tree-logo.gif (7741 bytes) Plant Taxonomy (BIOL308)  -  Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;

Plant Identification
(featuring Taxonomic Keys)

 I. Introduction to identification

A. Mustard and flamingos
    Let's start with a quote from Lewis Carroll's, Alice’s Adventure’s in Wonderland:

"Very true", said the Duchess: "Flamingos and mustard both bite. And the moral of that is – 'Birds of a feather flock together.'"
"Only mustard isn’t a bird", Alice remarked.
"Right as usual, said the Duchess: "what a clear way you have of putting things!"
"It’s a mineral, I think," said Alice.
"Of course it is" said the Duchess, who seemed ready to agree to everything Alice said: "there’s a large mustard-mine near here. And the moral of that is – 'The more there is of mine, the less there is of yours!'"
"Oh, I know!" exclaimed Alice, who had not attended to this last remark, "it’s a vegetable. It doesn’t look like one, but it is".

B. The need for identification
    We share the planet with at least 1.5 million (some estimates are as high 30 million) other species. In order to communicate, retrieve, store, and accumulate information about our co-inhabitants like flamingoes and mustard, it has been necessary for biologists and others to: (1) identify these organisms; (2) name them; and (3) place the organisms into groups that reflect our current knowledge of their evolutionary relationships. As we know, collectively these activities - identification, nomenclature and classification - make up the discipline of taxonomy. Here we'll focus here on identification.

II. Identification
    Identification is defined as associating an unknown entity with with that is already known; or at least recognize that the unknown doesn't have a known counterpart (= new species).  So what does this mean?  Let's give an example.  Suppose that you didn't know that the large bird Alice is carrying is a flamingo. How could you identify this creature that "bites"? The answer is simple in theory - we would compare the "mystery bird " to other "known" or "reference" or "type" birds until we find one that matches.  If, after an exhaustive search, we don't find a match we may be studying a species new to science. The process of matching a mystery organism to known ones can be accomplished by:

A. Asking an expert
    Consult an individual who has spent his/her life studying birds (ornithologist). This method usually provides a reliable and accurate answer because it is based on the wisdom and years of experience of a professional. "Experts" are typically found in botanical gardens, museums, herbaria, colleges, universities, etc. Unfortunately, experts are not always available (or willing) to help, so you must be prepared to do your own identifications. Note, it is common even for professional taxonomists to send "difficult" specimens to experts.

B. Hunting-and-Pecking (specimen comparison)
    In this case, we search through a field guide, museum or zoo for a bird that matches our mystery bird. If we were trying to identify a plant we could even search through the herbarium.  This can involve looking at pictures, actual specimens or descriptions.  Although we may get lucky, this method is the least satisfactory because of the small probability that we will stumble upon the match. This can be time-consuming and is not very efficient. Picture books are designed for hunting-and-pecking.

C. Using a taxonomic key
    A key is a device, which when properly constructed and used, enables a user to identify an organism. There are three types of keys that we will discuss; (a) dichotomous; (b) polyclave (also called multiple access or synoptic); and (c) probability.

III. Dichotomous keys
    (di - two; chotomy - forked). These keys, which are the most common, were probably first published by Jean Baptiste-Lamarck in 1778 (remember him, Mr. Acquired Traits?). They consist of a series of paired statements, termed couplets, that describe some feature of the organism. The statements, or leads, are in direct contrast (i.e., mutually exclusive). To use the key, begin with the first couplet and select the statement that best fits your specimen. This will direct you to another couplet and ultimately provide the identity of your specimen.

A. Types.
    There are two types of dichotomous keys. They differ in the method by which the couplets are organized and how the user is directed to successive choices.

