The Dichotomous Key Page 6

Dichotomous means “having two equal parts”. A key is a tool used to identify organisms. A dichotomous key provides a series of choices between two related characteristics. Each pair of choices is known as a couplet. Making the correct choice each time will ultimately lead to the name of the organism.

Two simple dichotomous keys:

Drawing contributed by Fan Jian Hong Canberra

What is the lady’s name?

1a. Person is a boy…………………..go to 2
1b. Person is a girl……………………go to 7

2a. Hair is black………..  ………..go to 3                           2b. Hair is not black              go to 4

3a. Boy is tall………..  ………..   Greg                            3b. Boy is short                   Steven                                                                 

4a. Hair is blond                 go to 5
4b. Hair is red………….. ………..go to 6

5a. Boy is tall………..  ………..   Paul                             5b. Boy is short                   David

6a. Boy l is tall………..  ………..   Tim                             6b. Boy is short                   John

7a. Hair is black………  ………….go to 8                           7b. Hair is not black              go to 9

8a. Girl is tall………..  ………..   Gail                             8b. Girl is short                   Julie

9a. Hair is blond                 go to 10
9b. Hair is red………….. ………..go to 11

10a. Girl is tall………..  ………..   Kylie                             10b. Girl is short                   Jennifer

11a. Girl is tall………..  ………..   Lily                             11b. Girl is short………..  ……….. Julia

The steps were as follows. The person was a girl so you go to step “7.” The person’s hair was not black so you go to step “9.” Her hair is red so you need to go to 11. She is short so her name must be “Julia.”

We worked this out from using elimination and following the correct path. Working through plant characteristics is exactly the same the same with more choices.

What tree is this?

1a. leaves broad…………………..go to 2
1b. leaves needle-like……………go to 3

2a. margin smooth, hemispherical woody capsules            Eucalyptus
2b. margin undulating, hemispherical non woody capsules    Waterhousia

3a. cone woody with 6 scales                                  Callitris    3b. cone soft or woody with protrusions                        Casuarina

There are two basic ways to set up a dichotomous key. Either of these forms is acceptable.

When using a key, keep the following in mind:

* You have 3 choices “go to a”, “go to b” or “return to the previous key if “A” or ”B” does not match the key exactly”.

* Always read both choices, even if the first seems to be the logical one at first.

* Be sure you understand the meaning of the terms involved. Do Not Guess – use a glossary or botanical dictionary for clarification of anything you do not understand.

* When measurements are given, use a calibrated scale. Do Not Guess.

* Living things are always variable. Beware of your conclusion if it is being taken from single observation or plant. Study several specimens to be sure that your specimen is typical.

* If the choice is not clear, for whatever reason, try both divisions. If you end up with two possible answers, read descriptions of the two choices to help you decide.

* Having arrived at an answer in a key, do not accept this as absolutely reliable. Check a description of the organism to see if it agrees with the unknown specimen. If not, an error has been made somewhere in the use of the key. The ultimate check of identifications is a comparison of the unknown with an authentically named Type Specimen.

* Use constant characteristics rather than variable ones. See glossary pages 4 to 23 for diagrams on the morphology of plants. These diagrams will assist in correct, precise language.

Identification takes great patience, time and methodology as outlined. The most important thing though is practice and expect to get many wrong before you become familiar with the language terms of reference. There are no short cuts.

For plants in New South Wales copy and paste the web site below for a full key to all of the specie in that state 

A sporangium (plural sporangia) (Is from the Greek σπόρος meaning sporos and αγγείον meaning angeion or a vessel as in a type of urn) is an enclosure in which spores are formed. It can be composed of a single cellular spore or can be multicellular.

A Synangia is a cluster of sporangia that are fused in development. This structure is most prominent in Psilotum and Marattiaceae, such as Christenisena, Danaea and Marattia families.

