides energy. "Today we are going to set up an experiment that
will help us think about all three of these hypotheses about how plants get their food. This
experiment will take a while, so we will watch and think about it for the next couple weeks."
ACTIVITY
Following the directions here, set up the experiment.
REFLECT AND CONNECT
Have the students use this exercise to consider (in discussion
and/or writing): What do they think will happen in the experiment and why?
Wrap Up: What might this experiment tell us about our question:
What is food for plants?
Explaining the Grass Seed Experiment
When is your class ready for this analysis of the results?
This discussion should occur after the students have completed
Activities Nine and Ten and after the grass plants have grown long enough to convince students that the
plants in the dark are dying.
Is it OK to do variations in the experiment?
If students suggest ways to change the experiment as you go
along, try out these variations such as:
Moving some plants from the light to the dark or moving plants
from the dark to the light
Stopping the watering process for some plants.
Trying the same experiment without soil.
Advance Preparation and Materials:
You may want to have a chart
or handout for the students to remind them about ways to act in a scientific discussion. Have plenty of
post-it notes and markers available to add evidence to challenge and evidence to support the hypotheses on
your class data chart.
Reflect and Connect
TEACHER SAY AND DO, STUDENT RESPONSES, AND SCIENTIFIC CONCEPTIONS
1. Have students write about their observations and their explanations, using the guide
questions here. Students will typically write that the plants in the dark died "because they need light." Some
students might add that they need the light "for energy." Students will not typically think
about how this experiment might challenge their ideas that water and soil are food for plants. They
do not usually wonder about why the plants in the dark died if they had water and soil.
Those who do wonder about the plants in the dark sometimes explain the results by saying:
"It's like people-- we need a balanced diet. Plants need a balanced diet, too. They use
water for food and soil for food, but they need light, too. If they don't get all three of these food
groups they will die. Scientific explanations focus on what is happening to the light
inside the plant. How is the plant using the light?
Scientists would explain these results differently. They would use the concept of
photosynthesis (which students have not been told about yet) to say: "Plants
need water and carbon dioxide and light energy from the sun, which they use in a chemical
reaction to create
energy-containing food. So light is not food for the plants, water is not food for the plants, and soil is
not food for the plants. Food for the plants is what they MAKE using water, carbon dioxide, and
light energy."
2. Prepare the class for a scientific discussion. Review the key
elements of a good scientific discussion. Assign observers to gather feedback about specific aspects of
your group's discussion.
(See Activity 5, Student's Pages, about scientific discussions).
3. Remind students that we are using a definition of food as
material that provides energy to help living things grow and live -- not just anything that the plant takes
into its "body."
TEACHER SAY AND DO AND STUDENT RESPONSES
4. "Does this experiment give us any evidence to say whether or not sunlight is food for plants?"
"Does anyone want to challenge what has been said so far?"
Have students put evidence on post-its and place in the appropriate column on the class
data chart. Students will overwhelming agree that this experiment shows that sunlight provides food
energy for plants because only the plants in the light lived.
5. "Does this experiment give us any evidence to say whether or not water is food for plants?"
Have students put their evidence to support or challenge on post-its and place on the class
data chart. There should be a good debate about this one.
To support the hypothesis students will say:
The plants in the light would have died if they had not had water; therefore, water is a food
for plants. In every experiment we've done or read about, the plants had to have water to live
(the bean experiment, Van Helmont experiment from Activity Nine, and the grass experiment).
To challenge the hypothesis students will say:
The plants in the dark died even though they had water. Water does not have any energy in it or any
calories in it (as they learn in Activity Ten), so it is not providing the grass seeds with
energy.
6. "Does this experiment give us evidence to say whether or not soil is food for plants?"
Evidence to challenge:
The plants in the dark had soil but they still died. So the soil must night be giving the
plant food. Students will also refer to Van Helmont's experiment (from Activity Nine).
Evidence to support:
The plants in the dark would have died eventually if they didn't have soil-- like the
bean seeds on our plates died after awhile.
They need soil at some point.
WRAP UP:
Have students write responses to the questions for this exercise.
Before they write, remind them to:
Use the definition of food as energy-providing material.
Use evidence from the experiment to support their answers.
Use this writing to assess student's understanding and use of the
discussion. Did they use ideas from the discussion in their answers?
WHERE TO NEXT?
At this point you should already have completed Activities Nine
and Ten.
Now use these exercises to help students synthesize the results
from the experiments in Activities Eight, Nine and Ten.
At this point students should be ready for the idea of
photosynthesis to make sense to them!