Grade Level:
Elementary, Middle School, High School
Ecological Concepts: Adaptation,
Resource
allocation
Arizona Science Standards: Science as Inquiry; Life Science
Materials:
1) Desert and non-desert plants or seeds (use annuals or quick-growing perennials)
2) Plant pots and desert soil
3) Potting soil or compost
4) Writing/drawing materials
5) Magnifying lenses*
6) Weighing scale
7) Trowels*
*May be borrowed from SCENE.
BACKGROUND
Plants have underground tissues called roots that stabilize the plant
and hold it in place, and absorb water and nutrients. Plants living
in ecosystems
with low levels of rain, such as the Sonoran Desert, have evolved
adaptations
to the dry conditions. One plant structure that has adapted is the
root system. (See
Plant Adaptations activity for others.) A few plant species
have adapted to the climate by growing very long tap roots that reach
deep into the ground for underground supplies of water in aquifers.
Mesquite trees use this adaptation. Most other plants have evolved
large root systems that lie close to the ground surface. Roots are
then close to the water when it rains and can absorb the moisture
in the soil surface before it evaporates under the desert sun.
GUIDED INQUIRY
Initial Observation/Exploration Period: Observe the
plants in the habitat. Carefully dig up some weeds (in the habitat
or elsewhere) to look at roots with the naked eye and magnifying lenses.
As you dig up the roots, try to notice how the roots are arranged
in the soil. The tiniest roots are root hairs. These root hairs do
most of the work of absorbing water and nutrients from the soil.
Group Discussion and Question Period: Do desert plants
differ from non-desert plants in root structure? Do some species of
plants grow more roots than others? How deep do roots grow into the
ground?
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 differ from non-desert
plants in root structure?" Our hypothesis could be, "Desert
plants will have roots adapted to collect as much water as
possible, therefore, desert plants will have larger root systems
than non-desert plant species." Or it could be stated, "As
plants are more drought
tolerant, their root systems will become larger,
either growing many roots near the soil surface or having
very deep roots."
Sample Experiment Design: The independent
variable will be the type of plant, desert species
versus non-desert species. The dependent
variable will be root biomass.
Grow five plants each of two different Sonoran Desert species
(recommend fast-growing, small plants) and five each of two
different, but comparable type and size temperate species.
(Note: Desert wildflower seeds such as Mexican poppy, bluebells
and globe mallow, to name a few, are available from Native
Seed Search. Temperate climate wildflower seeds
are available from various nurseries and seed suppliers.)
Each plant is an experimental
unit, and by having more than one of each species
you will have replicates
for statistical purposes. It would be best to grow the plants
from seed, or get them when very small so you can control
the environment they are grown in. Put plants in 1 gallon
pots (one plant/pot). Keep pots in the same area in the habitat,
and give them the same amount of water on the same days. The
amount of water should be comparable to the natural average
rainfall in the Sonoran Desert. An alternative procedure is
to grow the plants in the ground and carefully dig them up
later. The problem here would be the possibility of cutting
off some of the roots and not knowing it. Biomass measurements
would then be inaccurate.
Sample Prediction: Desert plants will have
larger/heavier root systems than non-desert plants.
Record Results: When plants begin to bloom,
remove them from pots, being careful to collect all root material.
NOTE: This means that not all plants will be harvested on
the same day. Each plant will likely begin blooming on a different
day. The important thing is that measurements are taken when
the plants are at the same life stage.
Wash the roots carefully
to remove all soil. (1) Lay the roots out and draw (or take
a photo) of the roots, with a ruler to establish scale. (2)
Dry the roots in a safe place. After they are thoroughly dry,
weigh them to measure the root biomass. Weight, or biomass,
is a measure of the amount of root available to soak up water.
Sample Analysis of Data and Presentation:
Compare the root biomass of the two different plant species.
Make a bar
graph of the numbers with plant species on the
horizontal axis and root biomass (weight in grams) on the
vertical axis. For students who can divide, calculate the
average
of the desert plant root biomass and the non-desert plant
root biomass. Graph the average numbers on the vertical 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.
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MORE:
(1) Elementary:
(a) Grow the plants in rich potting soil or compost
instead of desert soil. Compare the root biomass.
(b) Grow desert plants in desert soil and the same
species in potting soil. Compare the root biomass.
(2) Middle School:
(a) Find the mean,
median,
mode,
and range
of the data.
(b) Weigh the root hairs of each plant species.
Collect all the root hairs from each plant. Weigh the root hairs
of each plant separately, and then average the weights for all plants
within a species. Compare the two species.
(3) High School:
(a) Calculate the variance
and standard
deviation of the averaged data.
(b) Calculate the surface area of root available
for water absorption. Compare the plant species. |