Standard key for Dicotyledons is as follows: You would need to make yourself familiar with the 7 classes. Once the class has been found click on the class and it will lead you through one step at a time, just the same as we found Julia.

            KEY                                         CLASS

1a  Plants that do not reproduce by seeds.                   go to 2

1b  Plants that reproduce by seeds.                          go to 5

2a  Stems prominently jointed: leaves whorled,      

forming a sheath at stem nodes.                     SPHENOPSIDA

2b  Stems not jointed; leaves absent or if present,

whorled and forming a sheath at the nodes.              go to 3

3a  Sporangia borne otherwise on leaves or

in nut-like sporocarps.                               FILICOPSIDA

3b  Sporangia borne on stems, in the axils of

leaves or leaf like organs, or embedded in the

expanded bases of such organs.                          go to 4

4a  Sporangia fused to form synangia.                     PSILOPSIDA

4b  Sporangia not fused into synangia.                     LYCOPSIDA

5a  Ovules enclosed in a carpel;

carpels arranged in flowers (flowering plants).        MAGNOLIOPSIDA 

5b Ovules not enclosed in a carpel; ovules

arranged in cones or solitary on a short

fleshy axis (non-flowering seed plants).                    go to 6

6a  Leaves pinnate.                                      CYCADOPSIDA

6b  Leaves simple.                                       CONIFEROPSIDA

Now let’s follow the above path with our specimen. It fits into bracket 4 where the Sporangia are fused to form a synangia. Psilopsida

We left click Psilopsida and check any couplets that appear.

If the description still fits we left click on the family that our speciman fits into. If for one reason it does not fit return to the previous page.

We now see that the Psilotaceae family has 2 genres in Australia.

Again check the key and select the correct key. If however the speciman does not fit the couplet return to the previous page.

There are 2 genre to select from. Read the couplets and select the correct one. If however the specimen does not fit a couplet return to the previous page.

1. Stems much-branched, branching dichotomous;

synangia usually composed of 3 sporangia;

leaves scale-like.                                    Psilotum

2. Stems usually unbranched; synangia composed

of 2 sporangia; leaves usually at least 10 mm long.   Tmesipteris

We will see that the Psilotum Genus has 2 specie in Australia and our specimen is dichotomously branched. Again check the couplet and select the correct one. If however the specimen does not fit the key return to the previous page.

1. Aerial stems not flattened, ± circular to triangular

  in cross section, erect to somewhat pendent          nudum

2. Aerial stems flattened, ± elliptic in section, pendent  complanatum

Lastly check distribution maps for the species being checked. Check topography maps and habitats of the species to ensure it is found there or in similar areas outside the distribution maps. A good example is of plants that are found in the southern highlands of New South Wales can be found on the ranges at higher altitude levels  around Ebor to Guyra in the north of the state due to climate and habitat.

Ah we did it. Its name is Psilotum nudum it is in the Psilotaceae family which is a member of the Psilopsida class. What looks difficult and exhausting is really a matter of taking your time and completing one step at a time. If it does not fit into one of these categories or it is classified as rare and endangered and you are sure the plants name is correct check it out with a botanist at a Botanic gardens or from the AVH for positive identification and possible inclusion in the Herbarium’s listings. Herbariums like to know the exact location where the sample was found, the soil type, aspect and any other information out of the ordinary. See Science of Botany Chapter 7.

Problems will arise where similar specie overlap and hybridise.

A similar key exists for monocotyledon plants and ferns.

We have certainly come a long way from identifying the girl with the red hair as Julia. Good luck and happy identifying, oh and remember practice makes perfect.


  1. Dichotomous means to have into 2 equal parts while monocotyledon means to have a single seed leaf.
    2. A biology key is a tool used to identify organisms
    3. When constructing a key, always use constant known characteristics
    4. Each pair of choices on a key is known as a couplet
    5. When constructing a key, always make the choice positive, something is rather than is not.

6. When constructing a key, if possible start both choices of a couplet with the same word.

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