Teacher's Page
ACTIVITY 6: " Are Seeds Food for
Plants?"
ACTIVITY 6 - STUDENT'S PAGE
(It may be helpful to view the student page alongside this teacher
page.)
Purposes:
This activity begins the EXPLORE AND CHALLENGE phase of the
conceptual change model. With this activity we begin to explore the students' ideas, or hypotheses, about
food for plants and to collect evidence to support or challenge various hypotheses. Your students may or may
not have suggested the seed as a source of food for plants. If not, encourage them to consider
this as a new hypothesis. This activity challenges them to consider the seed as a food source for the
"baby plant" (the growing embryo).
Materials:
Approximately 25 lima bean seeds, 25 bush bean seeds, and
possibly 25 of a third kind of seed (assuming a class of 25-35; each group of students should have several of
each kind of seed).
Magnifying glasses
3x5 or 3x3 Post-it notes
Paper towels
NOTE: Students use finger nails or pencils to pry open bean seeds
Advance Preparation:
Soak 2-3 kinds of bean seeds (lima, bush, possibly one other) in
water for 24-36 hours prior to the lesson.
Set up a CLASS DATA CHART. Find a large space such as a bulletin
board or a section of the chalkboard. This chart will be used throughout the unit. The chart should have three
columns:
What is food for plants?
Our Hypotheses Evidence to Support Evidence to Challenge
List all your students' hypotheses in the left-hand column. Leave
plenty of space in the other two columns for students to place 3 x 5 post-it notes next to each hypothesis,. On these
post-its they will write down evidence they found to either challenge or support a particular hypothesis.
A Possible Teacher Narrative
FRAME
Show students a can of baked beans--ask for associations to their
personal experiences. Show the students a variety of dry bean seeds. Ask for their associations
to their personal experiences.
"We use beans to eat. They are food for us. But we also
plant bean seeds. And there are lots of different kinds of bean seeds. Today we are going to think about what the
seed does for the plant. Will our investigations about the seeds give us any new ideas about how
plants get their food?"
"With your group, explore the bean seeds you are given.
Identify as many parts as you can. Study them closely. Use your magnifying glasses. Draw and write about your
observations and ideas."
Sensitivity to Student Diversity
Be aware that students may use beans as a source of unkind
teasing. For example, we have heard students in a science class use the occasion of discussions about
beans to tease in a derogatory way Mexican American students, calling them "Mexican
burritos" and "Mexican jumping beans." An
appropriate response would be to call attention to this behavior, to clarify
the inappropriateness of this kind of name calling in a scientific (or any other) community. Letting it go
unacknowledged communicates to Mexican American students that this kind of behavior is condoned. It
further marginalizes them in the science classroom. I'm sure you can also imagine students teasing other
individuals regarding their flatulence problems! While some of this teasing might be done with the
intent of good humor, it is still inappropriate if it isolates and humiliates another student.
Common Student Responses and Suggested Teacher Actions
Common Student Responses
Suggested Teacher Interpretations and Actions:
Most students do not think about the seed as being a source of food for the plant. Do not expect students to be thinking about the cotyledon as a
source of food for the growing embryo. Accept all ideas at this point.
Most students will not know the terms "embryo" and "cotyledon." They will frequently call the embryo the baby plant. Do not introduce these terms yet. Some students have difficulty seeing the embryo. They believe that it is the cotyledon ("the big part of the seed") that will grow into the plant. Help students see the embryo and examine it closely. Students generally have very few ideas about experiments at this point, but they often come up with ideas after completing the bean seed experiment.
*Tell the students that it is OK not to have any experiment ideas
today, but that they should keep thinking about ideas of experiments that will help us answer our central question.
Some students come up with experiment ideas but they have little to do with the central question.
This is not surprising at this point. Students will need many opportunities to be involved in doing experiments and linking them to a central question before they will be able to generate
their own experiment ideas.
