How to use Dichotomous Taxonomic keys
Appendix 3
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A taxonomic key is like the game of Twenty Questions. When you've answered them all, the
key tells you what organism or group of organisms best fits the criteria. If you've never used a
"dichotomous" key, one where the choices typically occur in pairs, this example will help you
get oriented.
Key to Large Terrestrial Organisms
1a
1b
Has green leaves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Does not have green leaves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
5
2a
2b
Has flowers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Does not have flowers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
4
3a
3b
Has flower parts in 3's . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Has flower parts in 4's or 5's . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4a
4b
Has cones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Gymnosperms
Does not have cones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ferns
5a
5b
Has hair and gives milk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mammals
Does not have that combination of characters . . . . . . . . . . . . . . . . . . . . . . . .
6
6a
6b
Has feathers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Does not have feathers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Monocots
Dicots
Birds
Reptiles
The beauty of a key is that it only asks what is important, based upon your previous answers. If,
for instance, you choose option 1a, it would be pointless for the key to ask next a question about
hair. Leaves may go together with flowers, but never with hair.
The information in a key doesn't usually "define" the organisms it identifies. Reptiles, for
instance, are identified in this key entirely by negative characters. In this key they are just all
those "large terrestrial organisms" that don't have leaves or flowers or cones or hair, etc. That
they do have scales and lay eggs and have a peculiar inner ear structure, etc., is nowhere to be
learned. As a result, you may learn very little about your organism simply by finding its name in
a key. And if all you have is a name, how do you know it's the right name? After all, how do
you know you didn't take a wrong turn when using the key?
The only way to see if you came out in the right place is to check the answer it gives you. Most
keys are accompanied by longer, usually definitive, descriptions of the organisms they identify,
often including pictures. Check them out and see if the organism the key has led you to really
does seem to be the organism growing in your plate.
Another important reason to check frequently with the pictures that a key often provides is
because sometimes you won't know the meanings of some words in the key. If, for instance, the
key provides you with the choices of "globose sporangia" and "clavate sporangia", one way to
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How to use Dichotomous Taxonomic keys
Appendix 3
Page 2 of 2
figure out how those things may differ is to take a look at the pictures of some of the species that
are said to have either the one or the other sporangium type. Often, you can figure out from that
just what the words mean.
Of course, you should also try the glossaries of appropriate textbooks and dictionaries of
scientific terms. But even plain old Webster's Collegiate Dictionary will have the meanings of
"globose" and "clavate" and lots of other apparently obscure terms. For in fact many of them are
really just plain not-often-used English words that you haven't yet encountered. ("Globose"
means globe-shaped, and "clavate" means club-shaped, by the way.)
Sometimes, though, if you're lucky, you can get past a confusing choice without knowing exactly
the meaning of every term involved. For instance, consider the following choices:
33a
Sporangia clavate; spores with long spines connected at their bases to form a
reticulum, becoming smoky brown at maturity.
33b
Sporangia globose; spores spinose, remaining hyaline.
You don't really have to know exactly what "clavate" and "spinose" mean in this case.
(However, these words are found in the glossary, appendix 5.) Check the spore color: if they're
brown it is probably 33a; if they're clear (hyaline) it is probably 33b. Of course, if you do try to
figure out what the other terms mean, you'll be much surer of your decision.
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