Grade Level:
Elementary, Middle School, High School
Ecological Concepts: Adaptation
Arizona Science Standards Addressed: Science as Inquiry; Life Science
Materials:
1) Heavy white paper
2) Scissors
3) Magnifying lenses/loupe*
4) Writing/drawing materials
5) Measuring cup*
6) Plastic wrap
7) Rulers
8) Water
* 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. (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 (for example,
according to leaf characteristics— 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. Discuss what they think happens
when a plant is in the sun all day. What happens if a plant doesn't
receive any water? Do plant leaves have different shapes, colors,
etc? Does leaf color affect water use? 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, "Do desert
plants conserve water by having narrow leaves?" The hypothesis
could be, "Plants with narrow leaves will lose less water
than plants with broad leaves because narrow leaves expose
less area to the air." Or it could be written as, "If plants
lose water through leaves and we expose narrow and broad leaves
to the sun, then the narrow leaves will lose less water than
the broad leaves."
Sample Experiment Design: The independent
variable is shape. The dependent
variable is amount of water absorbed and then
lost by the leaf through evaporation. Using heavy paper, cut
out leaves in two different shapes, narrow and broad. Keep
the total leaf area the same, changing only the shape. Each
leaf needs an equal length stem, which is a continuation of
the paper leaf. Make five copies of each leaf shape for replication.
By keeping the leaf area and stem lengths the same you are
controlling
for these factors.
Figure 1. Leaf patterns. Each leaf is constructed
from a single piece of paper. The area (length x width) of
each leaf is the same. (For those conversant with areas of
ovals, circles and the use of calculus, the areas may be a
little less than exactly the same. Good math exercise!)
Place each leaf stem in a separate container with 500 ml of
water. Cover each container with plastic wrap to reduce surface
evaporation. Do not cover the leaves, just the top of the
container itself. Place all containers in the same location
in the habitat. After a set time (one hour, for example),
come back and see which leaf shape has used the most water
by measuring the water gone (evaporated) from the container.
By keeping all variables the same except leaf size, you are
conducting a controlled
experiment.
NOTE: This experiment uses
artificial materials to model an actual adaptation common
in desert plants: narrow leaves that reduce water loss. Theoretically,
it is possible to conduct an experiment to measure water loss
from the leaves of living plants in a natural setting, but
the protocol would be very difficult for students to implement
and the equipment required would normally only be available
in professional research labs.
Sample Prediction: Narrow leaves will lose
less water than broad ones.
Record Results: Calculate how much water
was lost for each individual leaf by subtracting the amount
of water left in the container from the beginning amount.
Measurements can be taken at different time points or just
once.
Sample Analysis of Data and Presentation:
Depending on mathematical level, you can calculate the evapotransipration
rate per leaf area. The equation would be:  .
Make a bar
graph with leaf shape listed on the horizontal
axis and amount of water lost on the vertical axis. For students
who can divide, calculate the average
water loss for each leaf shape. Graph the average number on
the vertical axis. If the water loss 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) Elementary:
(a) Simulate a waxy cuticle by making paper leaves
as above, but all the same size and shape. Coat half the leaves
in paraffin wax and then compare water loss out of the containers.
Be careful not to coat the stems or the leaves will be unable to
absorb the water.
(b) Soak washcloths of the same size in equal amounts
of water. Crumple half the cloths tightly and lay the others flat.
Place all the cloths in a plastic tray in the sunlight. Record how
long it takes for each cloth to become dry. This simulates thick
leaves with lots of mesophyll cells, and thin leaves without mesophyll
cells and how these compare in water loss over time.
(c) See All Levels, below, for more.
(2) Middle School:
(a) Find the mean,
median,
mode
and range
of the data.
(b) See All Levels, below, for more.
(3) High School:
(a) Calculate the variance
and standard
deviation of the averaged data.
(b) Perform a t-test
on water used from the containers. (T-test is a standard statistics
test comparing means
of two treatment groups.) Perform an ANOVA
when comparing three or more treatment groups.
(c) Dissect leaf surfaces and look at stomates
through a microscope. Estimate the number of stomates per leaf area.
Compare different plant species.
(d) See All Levels, below, for more.
(4) All Levels:
(a) Look at leaf and stem color.
Not all leaves are the same shade of green. Desert plants are quite
often a very light green to almost white (e.g., brittlebush). Very
light colored leaves are usually that way due to hairs (trichomes)
on the leaf surface. Look at leaves through a hand lens, loupe,
or dissecting microscope. Trichomes have several functions:
(1) to trap tiny herbivorous insects
and prevent herbivory;
(2) to reduce loss of transpired water to the atmosphere
by trapping the moisture close to the leaf surface; and
(3) to reduce heat load on the leaf by reflecting
light.
(b) Compare leaf sizes of desert plants (creosote,
brittlebush, ocotillo, hackberry, etc.) to each other and to non-desert
plants (such as citrus, mulberry, etc.). Microphylly (small leaves)
is an adaptation to heat.
(c) Design paper or cardboard boxes of various shades
of green, similar to those of plants in your habitat. Make the boxes
large enough to hold a small thermometer. Make five boxes of each
color, plus five white and five black boxes for comparison. Place
thermometers in the boxes and place all the boxes in the same area
of the habitat so that all boxes receive the same amount of sunlight.
Randomly place the boxes so that not all of one color are grouped
together. Record the initial temperature for each box as they sit
in the habitat. After a set time (fifteen minutes to one hour), check
the temperatures. |
