Dichotomous Keys
SBI3U
Dichotomous Key

A dichotomous key is a tool that allows the user
to determine the identity of items in the natural
world, such as trees, wildflowers, mammals,
reptiles, rocks, and fish.
Dichotomous Keys
Dichotomous means
"divided in two parts".
 Each step has two
choices; what you choose
leads you to another
question until the item is
identified.
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 Based on physical
characteristics.
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6
2
3
4
Dichotomous key
Example of a dichotomous key…
1 With flower.
Without flower
2 Seedless.
Seed bearing
3 Plant body do not differentiated into root,
stem and leaves.
Plant body differentiated into root, stem and
leaves
4 Without photosynthetic pigment.
With chlorophyll or other photosynthetic
pigment
5 No vascular tissues.
With vascular tissues
Angiosperms
Go to 2
Go to 3
Gymnosperms
Go to 4
Go to 5
Algae
Fungi
Mosses
Fern
Dichotomous key

the dichotomous key can also be expressed in a diagrammatic form
Pla nts
With flower
Without flower
Flo we ring Pla nts
(Ang io sp erm )
Non-flo wering p la nts
Se ed -b e a ring
Se ed le ss
Pla nt b o d y no t
d iffe re ntia te d into
roo t, ste m a nd
le a ves
Without p ho tsynthetic p ig m e nt
Alg a e
Pla nt b o d y
d iffe re ntia te d
into stem a nd
le a ves
With c hloro p hyll or
o the r p ho to synthe tic
p ig m ent
Fung i
No va sc ula r
tissues
With va sc ula r
tissues
Mo sse s
Ferns
Gym no sp e rm s
Dichotomous Key Activity
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Obtain 6 different shells and a bag.
Name your shells as a unique binomial species.
Create a dichotomous key that could be used to
identify each species of shells.
Exchange your dichotomous key with another
group in the class and use each other’s key to
properly identify the species according to the other
group’s assigned names.
Fill in the assessment sheet for the group whose
key you are using and return it to them when
finished.
GROUP TASK
Each group gets ONE bag
 Place 6 shells in the bag.
 Make a Dichotomous key for ALL the items
1
in the bag.
 Follow procedures on the
2
handout.
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5
6
PROCEDURES
1.
First make and answer key.
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Write the names of your partners and bag #
on your ANSWER key.
Assign a number to each object and make up
a new name “scientific name” for each object.
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2.
Example: #1--------Mollusca minuta (small shell)
May want to trace an outline of the object
Make a dichotomous key for all objects in
your bag.
Assessing the Effectiveness of Keys
 Switch
bags and keys with another
group.
 Use their key to identify their items.
 Write the scientific name next to the
object name.
 Give them a level grade on their
descriptions and accuracy of the
items.