Setting Up the Bean Seed Experiment
Purpose:
This activity is an exploratory opportunity which leads
to the understanding that the embryo only grows when attached to the cotyledon because it gets its food
source from the cotyledon.
Materials:
For each team of students: 6-8 lima or bush beans that have been
soaked overnight
ruler germination system OR Styrofoam plate and gallon size plastic bag
labels permanent markers paper towels
Advance Preparation: Soak lima or bush beans in water for 24-36
hours before class Have a large drawing of the experiment set up on the board or overhead
transparency.
A Possible Teacher Narrative
FRAME
Review terminology -- embryo, cotyledon, food, energy
***Possible Strategy for Review: Create magnet vocabulary cards
that can be used
throughout the unit and beyond. Get 4x6 inch index cards. On each
card print one vocabulary word or phrase in large letters using a marker. Glue a small
magnet on the back (magnets are available in craft and sewing sections of large discount
stores). Place the vocabulary words on the chalkboard or other magnetic surface. Have children
come to the board and create a meaningful sentence using 2, 3, or all the selected
worlds. Students quickly learn this strategy and like the challenge of trying to use all the
words and of ADDING new words to the group.
"What do you think about our question: Could the seed be
food for plants?"
"Today we are going to set up an experiment that will help
us gather some evidence about whether or not
the seed is food for plants."
ACTIVITY
"You will work in groups." Explain the groups and any
rules or assigning of group roles you want to establish for this activity.
Demonstrate for the students the system you are going to use for
seed germination. Options:
1. Germination disks or chambers available in science supply
catalogs.
2. A Styrofoam plate system
a. Place several layers of paper towel on a large Styrofoam
plate. Do not let the paper towel droop over the edge of the plate.
b. Use permanent marker on the paper towels to divide the plate
into 4 quadrants. Label each quadrant as directed on the student page.
c. Place the appropriate seed parts in their respective sections.
d. Water the paper towel so that it is thoroughly soaked but not
swimming.
e. Cover the plate loosely with plastic wrap or a plastic baggy.
Allow for air to get in to enable some evaporation to occur and to minimize mold growth. The plastic
covering should be loose -- not tightly fitted to the plate or drooping down on top of the seeds.
ACTIVITY
Have students use the chart to make predictions about which plant
parts they think will grow and why.
Then have students set up the experiment.
Where will you place the experiment? Most students will want to
place their seeds in the dark, although it is very interesting if some students choose to put theirs in the
dark. The seeds will germinate in the dark, but they will not continue to grow, turn green, and do photosynthesis
in the dark.
REFLECT
Do you think this experiment can give us any helpful ideas about
how plants get their food?
Monitoring and Observing the Seeds
It is important that the seeds remain moistened at all times in
the germination process.
However, if mold starts to grow, it will help to remove the plastic cover and let
the water evaporate.
Have the students use the data chart to record multiple
observations and measurements over the next couple weeks. The seeds should start showing noticeable growth
within three days of planting. It is interesting to let this experiment continue for awhile, even
after the general pattern of results has been discussed and used to make the point that there is food stored in
the cotyledon.
Use observations of this experiment to challenge students'
conception that plants get their food from the soil. Didn't the bean seeds germinate and grow just fine without
ANY soil?
A Measuring Strategy: It is easy to accidentally knock an embryo
off a cotyledon when conducting measurements of the plants. For this reason and also to improve
accuracy, we recommend a string strategy for measuring bean seed growth. The student takes a
length of string and lies it down next to the seedling to be measured. The string can help with measurements of
plants that curve. The string is marked at the beginning and end of the length representing the
plant. Then a ruler is used to measure the
straightened out string.
Making Sense of the Experiment
FRAME
TEACHER DO AND SAY STUDENT RESPONSES INTERPRETING STUDENT
RESPONSES
"What is the hypothesis that we have been testing with this experiment?"