    1. Indented Keys (also called yoked) - indents the choices (leads) of the couplet an equal distance from the left margin. The two choices of the couplet are usually labeled; e.g.,  1 and 1' or 1a and 1b. It is not necessary that the choices are numbered, but it helps. The user goes to the next indented couplet following the lead that was selected. For an example, check out Figure 1.
    2. Bracketed keys - provides both choices side-by-side. The choices of the couplet must be numbered (or lettered). It is very helpful if the previous couplet is given. Note: in some bracketed keys alternate couplets are indented; in others, all couplets begin at the left margin. The user proceeds to the couplet that is indicated by the lead selected.  For an example, check out Figure 2.
    3. Examples of both types of keys will be shown in class. In general, botanists prefer indented keys. Advantages of indented keys include: (a) similar specimens are grouped together; (b) it is harder to get lost or loose your place; (c) they are faster to use; and (d) it is easier to retrace your steps if you make a "wrong turn". Which kind is used in our texts? Which type do you prefer? Why?

B. Using a dichotomous key
    To demonstrate how a key works we will identify some of the creatures (i.e., Flamingo, Gryphon, Mock turtle, Cheshire Cat, Dormouse) from Lewis Carroll (see Figure 1 and Figure 2). Tips for using a key:

  1. don't guess - be sure that you understand the meaning of any terms that are used in the key;
  2. read both choices;
  3. measure;
  4. watch for "weird" (abnormal) specimens and remember the natural variability in all organisms, e.g., check several specimens to make sure you have a normal one;
  5. when in doubt, try both choices;
  6. check your answer with a description or photo or herbarium specimen.

C. Problems using dichotomous keys
    Aside from being poorly written (see below), a key may be difficult to use because:

  1. the key doesn't include all potential variations in the species;
  2. the key relies on features not present in that season;
  3. the key doesn't include "all" species of interest;
  4. you misinterpret a feature or make a mistake.

    All of these mistakes can lead to frustration. But, try, try again!

D. Writing a dichotomous key
    First collect your data by: (a) laying out the plants to be keyed in front of you; and/or (b) recording data on note cards or in a computer spreadsheet or database; and/or (c) creating a table listing the species to key along one side and the characters to study along the other side (see Table 3 for the data used to created the keys to Lewis Carroll's creatures). Once you have collected your data, start to group the objects. It is best to start with a feature that separates the things to be keyed into two groups of similar number and then subdivide these groups until individuals are distinguished.

Other tips for key writing:

Figure 1:  Bracketed Key to Some Characters from the "Alice" stories by Lewis Carroll

2a (1) 

3a. (1)

4a. (3)

Wings present ..................................2
Wings absent ...................................3

Body covered only with feathers............Flamingo
Body covered with feathers and fur........Gryphon

Fur absent, animal usually crying...........Mock Turtle
Fur present, animal rarely crys.............4

Able to disappear, usually with a grin......Cheshire Cat
Unable to disappear, usually asleep........Dormouse


Figure 2:  Indented Key to Some Characters from the "Alice" stories by Lewis Carroll
1a.     Wings present
                2a.      Body covered only with features........Flamingo
                2b.      Body covered with feathers and fur...Gryphon
1b.     Wings absent
                3a.      Fur absent, animal usually crying........Mock Turtle
                3b.      Fur present, animal rarely crys
                              4a.   Able to disappear, with a grin.......Cheshire Cat
                               4b. Unable to disappear, usually asleep...Dormouse


Table 3:  Table of characters and states for characters from the "Alice" Stories


Wings Fur Unique Behavior(s)
Cheshire Cat absent + disappears, grins
Dormouse absent + usually asleep
Gryphon present +  
Flamingo present -  
Mock Turtle absent - usually crying

E.  Interactive Keys
    Dichotomous keys have traditional been written out.  However, in recent years, interactive dichotomous keys have been developed for the computer.  In these keys, the user clicks through a series of questions, often featuring images of the character/state until reaching an identification.  A good example is a key to the trees by the Iowa State University Extension Service.   An interesting hybrid between an interactive dichotomous key and a polyclave (see next section) is posted by the the Forestry department at Virginia Tech (A multichotomous key to the trees). 

