Taxonomic Keys

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Taxonomic Keys

Taxonomic keys are a written means of helping people to identify an unknown plant. Looking randomly through a flora that includes thousands of plants would take far too much time. A key provides a structure for sorting through a great deal of information, so that the user can quickly and automatically skip over many species that do not resemble the plant.

A key is written as a series of couplets. Each couplet consists of two opposing descriptions of some features of a plant. The user chooses the description that best fits the unknown plant, and is guided by that choice to another couplet or to an answer. The two halves of the couplet lead the user to different parts of the key, dealing with different subgroups of the plants included in the key. All of the plants in the half that was not chosen are instantly rejected. Because the key is constructed of pairs of contrasting choices, it is often referred to as a dichotomous key.

A taxonomic key begins by looking at large, important features that can divide the possible answers into a few large groups, thus quickly ruling out most of them. Later couplets, which divide those groups into smaller and smaller subgroups, use tiny details to help the user tell the difference between very similar species.

see also Flora; Plant Identification; Systematics, Plant; Taxonomist; Taxonomy.

Wendy L. Applequist


Fernald, Merritt Lyndon. Gray's Manual of Botany, 8th ed. New York: American Book Company, 1950.


This is a simple taxonomic key allowing the user to identify the following common grocery store fruits: apple, banana, orange, peach, tomato, and watermelon. To use the key, pick one of these as your unknown, then read both halves of the first couplet. Which half better describes your fruit? There will be a number after that half. Go to the couplet of that number, and deal with each couplet similarly until, instead of being led to another couplet, you find the name of your fruit.

1a. Skin of fruit is thin, soft
or at least flexible, edible 2

1b. Skin of fruit is thick,
leathery or hard, inedible 4

2a. Seeds in several liquid-filled
chambers; fruit soft
throughout Tomato

2b. Seeds in hard or papery
structure in center of fruit 3

3a. Seed enclosed in hard,
stonelike pit; flesh soft Peach

3b. Seeds enclosed in
, papery core; flesh crisp Apple

4a. Fruit weighs more
than 1 pound (0.5 kg);
skin does not peel off Watermelon

4b. Fruit weighs less
than 1 pound (0.5 kg);
skin can be peeled 5

5a. Fruit long and narrow,
yellow; flesh not divided
into sections Banana

5b. Fruit round, orange;
inner flesh divided into
several segments Orange