A key to creating plant identification keys

If you have ever used a field guide or any kind of identification tool for plants (or animals), you might have come across a “key” to the different species. A key is a step by step process to figuring out which species you have found, based on different characters. The term “key” is actually quite appropriate, since you are basically unlocking the correct identification of the unknown specimen.

Dichotomous keys give you two options at a time, and your plant will either agree with the first option or the second option. There are also smart keys which are more user friendly, but these require some sort of device (cellphone, tablet or computer) and will be looked at at another time.

Let’s work on a very simple dichotomous key together using the five plants below.

To make a key for these plants, you will need to start grouping them according to shared characters. You can see that plant 1 and plant 2 both have thorny stems, while the rest of the plants rather have long soft hairs on their stems. Plant 1, plant 3 and plant 5 have arrow shaped leaves while the rest of the plants have heart shaped leaves. Plant 1 and plant 5 have yellow flowers, while the rest of the plants have red flowers.

It is usually a good idea to make your first groupings based on characters that will always be available, such as stem and leaf characters (although leaves might fall off in certain seasons). If you start your key with a character such as flower colour, then your user might get stuck if the specimen has no flowers. So let’s start our key by grouping the plants according to leaf characters. (Take note of the two steps and how they are numbered the same, with letters indicating option a or option b for each step. As the key progresses, the numbers to each two options increases.):

1a. The leaves are shaped like a heart (also known as cordate leaves)…plant 2 and plant 4 (Take note, when a step leads to more than one plant, a colon will be used to indicate that more steps follow. You will see this format being used below.)

1b. The leaves are shaped like an arrow (also known as hastate leaves)…plant 1, plant 3 and plant 5

Now you can look at the first group of plants that both have heart shaped leaves. You can see that both of these plants have red flowers so you will need to use the surface of their stems to key them apart:

1a. The leaves are shaped like a heart (also known as cordate leaves):

2a. The stems are thorny…Plant 2

2b. The stems are not thorny and rather have long soft hairs…Plant 4

Now that you are able to key out to plant 2 and plant 4, let’s look at the next group of plants with arrow shaped leaves. You can see that plant 1 and plant 5 both have yellow flowers, while plant 3 has a red flower. You can also see that plant 1 has thorny stems while both plant 3 and plant 5 have long soft hairs on their stems. You can make two groupings based on either of these two characters, but it is better to use the stem characters first:

1b. The leaves are shaped like an arrow (also known as hastate leaves):

3a. The stems are thorny…Plant 1

3b. The stems are not thorny and rather have long soft hairs…Plant 3 and plant 5

Now you can use the flower colour to separate plant 3 from plant 5:

1b. The leaves are shaped like an arrow:

3a. The stems are thorny…Plant 1

3b. The stems are not thorny and rather have long soft hairs:

4a. The flowers are red…Plant 3

4b. The flowers are yellow…Plant 5

This key could have looked very different if you decided to start off with using stem surface instead of leaf shape and there are many other characters that can be used to make different groupings. The more species that you have, the more complicated your groupings might become.

It is also important to note that one of the above plants might have soft hairs in one area while it is thorny in other areas. This variability must be accommodated in your key to make sure that the species can be keyed out in both instances.

There are also many groups of plants that can only be distinguished from each other by using much smaller characters such as those characters related to flowers and fruits. Now that you know the basic structure of a key, you can tackle those tiny little flower characters one grouping at a time!

Acknowledgment: Thank you to Marinda Koekemoer for her valuable review of this article.