By Aaron D. Isabelle and Cornelis de Groot
O
ne of the most captivating things about plants
is the way they capture the Sun’s energy, but
this can be a difficult topic to cover with
elementary students. We recently had the
opportunity to work with a motivated group of third and
fourth graders in a one-day after-school science enrichment program. We chose plants as the topic of the day’s
investigations and decided to focus on leaves and how
they act as “solar collectors,” something we thought our
students could make a concrete connection to. Our result
was this series of solar-energy lessons focused on surface
area. In the first lesson, students explored the idea that
an increased surface area collects a larger amount of solar energy. In the second lesson, students measured and
compared the surface areas of various leaves and learned
that this affects a plant’s ability to conduct photosynthesis.
In the third lesson, students investigated the relationship
between leaf size and location by observing the amount
of light that strikes a leaf. As students pondered the mechanics of leaves’ solar-collecting abilities, they began to
understand plant behavior in a meaningful way.
Solar Collectors Setup
The first lesson establishes the relationship between heat
absorption and surface area (see Internet Resources). Because we were dealing with time constraints, we set up the
experiment in advance; however, in a science class setting
you could involve students in planning the experimental
design. The day before the demonstration, pour approximately 1 L of water in a container. One pan should have
approximately twice the surface area of the other. (We used
a 4 × 8 in rectangular baking pan and an 8 × 8 in square
baking pan.) Let the water rest overnight so that it will be
room temperature the next day. Trace the bottom of each
pan on a piece of centimeter graph paper. Cut out each
shape and place it in the bottom of each pan. Measure
the surface area of each pan using the centimeter squares
and record it on the back of the graph paper. Now place
a piece of Styrofoam insulation approximately 2 cm thick
underneath each pan and a desk lamp next to each pan so
that the bulb of each lamp is almost parallel with the pan
and about 15 cm away from the surface of the pan. (We
obtained the Styrofoam from packing material in shipping
boxes; the insulating material is used so that the metal
pans are not in direct contact with the table, resulting in a
loss of heat energy). Make sure the lamps are of the same
type and the bulbs are of the same wattage. The next day,
Learn about plants in a meaningful way with this series of lessons.
38 Science and Children
Leaves: Nature’s Solar Collectors
pour two cups (500 mL) of room-temperature water into
two separate containers and place a container next to each
pan. Insert a thermometer into each container; the water
should have the same initial temperature at the start of the
demonstration. The setup is now ready.
Making Predictions
On lesson day, present the setup to students and discuss
the idea of surface area. Direct the students to compare
the graph paper cutouts created by tracing the shape of
each pan. Ask the students, “Which pan has the greater
surface area? How do you know?” By directly comparing the cutouts by putting one on top of the other, students concluded that the square pan, which contained
many more centimeter squares than the rectangular
pan, had a greater surface area.
Next, we explained that the same amount of water
at the same temperature would be poured into the
containers at the same time. (The students already
had a good understanding of the idea that conducting
an accurate science experiment requires changing only
one factor while keeping all other factors the same.)
Figure 1.
Solar collector worksheet.
In front of you are two pans
and two light sources (lamps).
One pan is rectangular and
the other is square. The
square pan has twice the surface area of the rectangular
pan. Two cups (16 oz. or approximately 500 mL) of
water at room temperature are placed in each pan. If
we leave the two pans with the water under the lamps
for 15 minutes, then what do you think will happen?
Circle your choice below:
The final temperature will be higher in the rectangular pan.
The final temperature will be higher in the square pan.
The temperature will be the same.
Explain your thinking and why you chose the answer
that you did:
November
February 2009 2008 39
We then elicited students’ predictions with
Figure 2.
the question, “If we leave the two pans with
the water under the lamps for 15 minutes,
Leaf surface area and leaf size data table.
then what do you think will happen to the
Leaf
Number of X-ed squares:
Leaf Size:
temperature of the water in each pan?” We
Sample Leaf surface area in sq. cm.
Small, Medium, Large
gave the students three choices to focus their
thinking:
Leaf 1
•The final temperature will be higher in
Leaf 2
the rectangular pan.
Leaf 3
•The final temperature will be higher
Leaf 4
in the square pan.
•The final temperature will be the same in
Leaf 5
both pans.
