Grade: 3 to 5
Length: one hour
Subjects: life science, classification
Topics: weed identification, dichotomous keys
Objectives
Exercises in this lesson help students achieve the
following objectives:
Practice observation skills, distinguish similar
characteristics, and identify plants by using two
simple dichotomous keys
Create and use their own dichotomous keys
Introduction
Dichotomous keys are one of the tools available for
weed identification. The use of keys encourages
close examination of a subject and helps to sharpen
observation and classification skills. Students will
learn about dichotomous keys, apply this method
for classification and identification to simple objects, and create their own keys. Before teaching
this lesson, read the entire lesson, and make sure
all materials are available.
Background
Dicho comes from the Greek word meaning in two
or into two. When using a dichotomous key, you
choose between two characteristics, follow the appropriate path, and go to another set of characteristics until the item you are examining is in a class by
itself. At this end point, you find a scientific name
for the item.
have its own common names for plants, and the
results are often confusing.
In the mid-eighteenth century, a Swedish botanist
named Carolus Linnaeus developed the scientific
naming system that is in use today. Names are
based in Latin, and names may have Greek roots.
The scientific name provides information on the
genus and species of the life form.
The first part of a scientific name is the genus.
The genus is always capitalized, and it is usually
assigned to a number of species with many characteristics in common. The second part of the scientific name describes the species. It is usually lower
case, and it often describes a particular species with
regard to color, size, or location in which it grows.
Together, the two parts form the scientific name for
the species.
Scientific names, which are usually italicized in
print, are used worldwide. Genus and species are
the two final categories in a larger system of classification that includes the following categories:
Kingdom
Phylum
Class
Order
Family
Genus
Species
Plants are often best known by their common
names. However, common names are not univer-
sally accepted. Each geographical area tends to
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Preparation
Materials
five different kinds of small candies, such as
dum-dums, Hershey kisses, smarties, starbursts, or jolly ranchers in approximately equal
quantities – Have available two pieces of candy
for each student.
students a bowl of candy, and ask
1 Show
students to identify the candy by its common
name.
that you have assigned a scientific
2Explain
name to each type of candy, and students may
determine the scientific name only by using a key
correctly. Students may keep the candy if they
correctly identify the candy by its scientific name.
Activity
Materials
five different weed samples common to your
area in approximately equal quantities – Have
two weeds available for each student. If live
weed samples are unavailable, use three-dimensional models.
copies of the Candy Key and Weed Key Template
worksheets – Have available one copy for each
student. Complete the Weed Key Template with
local plant species before making copies.
that a dichotomous key classifies
1 Explain
objects based on similarities and differences.
We rely on these similarities and differences to
identify objects or organisms. Students will use
dichotomous keys to help identify weeds growing
in the community.
that before using a dichotomous
2Explain
key to identify weeds, students will practice
using a key to identify candy. Give a copy of the
Candy Key to each student. Using a piece of candy,
demonstrate how to use the key.
around the bowl of candy. Ask each
3Pass
student to take two different kinds of candy.
students to use the Candy Key to determine
4Ask
the scientific name you assigned to the
different types of candy. Check students’ work for
accuracy.
each student a copy of the Weed Key
5Give
Template that you prepared in advance.
each student two different weed
6Give
specimens, and ask students to use the Weed
Key Template to determine the scientific names of
the weeds. Check students’ work for accuracy.
Conclusion and Evaluation
Conclude the lesson by reviewing the common and/or scientific names for the five weeds
used in the Activity. Tell students where you
collected the weeds. Discuss other locations
where students have seen these weeds growing.
Review some of the problems associated with
invasive plants. Suggest that properly identifying plants is the first step in controlling invasive
plants.
To evaluate students, have students identify all
five species of weeds used for the Activity.
Independent Practice and Related
Activities
Ask students to develop their own dichotomous keys for other groups of objects or organisms. Have other students use the keys to test
their accuracy.
Take students into the field to practice identifying weeds in a field setting.
Vocabulary
classification, common name, dichotomous, identification, key, organism, scientific name
Resources
Peterson, Roger Tory and Margaret McKenny. A
Field Guide to Wildflowers of Northeastern and Northcentral North America. Boston: Houghton Mifflin
Company, 1968.
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Spellenberg, Richard. National Audubon Society Field
Guide to North American Wildflowers, Western Region.
2nd ed. New York: Knopf, 2001.
Weed identification guides from local extension
offices or weed control departments of state and
federal agencies.
For three-dimensional models of weed specimens,
contact the Center for Invasive Plant Management,
733 Leon Johnson Hall, MSU Bozeman, P.O. Box
173120, Bozeman, MT 59717-3120; 406-994-6832.
National Science Education
Standards:
As a result of their activities in grades 3–5, students
should develop abilities in and an understanding
of the following areas:
Life Science – Content Standard C: characteristics
of organisms
cipm@montana.edu, www.weedcenter.org
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Weedy Definitions
classification – placing plants and animals into groups, based on characteristics they have in common
common name – the name organisms are usually called; can vary from region to region
dichotomous key – method of classifying organisms by choosing between two characteristics
identification – something that proves what a plant or animal is During the lesson, you used a dichotomous
key for plant identification.
key – a logical, orderly listing of characteristics used to identify organisms
organism – a living plant or animal
scientific name – the name an organism is given by the scientific community; made of two words, usually
of Latin or Greek origin, the first of which lists the organism’s genus and the second of which lists its species
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