IV. Polyclave/Random Access/Synoptic Keys

A. General
    Another type of key is termed multiple access or polyclave or synoptic key. The advantage of these keys is that they allow the user to enter the key at any point and the user can chose the most obvious/important characters of the specimen to identify. These keys are a relatively new alternative to dichotomous keys and are becoming increasingly popular, especially because of the ease of computerizing them.

    Identifying organisms with a polyclave is a process of elimination. In a written polyclave key there are a series of characters and character states. Each state is followed by a number or code for the species that possess that feature (see Pollination Key). The user selects any character and then copies down the list of species that possess the feature. Then the user selects another character and eliminates any species not common to both lists. This process continues until the specimen is identified.

    It's easy to imagine how these keys are computerized. Consider a series of standard playing cards. Imagine each card has four holes punched into it along the top margin, one each for the suit - spades, hearts, clubs, diamonds. If the card is a spade we cut the "spades" hole through to the margin; if it is a club, the "clubs" hole is notched to the margin and so on.  Further imagine that along the bottom of the card we punch 14 holes representing the card face value (2 - 10, J, K, Q, A).  We would  cut a notch for the appropriate number. Thus, the Queen of Hearts will have a notch cut into the "hearts" hole on the top of the card, and the "Queen" notch on the bottom of the card.

    Now, let's use our specialized deck of "punched cards" to identify an unknown card. Shuffle another deck of cards and pick any card. Let's assume that this "unknown" card is the Ace of Spades. To identify this unknown, we analyze the characters and two are obvious, suit and number. Let's start with suit - take a needle and stick it through the "spades hole" of the punched deck. Since, only spades are notched, the other suits will remain on the needle and spades will drop out of the deck. Cool. Now, collect the spades cards and put a needle through the next character, the Ace. And, viola, the Ace of Spades falls out. This is the general principle of how the computerized version of polyclave keys work. The main difference is that a computer allows for infinite holes (characters) and notches (states) to be included and does the needlework for us.

B. Advantages/Disadvantages
    The advantages of a polyclave (multiple-access) key are:

    1. easy to use;
    2. multi-entry - meaning the user can start anywhere. This is a significant advantage because the user can rely on characters that are most easy to observe, rather than having to deal with characters that may not be present in the specimen or are poorly developed;
    3. order-free - meaning the user can work in any direction with any character;
    4. faster (sometimes); and
    5. easily computerized. In fact, these keys are most commonly used in this form. Paper versions are typically large and unwieldy because each character needs to list all possible taxa.

    Disadvantages of polyclave keys include: (a) few keys are currently available; and (b) requires a computer be handy when you are making your identification (which is no problem in a lab but may be unwieldy to use in the field).

C. Example
    Click here for an example of a "paper version" of a synoptic key to pollination systems follows. To use this key: 

  1. Read through the list of characters to become familiar with the possibilities; 
  2. Scan the list to find a character with a state that you observe in your specimen. Start with a readily identifiable character that has only a few numbers (taxa) associated with it; 
  3. Write a brief description of the character and state and the numbers of the taxa that can be described by this state; 
  4. Choose another character and state that describes your taxa. Write a brief description of this state below the name of the first state chosen. Then, scratch off the original list any taxon that doesn't appear in the second;
  5. Continue this process until just one taxon remains for all of the states. If there is no single taxon described by the states chosen, and two or more remain, go back and check for errors;
  6. Read the name of the taxon after its number in the list of taxa. Check your identification with a description in a manual or the herbarium.

    There are several computer programs  for preparing interactive computerized keys (see Principles of Interactive Keys - MJ Dallwitz et. al. and Programs for Interactive Keying and Information Retrieval compiled by MJ Dallwitz) and these have been reviewed (Comparison of interactive Keys - MJ Dallwitz).  Some of the available programs includes:

Examples of multiple access/polyclave keys include:

IV.  Probability Keys - we will show the use of one example from the MN DNR.

V.  Confirming Your Identification
     Once you have identified your unknown it needs to be confirmed either by: (a) comparing it to a written description; (b) image; (c) actual specimen in the herbarium; or (d) discussion with an expert.