Students approached the setup, read
the temperature on the thermometers and recorded
don’t have such a resource, there are good examples of
the starting temperature of 74 ° F (23.3° C) on
solar panels at www.californiasolarco.com/power-systemsphoto-gallery.html. We asked students, “What do you
a data table. Desk lamps get hot and can burn
know about solar panels?” Students shared that “Solar
skin! Make sure students do not touch the bulb
panels collect energy from the Sun to heat things.” We
or lamp.
explained that these particular collectors (photovoltaic
Students recorded individual predictions about the
panels) are used to convert solar energy into electricity,
outcome of the experiment and explained their thinking
to heat water for example. Then, we asked probing queson a worksheet (Figure 1, p. 39). They also predicted
tions, such as: “Why do you think the town installed so
the final water temperature.
many solar panels? Why didn’t they just install a few?”
Next, students discussed their ideas in groups. After
Students responded that “solar panels are supposed to
five minutes, each group reported to the class what was
collect as much sunlight as possible.” Then, we led studiscussed. One group shared, “We think the water in
dents to make a connection between the solar panels on
the square pan will have a higher temperature because
the building to the classroom experiment by asking, “How
the square pan is bigger, but the water isn’t as deep, so
are the pans of water in the classroom similar to these solar
there is more area for the lightbulb to heat.” Another
collectors? Would you want a solar panel that has a large
group thought differently, “We think the water in the
surface area or a small surface area to heat your home?”
rectangular pan will be warmer because it is smaller, so
Students agreed that to collect a larger amount of solar
the water is higher. That means the light hits it first so it
energy, the solar panels should be as large as possible.
gets warmer faster.” We told the students, let’s find out.
Then, we poured the water into the pans, turned on the
lamps, and started the stopwatch.
The Results
Outside Connections
photographs courtesy of the authors
For the next 15 minutes, we went outside to observe the
solar panels on our town maintenance building. If you
Students observe the solar panels on a maintenance building.
40 Science and Children
Back in the classroom, students were eager to find out
the results of their experiment. They gathered around the
setup to read and record the final temperatures on the
thermometers. Simultaneously, we shut off the setup’s desk
lamps and then poured the water from each pan back into
the plastic containers. Students recorded a final temperature of 80° F (26.7° C) for the rectangular pan (with smaller
surface area), and a final temperature of 82° F (27.8° C) for
the square pan, a 2° F (1.1° C) difference.
We guided students in understanding this outcome
by discussing real-world examples that operate on the
same underlying principle. For example, we revisited
the idea that solar panels have maximum surface area to
absorb as much energy as possible. We also discussed
how laundry dries faster with its maximum surface
exposed and how some birds spread their wings while
warming in the Sun. By the end of our discussion,
students were beginning to make a direct connection
Leaves: Nature’s Solar Collectors
dents can identify poisonous plants in your
area, such as poison ivy or poison oak, and
remind students never to touch or pick any
Leaf size and plant location data table.
leaves they are unsure of.
Leaf
Leaf Size:
Leaf Location:
Students brought the leaves back to the
Sample Small, Medium, Large Full Sun, Partial Shade, Full Shade
classroom, and we instructed each pair of
students to determine the surface area of
Leaf 1
each leaf by tracing the leaf onto a piece of
Leaf 2
centimeter graph paper and counting the
Leaf 3
squares inside the outline, marked with an
Leaf 4
“X”. Students recorded the number of Xs
beside the leaf outline and recorded it on
Leaf 5
the data table (Figure 2).
Next, on the board, we guided the students in categorizing the leaves according to small surbetween increasing surface area and the collection of a
face area, medium surface area, and large surface area.
greater amount of solar energy.
To further classify the data, the students compared and
contrasted their leaf samples to determine if a particular
leaf was small, medium, or large relative to the other
Leaf and Surface Area
leaves. Students recorded this information in the last
To begin the second lesson, we presented leaf samples
column of the data table.
we had previously collected from plants found in the
Then we asked, “Why do leaves have different sizes?
schoolyard. During 10 minutes of free exploration,
Isn’t it the purpose of a leaf to capture the most sunlight
students observed leaf structure and shape using magpossible?” Students commented, “We noticed that if a
nifying lenses. Then, we asked, “What do you already
plant has small leaves, it has many small leaves.”
know about the function of leaves? What do you think
We also asked, “Do you think that leaf size deterwould happen if the leaf was placed in the dark for a
mines where a particular plant lives?” The students
long period of time?” The students responded, “The
seemed a little unsure. We shared the example that
leaf allows the plant to conduct photosynthesis; the
many plants growing in the lower levels of dense tropileaf and the plant would die without sunlight” (Note:
cal rain forests produce extremely large leaves. One
students should have a base understanding of photostudent stated, “Since so little light gets to the bottom
synthesis before conducting this particular series of
of the rain forest, the leaf needs to be really big to collect
lessons. If needed, you may wish to show the students
as much sunlight as possible.” We helped the students
a web-based animation for a quick review of the process
make a connection to the solar panels by asking whether
of photosynthesis [See Internet Resources]).
