Remind students to check on their cotton
seeds. The plants may not need watering, but daily checking
gets students in the practice of taking care of their
Before class place a seed, a young plant and a chunk
of wood on each group’s table. On the board pose
this question, “How does a seed gain the mass necessary
to develop into a sapling and then into a tree?” “How
does this compare to how people grow taller and larger?” Students
write their individual responses in their notebooks before
discussing them as a group. Students often don’t
recognize photosynthesis as the method by which plants
obtain their food to grow. Discuss this as a class, writing
on the board student’s ideas. Examples might be, ‘the
plant obtains food from the soil.” At this time
all responses are accepted, the class will return to
their ideas again at the end of class. Photosynthesis
is not introduced at this time.
Baptista van Helmont exercise
2. After students have proposed their own ideas about how a plant grows and
gains food, challenge them by recounting Jan Baptista van Helmont’s experiment.
Helmont was a 17th century scientist who attempted to
answer the same question posed to you. He planted a tree
weighing 5lb, in 200lbs of soil. Over five years he added
only rain water. After five years he weighed the tree,
including its roots and the soil. These were the results:
the tree and roots weighed 169 lbs, the soil weighed
199lbs 14oz. How would you explain how the tree gained
164 lbs in mass? What made up that mass? Be prepared
as a group to defend your answer. Write your answers
down in your notebook. How does this change your answer
to the first question? Discuss and write up possible
ways you might test your group’s ideas.” After
10 minutes encourage students to share some of their
ideas about where the tree’s mass came from after
reviewing van Helmont’s experiment? This might
also be a good time to add van Helmont’s experiment
to your science time line if you maintain one.
Ask students to identify some common themes in their
responses explaining van Helmont’s experiment then
come up with a general class explanation. Ask students
to identify what might seem scientifically implausible
in their explanation of van Helmont’s experiment.
This is an opportunity for you to really question students’ models,
to address issues that the students have with the larger
idea of photosynthesis although not directly identified
as such at this time, unless addressed by the students.
At this time share with the students that scientists
have indeed come up with an explanation of how plants
get the food they need to grow from a small seed or as
in van Helmont’s experiment from a sapling to a
tree. Share with them that the plant uses carbon dioxide
from the air plus water to produce sugar and as a by-product,
oxygen. If they identified some part of this already,
recognize that. However, van Helmont identified water
as the sole source of nutrition for the tree. Explain
to the students that at the time he was performing his
experiments, the composition of the air was unknown and
van Helmont didn’t know about carbon dioxide in
While students may have heard that carbon dioxide and
water produce sugar and oxygen before and may have accepted
it, does it make sense to them? Using overhead 2, have
the students working in groups, review the statements
and agree or disagree and list 3 reasons justifying their
decision. You may want to add statements that the students
have proposed or use their explanations as additional
statements. Have each group share their reasons as to
agreeing or disagreeing with at least one statement.
You may use this time as an opportunity to note a group’s
conceptions about the growth of a plant. At the end of
the class discussion let students know that they will
be addressing these issues over the next two days.
Students analyze van Helmont’s experimental technique.
How would they change the procedure? Provide the homework
sheet for this exercise. Are they able to identify lack of
control of variables? What are the independent and dependent
variables? How repeatable are the results?
Today partially cover up leaves on several geranium plants
using black construction paper or aluminum foil and paper
clips. Strips of paper or foil three by five centimeters
typically work well. Fold the strips over the geranium
leaves so that they cover approximately half the leaf.
Slide a paper clip over each piece of foil or black construction
paper to secure to leaf. Both the top and underside of
one half of the leaf should be covered. Make sure the plants
are in bright light. You will use these leaves on day 3.