Objective Students will be able to
make and use a dichotomous key to
classify a group of items.
Skills Focus Observing,
Classifying, Forming
Operational Definitions
Time 45 minutes
Advance Prep
• For Part B, obtain graphite pencils
(with and without erasers) and
ballpoint and felt-tip pens. Other
groups of items may include:
different types of nuts and bolts or
a group of coins.
Teaching Tips
• Explain that the term dichotomous
means “having two forms.” Add that
a dichotomous key uses two forms of
each of several characteristics to
identify a species.
• Ask: What characteristics are
used in the dichotomous key for
leaves? (Whether a leaf is simple or
compound, how the leaflets are
arranged, leaflet shapes, how the leaf
veins are arranged, the shape of the
leaf, and the appearance of the leaf
edge)
• Ask: How many choices does the
key give for each characteristic?
(Two)
• Ask: Could you give the same set
of choices to identify leaves of
species other than the ones shown
in the book? (Yes, if they belong to
the genera named in the key)
Procedure
1.–3. Make sure students take time
to familiarize themselves with the
two forms of each characteristic so
they will be less likely to make a mistake in the identification process.
5. Characteristics students select will
vary depending on the group of
items. For example, if writing implements are used, they might select
the characteristic ink/inkless. Each
characteristic they select should vary
in at least one of the items.
6. Make sure that all of the characteristics students select are
dichotomous. For example, color is
not a dichotomous characteristic, but
black or nonblack is a dichotomous
characteristic.
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Chapter 18
Classifying Organisms Using
Dichotomous Keys
One tool used to identify unfamiliar organisms is a dichotomous key. A
dichotomous key is a series of paired statements that describe physical
characteristics of different organisms. In this activity, you will use a
dichotomous key to identify tree leaves.
Problem
How are dichotomous keys used and made?
Materials
• 6–8 writing implements or other group of common items
Skills
Observing, Classifying, Forming Operational Definitions
Procedure
Part A: Using a Dichotomous Key
1 To use the dichotomous key for leaves, begin
by reading paired statements 1a and 1b.
Notice that the statements are opposites.
2 Carefully observe the leaf labeled I on the next
page. Decide which statement, 1a or 1b, applies
to this leaf. Then, follow the direction at the end
of the statement. In other words, because the
leaf is a simple leaf, go to statement 4.
3 Continue reading the paired statements and
following the direction at the end of the
applicable statement until you determine the
identity of leaf I.
4 Repeat steps 2 and 3 for leaves II through VII.
Part B: Constructing a Dichotomous Key
5 Examine the group of items your teacher gives
you. List some characteristics you could use to
classify these items into groups.
6 Using the dichotomous key from Part A as a
model, construct a dichotomous key for your
group of items. You may wish to use some of
the characteristics you listed in step 5 to construct your key. Make sure that each of the
paired statements in your key are opposites.
7 Once your dichotomous key is complete, test it
with each item and revise your key, if necessary.
8 Exchange keys and items with a classmate.
Use your classmate’s key to identify his or her
items. Then, suggest ways to improve that key.
Dichotomous Key for Leaves
1. Compound or simple leaf
1a) Compound leaf (leaf divided into leaflets)
.............................................go to step 2
1b) Simple leaf (leaf not divided into leaflets)
.............................................go to step 4
2. Arrangement of leaflets
2a) Palmate arrangement of leaflets (leaflets
all attached at one central point)
...................................Aesculus (buckeye)
2b) Pinnate arrangement of leaflets (leaflets
attached at several points) ....go to step 3
3. Leaflet shape
3a) Leaflets taper to pointed tips
..........................................Carya (pecan)
3b) Oval leaflets with rounded tips
........................................Robinia (locust)
4. Arrangement of leaf veins
4a) Veins branch out from one central point
.............................................go to step 5
4b) Veins branch off main vein in the middle
of the leaf .............................go to step 6
5. Overall shape of leaf
5a) Leaf is heart shaped ..........Cercis (redbud)
5b) Leaf is star shaped
........................Liquidambar (sweet gum)
6. Appearance of leaf edge
6a) Leaf has toothed (jagged) edge
...........................................Betula (birch)
6b) Leaf has untoothed (smooth) edge
................................Magnolia (magnolia)
I
Expected Outcomes In Part A,
students should correctly classify the
leaves using the dichotomous key. In
Part B, they should develop a
dichotomous key to correctly classify
another group of items.
II
III
IV
VI
Analyze and Conclude
1. Classifying In Part A, identify leaves I through
VII.
2. Applying Concepts In Part B, how did you
choose the characteristics for your key? How
did you decide on the key’s order?
3. Evaluating and Revising Based on your
classmate’s feedback, does the key you developed in Part B need to be revised? If so, how?
4. Inferring Why is it important that the paired
statements in a dichotomous key be opposites?
Analyze and Conclude
1. I: Betula (birch); II: Aesculus (buckeye);
III: Carya (pecan); IV: Liquidambar (sweetgum);
V: Cercis (redbud); VI: Magnolia (magnolia);
VII: Robinia (locust)
2. Characteristics chosen will depend on
the writing implements or groups of items
provided.
V
VII
Classifying Develop a model of a hierarchical
classification system for a group of small
objects, such as nuts, bolts, and screws. Your
classification should be based on observable
similarities and differences. Invent your own
taxonomic nomenclature for the levels of your
classification system. Then, develop a dichotomous key that someone can use to identify the
items in your classification system. Use the
dichotomous key in this lab as a model.
Additional Activity
Collect 3 or 4 leaves from different
trees in your neighborhood. Then,
use a field guide to trees in your area
to determine the identity of each
tree. Which characteristics of the
leaves were useful in determining
their identity? In addition to leaves,
does the field guide use other characteristics of the trees to help
identify them? If so, which ones?
(Answer: Students may indicate that
the descriptions and drawings of the
basic leaf types and tree shapes helped
to determine the identity of each tree.
Field guides may also provide information about types of fruits and flowers
as well as habitats.)
Examples of groups of objects students might use include eating
utensils, postage stamps, and model
vehicles. They should select only
observable characteristics, such as
color or shape, and make all the
choices dichotomous; for example,
black or silver, car or truck, and
round or square. Have pairs of students exchange completed keys to
check that the keys work correctly to
classify the selected objects.
3. Students’ answers will depend on which
characteristics they chose for their classifications.
4. Paired statements must be opposites
because each statement leads to either another
step or identification of the species.
Classification
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