DICHOTOMOUS KEYS
Introduction
• 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" means "divided into two parts". Therefore,
dichotomous keys always give two choices in each step.
• A dichotomous key is a listing of characteristics, such as structure
and behavior, organized in such a way that an organism can be
identified or classified.
• Think of a dichotomous key as a type of scavenger hunt.
Using a Dichotomous key
At each step of the process of using the key, the user is
given two choices; each alternative leads to another
question until the item is identified.
1a. If the leaves are flat….go to question 4.
1b. If the leaves are needle-like….go to question 2.
2a. Are the needles in a bunch? Go to question 5
2b. Are they spread along the branch?“…pine tree
Eventually, when enough questions have been answered,
the identity of the tree is revealed.
Dichotomous Key Example
1. a. wings covered by an exoskeleton – go to step 2
b. wings freely observed – Go to step 3
2. a. body has a round shape ……….ladybug a red beetle with black spots
b. body has an elongated shape ……….grasshopper a green insect that hops
3. a.. wings point out from the side of the body ……….dragonfly an insect
that is 10- 15 cm long and lives in marshes
b. wings point to the posterior of the body ……….housefly a flying insect with
red eyes and an annoying buzz
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
Pla nts
form
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
Making a Dichotomous Key Hints:
Use constant characteristics rather than variable ones.
(Flowers change with the seasons)
Use measurements rather than terms like "large" and "small".
Make the choice a positive one - something "is" instead of "is
not".
If possible, start both choices of a pair with the same word.
Finish the dichotomous key with a description of the organism