Teacher Name: Caryl Crowell
Grade Level: 2-3
Summary:
School/District: TUSD – Borton
Primary Magnet
School
Title of Lesson Dichotomous Keys
Plan:
Subject Area(s): Science
Time Frame to 45 min – 60 min
complete lesson:
Students will learn to use a dichotomous key to identify plant species by first constructing a
dichotomous key of their shoes. The shoe key will be in the form of a branching diagram,
while most dichotomous keys used to identify plant species are in book form. The follow-up
to this lesson helps children transition to using a key presented in book form.
For the shoe key:
*Formulate relevant questions about the properties of objects, organisms, and events of the
environment using observations and prior knowledge.
* Record data in an organized and appropriate format (e.g., t-chart, table, list, written log).
*Identify ways that people use tools and techniques to solve problems.
For our outdoor work:
* Identify adaptations of plants and animals that allow them to live in specific
environments.
* Describe how environmental factors (e.g., soil composition, range of temperature,
quantity and quality of light or water) in the ecosystem may affect a member organism’s
ability to grow, reproduce, and thrive.
Assessments:
Systems Tools:
Standards:
The following standards come from the Arizona Standards in Science for 3 rd grade:
Children will be able to accurately identify a classmate’s shoe by following the dichotomous
key.
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Pre-Requisite
Knowledge:
Learning
Environment:
Printed
Materials:
Supplies:
Research Materials &
Books:
Nothing specific, although the opportunity to have observed differences among plant species
is helpful.
Classroom setting for the shoe key
Outdoor area appropriate for the plant identification guide you are using
A map of the outdoor area you will be using that shows any major landmarks, paths,
buildings, etc. that will help children locate themselves and plants on the map
Butcher paper
12 x 18 white drawing paper
Markers
crayons
A dichotomous key appropriate for the area in which children will do their outdoor work. For
example, Easy Field Guide to Common Desert Cactus of Arizona, by Richard C. Nelson and
Sharon Nelson. Glenwood, NM: Tecolote Press or Desert Tree Finder, by May Theilgaard
Watts and Tom Watts. Berkeley, CA: Nature Study Guild. The Nature Study Guild lists
guides for other areas of the country.
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On-line resources for dichotomous keys:
I found many resources available on-line for a variety of dichotomous keys, from very simple
ones using just four dichotomies, to ones more suitable for middle and high school students.
Here are just a few:
Other:
http://www.uwsp.edu/cnr/leaf/Treekey/tkframe.htm
1. Trees of Eastern, Central, and Western U.S. - www.arborday.org
2. Key to the Vertebrate Classes - http://pc65.frontier.osrhe.edu/hs/science/zochord.htm - key
3. Key to the Insect Orders - http://pc65.frontier.osrhe.edu/hs/science/binsect.htm
4. A Fruit Key - http://130.17.2.215/key/key.html
5. Trees of the Pacific Northwest - http://www.orst.edu/instruct/for241/index.html
6. Several Other Keys - http://hermes.mbl.edu/BiologicalBulletin/keys.html
http://cbe.wisc.edu/assets/docs/pdf/biolearn/Classification/WhatIsLife/dichotomous_key.pdf
http://www.dnr.state.wi.us/org/caer/ce/eek/veg/treekey/index.htm
Procedures
Procedure:
1. Show the class a dichotomous key and explain how it helps us identify cacti (trees, etc.) by considering
one characteristic of a plant at a time and asking if the plant has it or not (the dichotomy).
2. Explain that one way to learn to use a dichotomous key is to construct one, what the class will do using
our shoes.
3. Ask everyone, including the teacher, to remove one shoe and put it in the center of the circle.
4. Invite students to suggest rules that would sort the shoes into one group or another. Some examples might
be shoes with laces and shoes without laces, or leather shoes and not leather shoes.