VI. References:

Polyclave Key to Pollination Type

    Pollination is the process of transferring pollen from one flower to another. Since plant can't move, they utilize vectors such as wind, water and animals to accomplish this process for them. Flowers are specialized by shape, color, odor, nectar reward in order to maximize the chance that a certain vector will accomplish pollination. These flower adaptations are collectively known as pollination syndromes or systems.

    Plants differ in the degree of their specialization for a particular pollination system. For example, many orchids are pollinated by only a single type of bee. Other flowers are not as specialized and may be pollinated by a variety of bees or perhaps beetles. In other cases, insects may visit flowers without actually transferring pollen. These factors make it difficult to determine with absolute certainty the pollination system by the polyclave key.

    To illustrate how to use a polyclave key, let's determine the pollination system of a dandelion.


1. Select any one of the FLOWER CHARACTERS in the key. Note: you can choose the characters in any order (i.e., multi  - entry)

  Let's choose FLOWER COLOR

2. Choose the character state (description) that matches the flower you are observing

  Dandelions are Yellow

3. Write down the possible pollination systems for this feature

4. Select another feature   NECTAR

5. Choose the character state description that matches your flower

  b. present...WI,FM,BE,BA

6. Eliminate from the first character state selected the pollination systems not found on both lists. Continue this process until the pollination system is identified. Note: in some cases it will not be possible to narrow the choices to just one.

Thus, dandelions are bee pollinated

Polyclave Key to Pollination Systems.

(Adapted from Tyrell, L. 1989. Journal College Science Teaching, May 378-383)


a. radial ....... WI,BT,FM,FS,BE,BU,MO,BI,BA
b. bilateral .............. WI,BE,(BU),BI,BA


a. tubular ............... FS,BE,BU,MO,BI,BA
b. not tubular .....BI,BT,FM,FS,BE,(BU),(BI)


a. small/inconspicuous ............ WI,FM,FS
b. showy, large, conspicuous, or small in a conspicuous group .......BT,(FM),FS,BE,BU,MO,BI,BA


a. white .......... BT,FM,(BE),BU,MO,(BI),BA
b. yellow ......................... BE,BU,BI
c. blue ................. BT,BE,BU,(MO),(BI)
d. red ......................(BE),BU,(MO),BI
e. dull or dark ..... WI,BT,FM,FS,MO,(BI),BA


a. night only ...WI,BT,FM,FS,(BE),(BU),MO,BA
b. day only ...... WI,BT,FM,FS,BE,BU,(MO),BI
c. day and night..WI,BT,FM,FS,BE,BU,MO,BI,BA


a. no odor ............... WI,BT,FM,BE,BU,BI
b. putrid ................... BT,FM,FS,BE,BU
c. fragrant .............. BT,FM,BE,BU,MO,BA


a. few grains ......... BT,FM,FS,BE,BU,MO,BI
b. abundant ..... WI,BT,FM,FS,BE,BU,MO,BI,BA


a. none ........................ WI,BT,FM,FS
b. present...................... WI,FM,BE,BA


a. present .......................... BE,BU
b. absent ............ WI,BT,FM,FS,MO,BI,BA


a. low sugar............. WI,BT,FM,FS,BU,MO
b. low amino acids ....WI,BT,FM,FS,BE,BI,BA


* May only appear under UV light ** Taste can provide a clue.

 Abbreviation Code::

WI Wind pollination (anemophily); BT Beetle pollination (cantharophily); FM Fly pollination (syrphid and bee flies; myophily); FS Fly pollination (carrion and dung flies; sapromyophily); BE Bee pollination (mellittophily); BU Butterfly pollination ; (psychophily); MO Moth pollination (phalaenophily & sphingophily); BI Bird pollination (ornithophily); BA Bat pollination (chiropterophily)

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Last updated:  03/24/2009 / Copyright by SG Saupe