In this discussion, we emphasized
that plants are the only living things
that can make their own food and that
leaves are an essential part of the process. Then, we reminded students about
their discovery from the first lesson that
a larger surface area absorbs a larger
amount of heat. When applying this
idea to leaves, students noted that a leaf
with a larger surface area would absorb
a larger amount of solar energy. Students
came to think of leaves as natural solar
collectors.
Next, we went outside and student
pairs collected five leaves from different
plant species and different locations.
Students were asked to take note of the
location of each leaf sample as
Students traced each leaf onto centimeter graph paper and counted the
this would play an important role
squares inside to determine the leaf’s surface area.
in the next lesson. Make sure stu-
Figure 3.
February 2009 41
Leaves: Nature’s Solar Collectors
large or small solar panels would be more effective at
the bottom of a rain forest.
Connecting to the Standards
Leaf Size and Plant Location
This article relates to the following National Science
Education Standards (NRC 1996):
Next, students explored the relationship between leaf
size and plant location in the environment. We led students back outside to more closely investigate the locations of the various plants from which the leaf samples
were previously taken. We explained that we wanted the
students to investigate the relationship between leaf size
and plant location (sunny, shady, etc.).
First, we had the students copy the leaf size data into
the appropriate column in another data table (Figure 3,
p. 41). Then we asked students to determine if a particular leaf was in full Sun, partial shade, or full shade.
The students also recorded this information in a second
column of the table.
Back in the classroom, students looked for relationships in their data. They noticed that leaves located in
direct sunlight came in a variety of sizes. One student
commented, “If a plant is in full Sun, it really doesn’t
matter if its leaves are small, medium, or big. It will still
get enough sunlight to make food.” The students also
noticed that leaves located in shady areas were consistently larger than leaves found in sunny areas.
We verified the students’ findings by explaining
that a critical factor influencing photosynthesis is the
amount (intensity and duration) of light that hits a leaf.
The video “Photosynthesis,” available from the Teachers’ Domain website, has a good explanation of this (see
Internet Resources).
We then discussed the idea that many plants have
evolved leaf and branch structures that minimize overlap and shading. This, in turn, helps to maximize the
rate of photosynthesis in the plant. Some extension
ideas based on these lessons are available online (See
NSTA Connection).
Lesson Synthesis
To bring the sequence of three lessons together, we asked
students to tell us what the phrase, “Leaves are nature’s
solar collectors,” meant to them. The students agreed that
“Leaves are like the solar collectors that we saw on the roof.
The larger the area that can get sunlight, the better job a
leaf can do in making food for the plant, just like the solar
panels make more electricity from the Sun’s energy.”
When we asked students what they most enjoyed
about the lessons, they commented that they liked investigating the leaves the most, but it would not have been
as much fun if they had not first thought about how solar
(light) energy is absorbed by things such as water and
solar panels. One student said, “I never thought plants
were so clever to know how large to make their leaves.”
42 Science and Children
Content Standards
Grades K–4
Content Standard C: Life Science
• Organisms and environments
National Research Council (NRC). 1996. National science education standards. Washington, DC: National
Academy Press.
When we designed these lessons, we thought about
the fundamental underlying concepts that relate to photosynthesis. Our objective was not to teach the process of
photosynthesis, but to more deeply investigate an aspect
that promotes photosynthesis, namely surface area.
What we learned from doing this with young scientists
is that they appreciate being guided toward this concept,
but liked that their investigations were relatively open
when it came to the leaves. It is this transition from more
guided to more open inquiry in a set of three lessons that
we (and the students) feel made it a successful learning
experience. The students clearly saw the “story” in the
three lessons as we altered their idea of plants from inanimate objects to adaptive creatures. n
Aaron D. Isabelle ([email protected]) is an assistant professor of science education at the State University
of New York at New Paltz in New Paltz, New York.
Cornelis de Groot ([email protected]) is an associate professor of mathematics education at the University of Rhode Island in Kingston, Rhode Island.
Internet Resources
BrainPOP Photosynthesis Cartoon
www.brainpop.com/science/cellularlifeandgenetics/
photosynthesis/preview.weml
Collecting Solar Energy: Is Bigger Better?
www.wattsonschools.com/pdf/ue-1.pdf
Leaves: All-Natural Solar Collectors
www.powernaturally.org/Programs/pdfs_docs/17_
LeavesAsCollectors.pdf
Teachers’ Domain: Photosynthesis
www.teachersdomain.org/resource/tdc02.sci.life.stru.
photosynth
NSTA Connection
For extension ideas based on these lessons,
visit www.nsta.org/sc0902.