Page 139 - A Field Guide to Fairfax County's Plants and Wildlife
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ctivity: The key to success (Part 1)

A dichotomous key (from dichotomy, meaning divided into two parts) is
a tool scientists use to identify organisms based on their traits. It asks
a series of paired questions, each of which narrows down the set of
species. There are several kinds of dichotomous keys. In one kind, the
questions are written out as descriptive sentences. For example:
1a. Organism has a backbone............................................................................ 2
1b. Organism lacks a backbone..........................................................................3
If the organism you’re trying to identify has a backbone, you would
choose Question 1a, which directs you to Question 2. If the organism
has no backbone, you would choose Question 1b, which tells you to skip
Question 2 and go to Question 3.
Another kind of dichotomous key uses the same narrowing-down
approach in a more visual way. A classification tree also asks paired
questions, but the distinguishing traits are described very briefly (or
pictured); arrows show you which “branch” of the “tree” to follow to the
next set of questions. Whatever the type of key, the basic idea is the
same: “If the organism has this trait, then go this way; if it does not (or has
a different trait), then go the other way.”
Select two plants from the Field Guide. Use the classification tree shown
on the next page to identify them.

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