How to Use the Advanced Keys

A plant is usually identified by means of the differences between the form of its parts and those of other closely related plants. In a dichotomous key, these differences are arranged within paired couplets. For example, in the couplet below, you are asked to make a decision as to whether or not the plant in question has leaves. If leaves are present, you can proceed to couplet 2; if they are absent, you should go to couplet 130. This procedure of choosing between paired alternatives is repeated with each new couplet until you arrive at the name of a tree.

Two special features of the advanced keys on this web site aid identification. When the species name is eventually reached, additional characters not included in the other member of the couplet pair are often given as a double check on the keying out process to make sure the identification is correct. When it is clear from this extra description that the tree has been misidentified, it is often helpful to work the key backwards to try to find that couplet where the wrong alternative was chosen. To do this, after the number of every couplet, a second number is given in parenthesis indicating the number leading to the present one, e.g., 178 (157) means that you arrive at couplet 178 through 157.

Although the key was constructed using the most diagnostic features to separate one plant from another, natural variation may create difficulty in using the key. Age and growing conditions often cause great differences in leaf size and shape. When variation is present, it is best to (1) try to obtain a representative leaf by looking at a number of leaves on the plant and then to (2) follow through both parts of the couplet where the variation has caused doubt as to which alternative to choose.