5. Choose one of the suggestions that allows all shoes to fit into one of the two categories without any
ambiguity. It might be necessary to demonstrate how rules related to size, for example, are relative and
won’t work. To do this, draw a circle on the board and surround it with larger circles. Ask the children if
the original circle is small or large compared to the other. Then erase all but the original circle and surround
it with smaller circles. Show the children that the circle that was small before is now large in comparison.
Such descriptors are “relative” and for this activity, we need descriptors that are constant, that don’t change
when the surroundings change.
6. Sort all the shoes into the two categories. This lesson will use “shoes with laces that tie” and “shoes
without laces” as an example. Begin a tree diagram on a large piece of butcher paper, with the first two
branches labeled for the chosen rules.
7. Now work just with the group of shoes in the “shoes with laces” category. Decide upon another rule that
will sort this group of shoes into two sub-groups. For our example, we’ll use “flat laces” and “not flat
laces.” Once more, sort the shoes from this group into two smaller groups and add the branches to the chart.
8. Keep sorting the ever-smaller groups of shoes into dichotomous categories until only one shoe remains in
each sorted group. Then, identify to whom the shoe belongs and write that person’s name under the
descriptor on the tree diagram.
9. Repeat these procedures with the group of shoes set apart in step 6, for this example, “shoes without
laces.” Continue sorting until each of these shoes is in a category by itself and can be identified with a
child’s name.
10. Ask some children to choose a shoe at random, one that is not their own, and follow the dichotomous
key tree to identify the shoes.
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Some of the descriptors my students used: color of laces, logos on shoes, pull-on loops on the heel, number
of eyelets, buckles, Velcro, patterns on soles, decoration, slip-on shoes, and anything else that is not a
relative descriptor.
11. Before the children put their shoes back on, have them trace the outline of their shoes on white drawing
paper and add all the details, colors logos, etc. needed to create a reproduction of their shoes. They should
write their names on the back.
12. Post the shoe key and shoe drawings in a spot where the children can gain experience using the key by
choosing one of the shoe drawings and trying to identify to whom it belongs.
Special
Education:
Modification for Differentiated Instruction
Students with visual-motor difficulties may need assistance tracing around their shoes or
representing the details that sorted that shoe for the dichotomous key. They might ask
another child or adult to help as needed.
Gifted:
ESL:
Words describing different parts of a shoe might be unfamiliar to English language learners.
Consider posting a diagram of a shoe with the parts labeled: eyelets, laces, tongue, sole, etc.
These children might be challenged to think about other descriptors that would be
considered “relative,” like size.
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Suggestions for using a dichotomous key in the field:
Most field identification guides that are set up in dichotomous keys will be in book form, not
in a tree form. The children will need a bit of support in understanding how the two forms
of dichotomous keys relate.
Other/Comments:
When you take the keys outdoors for the first time, work together as a class, or in small
groups to identify a few plants. Read through the descriptors together, make a group
decision, and then follow the directions in the key to turn to the appropriate page and make
the next decision in the dichotomy. It won’t take long until the children are able to use the
keys on their own.
There may be some terms in the key that are new to the children. For example, in our cacti
key, the children needed to understand the difference between columnar forms and flat stems
and between central spines and lateral spines. If you are working with a tree guide, the
children will need to know leaf types and ways to describe the vein patterns and leaf edges.
The children also needed to know how to gauge size without a ruler. Our field guides had an
inch measurement along one edge of the binding, but you could always take rulers into the
field with you. Another option is to develop beforehand a series of body measurements that
children will always have available. For example, the distance between the first and second
knuckle on the index finger is about an inch. The distance between the elbow and fingertips
is about a foot, depending of course, on the size of the child. Developing this set of body
measurements is an interesting and fun activity in and of itself that has a practical
application in many areas.
We also gave our students maps of the area we were investigating and showed them how to
use a legend to locate each plant and use the same number on their maps to indicate the
location of each plant of a single species. It’s our intent to use these maps to explore why
our area of investigation varies so much in plant type and density from one part to another.
Used by permission from Project Venture, 1998 Technology Innovation Challenge Grant, Phoenix, AZ
http://www.creighton.k12.az.us/projectventure/index.html
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