Grade Level:
Middle School, High School
Ecological Concepts: Adaptation
Arizona Science Standards: Science as Inquiry; Life Science
Materials:
1) Plastic wrap
2) Scissors
3) Magnifying lenses/loupes*
4) Writing/drawing materials
5) Measuring cups*, beaker, or graduated cylinder
*May be borrowed from SCENE.
BACKGROUND
This activity would fit well with a unit on plants, how they grow
and produce food through photosynthesis. Provide as much of this
information as is needed for the students to have a beginning grasp
on the subject of photosynthesis.
Plants have green structures called leaves whose primary purpose
is to collect the energy of sunlight and transform it through a
chemical process called photosynthesis
into food for the plant. Leaves come in many shapes, sizes, colors,
and textures. Sometimes they do not even look like what we think
of as typical leaves; for example, cactus spines are a form of leaf.
Water is necessary for photosynthesis and food transport throughout
the plant. Plants wilt and will eventually die if they do not have
enough water. Where does the water go that a plant takes up through
its roots? Is it all incorporated into the plant's physical structure?
On a typical plant, leaves are the part with the most area exposed
to the air. Leaves have microscopic
stomates
to release water as vapor during photosynthesis. Carbon dioxide,
a gas, is needed for photosynthesis. Carbon dioxide is taken into
the leaf through the stomates. Just as we use water in our cells
to survive, plants do too. We inhale oxygen and exhale water vapor
in our breath as a by-product, while plants take in carbon dioxide
and release water through the stomates. This water loss through
the stomates is transpiration.
(See
Energy Flow for more details.) This creates a problem
for plants in dry environments: how to conserve water.
Desert plants have adapted to environmental conditions of low water
availability and high temperatures in a variety of ways, on the
macroscopic
as well as microscopic scale. Some characteristics are having a
waxy cuticle,
stomates on the under leaf surface, opening of stomates only at
night, and a thick layer of mesophyll
cells, among others.
GUIDED INQUIRY
Observation/Exploration Period: Observe the various
types of plants in the habitat using eyes, magnifying lenses, flashlights,
brushes, fingers, etc. Focus on leaves, but other plant structures
can be observed as well. Have students categorize findings (example:
plants with spines, thin leaves, oval leaves, etc., or use another
scheme) as they see fit.
Group Discussion and Question Period: Guide students
to look for patterns among and between different types of plants,
focusing on leaves in terms of size, shape, color, texture, and
other characteristics they may have noticed. What happens when a
plant is in the sun all day? How much water does a plant lose through
it leaves? What happens if a plant doesn't receive any water? Do
plant leaves have different shapes, colors, etc? Does plant leaf
color affect water loss? Which leaf size would survive the desert
heat best?
Important aspects of guided inquiry are encouraging students
to generate multiple
hypotheses, and letting students make decisions about
what data are important and create their own data sheets. Keeping
these ideas in mind, the sample in the box below illustrates how
ONE OF MANY possible investigations around this topic might develop.
Sample
Hypothesis: Let's use the question, "How
much water do plants lose through their leaves?" Our
hypothesis could be, "If plants transpire more water
during the day than the night and we collect the transpired
water, then we will collect more water from the plants in
the afternoon than the morning." Or, "Plants transpire
more water during the day than at night because the heat evaporates
more water from the plant."
Sample Experiment Design: The independent
variable is time of day, daylight versus dark.
The dependent
variable is amount of water collected. Use at
least three plants for replication,
of the same species and as close to the same size as possible.
The plants also need to be in the same amount of light and
shade throughout the day. By keeping these factors the same
you are controlling
variables different from the one you are testing. Use clear
plastic bags or plastic wrap to completely cover all the leaves
of each plant. Tie the bottom of the plastic around the main
stem of the plant, leaving some droop at the bottom to collect
water. Try to not crush the leaves with the bag.
Bag the plants early in the morning. Leave on all day. Check
the bags at the end of the day for signs of water drops inside.
If water has collected, gently shake the drops to the bottom
of the bag, carefully remove the bag and pour the water into
a measuring device such as a beaker or graduated cylinder.
Record the amount. Dry the bags off and replace on the plants
as before. Check the bags first thing in the morning and collect
and measure any water.
Figure 1. Plant bagged with plastic with
water collection areas at bottom.
Sample Prediction: More water will be collected
at the end of the day than in the morning.
Record Results: Record how much water is collected
from each plant, once in the afternoon and once the next morning.
A graduated cylinder marked in milliliters will be most accurate
since the amounts are likely to be very small.
Sample Analysis of Data and Presentation:
Make a bar
graph with time of day (day and night) on the
horizontal axis, and amount of water collected on the vertical
axis. For students who can divide, calculate the average
amount of water collected for each time period. Graph the
average number on the vertical axis. If the water collected
is tracked over time, make a line
graph with time on the x-axis and average water
loss in ml on the y-axis.
Discussion: Was your hypothesis supported?
If yes, go on to test other hypotheses. If not, why not? What
did happen? Why? This is a great opportunity to revise your
hypothesis and do another test.
|
MORE:
(1) Middle School:
(a) Find the mean,
median,
mode
and range
of the data.
(b) Compare transpiration rates
between plants with different shaped leaves. Choose plants close
to the same size, but of two different species. Run an experiment
similar to the one above.
(2) High School:
(a) Calculate actual leaf surface
area and evapotransipration rate per leaf area. Calculate the
standard deviation of water loss for each leaf size and plot on
a graph.
(b) Calculate the variance
and standard
deviation of the averaged data.
(c) Perform a t-test
on water collected. (T-test is a standard statistics test comparing
means
of two treatment groups). Perform an ANOVA
when comparing three or more treatment groups.
(d) Dissect leaf surfaces and look at stomates
through a microscope. Estimate the number of stomates per leaf
area. Compare different plant species. |
