Plant Taxonomy
Lab 3: Plant Identification
A. Methods of Identification
1. Ask an expert
2. Compare with illustration, photograph or herbarium specimens
3. Recognize
4. Use an identification key
B. Use of Taxonomic Keys
A taxonomic key is a device for quickly and easily identifying to which species an unknown plant belongs. The key consists of a series of choices, based on observed features of the plant specimen. By making the correct choice at each level of the key, you will eventually arrive at the name of the unknown plant.
Keys in which the choices allow only two (mutually exclusive) alternatives are known as dichotomous keys. In constructing a key, contrasting characters are chosen that divide the full set of possible species into smaller and smaller groups. Each time a choice is made, a number of species are eliminated from consideration and the range of possible species to which the unknown specimen may belong is narrowed. Eventually, after sufficient choices have been made, their range reduces to a single species and the identity of the unknown plant is revealed. Taxonomic keys are usually of two kinds: bracket keys and indented keys. Both types are demonstrated below for the same group of organisms. In this class you will make use of indented keys.
Bracket Key to Three Dicot Genera Examined in Class
1. Leaves lanceolate to oblong; flowers yellow………………….………………….Ludwigia (2)
1. Leaves sagittate to palmatifid; flowers pink………………………………….Ipomoea lacunosa
2. Leaves sessile and decurrent; capsule obpyramidal……..………...……….Ludwigia decurrens
2. Leaves sessile to subsessile but not decurrent; capsule obconic, many ribbed…L. bonariensis
Indented Key to Three Dicot Genera Examined in Class
1. Leaves lanceolate to oblong; flowers yellow
2. Leaves sessile and decurrent; capsule obpyramidal……..………....Ludwigia decurrens
2. Leaves sessile to subsessile but not decurrent;
capsule obconic, many ribbed………….……………………………...….L. bonariensis
1. Leaves sagittate to palmatifid; flowers pink………………………………….Ipomoea lacunosa
Guidelines on Using or Making Taxonomic Keys:
Before using or making a taxonomic key, it is good practice to examine the plant specimens carefully and make note of their characteristics. Some other helpful hints for success in using a key are:
1. Read the introductory comments, abbreviations, glossary etc before using the key.
2. Always read both of the choices presented at each step in the key before making a decision,
Even if the first choice seems correct, the second may turn out to be even better.
3. Look up the definition of any term with which you are not familiar.
4. If measurements are needed, use a ruler: do not guess.
5. Living organisms are always somewhat variable, so try to examine several parts of the
specimens and base your choices on typical or average features.