If students mention that the experiment is testing hypotheses about soil or water, get them to
describe how the experiment gives us evidence about soil and water. Have students add this evidence on the
post-its for the class data chart. Whether the seed provides food for plants.
Students might also say the experiment is testing whether soil or water provides
food for plants. SOIL The experiment does provide some evidence to support the idea that
SOIL is not food for plants, since the plants grow WITHOUT SOIL.
WATER Students are usually convinced that this experiment proves water is food for plants. However, the
experiment does not say too much about water. Students might want to set up a control experiment which is
given no water. Then the comparison would suggest that water is important in enabling seeds to grow. There is really
no evidence from this experiment to show that water is NOT food for plants. So accept students' assessment at this
point that water does seem to be food for the growing seeds. HOWEVER, you can try to push them to think about the
embryo -- it got water but it did not grow! How might they explain that result?
ACTIVITY
TEACHER DO AND SAY, STUDENT RESPONSES, INTERPRETING STUDENT
RESPONSES
Have students work in their groups, using p. 20 or their science journals
to draw what happened in their experiment and to begin to develop an explanation of
what they think the experiment says about the food source for the growing
embryo Some groups may have had no growth with any of their four seed conditions.
Talk to students whose seeds did not grow at all. Do they have explanations about why theirs did not grow and those
of other groups did grow? These students may be very frustrated and start to lose enthusiasm for the investigation.
Do they want to set up the experiment again? If so, let them do this now.
MAKE SURE THEY GET TO OBSERVE SEEDS FROM OTHER GROUPS.
While students are working (or before class), put a diagram like the one on
p. 20 on the board or overhead.
Whole class discussion:
"Let's look at our results for the class as a whole. How many groups had the
whole seed grow?" Record YES underneath the whole seed for each
group that had the whole seed grow. Most groups should say YES
Exceptions might be explained by a nonviable seed, too much water
leading to excessive mold growth, or the seed getting dried out. "How many had the cotyledon alone
grow?" Record results. Most groups should say NO.
"The embryo alone?" Record results. Most groups should say NO. Exceptions might be explained by the
embryo having a little bit of cotyledon still attached to it. "The cotyledon with the embryo
attached?" Record results. Most groups should say YES.
"Let's look at the general pattern here. In general, what seems to have happened?"
The whole bean and the embryo attached to the half cotyledon grew. Our group did not have that
happen. We had the embryo alone grow! I think ours didn't have enough
water -- it got dried out. Ours had too much mold growing on it -- I think the mold
killed the bean.
"How can we explain those results that did not follow this pattern? "
I think our embryo grew because it had a little cotyledon still attached to it. Usually students will come up with
reasons to explain why their results did not follow the pattern. However, some students will stick with the results of
their own seeds even if that does not follow the pattern of the class. Encourage these students to re-test. "What are your ideas about how the
embryo is getting its food?
What ideas does the experiment give you?"
Don't tell students at this point that they are WRONG about water. Instead comment on the logic of their inferences -- based on the data are they making a logical explanation? I
think the embryo gets food from the cotyledon -- it's like the a baby gets its food from the mother through the
umbilical cord. I think the embryo gets its food from the water, and the cotyledon helps it get the
water. Some students will suggest that the embryo is getting food from the seed.
However, they may be thinking that the food in the cotyledon is water that the cotyledon has soaked up. They think
that the water is the food, but that the water has to go through the cotyledon to get to the embryo.
REFLECT AND CONNECT
TEACHER DO AND SAY STUDENT RESPONSES INTERPRETING STUDENT
RESPONSES
Use pp. 21-22 to help students think about the nature of a good explanation and the distinction
between an observation and an explanation.
Push for consensus that the cotyledon is supplying food for the embryo.
Point out points of disagreement among the students about the nature of that food -- is it the water? is it
something else?
Encourage them to question and challenge and to look for reasonable inferences based on the experiment.
Students will have difficulty seeing the difference between an observation and an explanation.