Key: A plant Identifying Method

It is estimated that in the present world, the number of plant species is as many as five million. Among them about 391,000 species of vascular plants-are currently known to science of which nearly 369,000 species (94%) are phanerogamic or flowering plants as other 6% is consisted with cryptogamic or non flowering plants (according to Royal Botanical Gardens, UK).

The millions of these plants are different from one to the other. In the same species, again natural variation can be observed. By identifying plants we could easily find out the proper ways of using these plants e.g. medicinal uses, industrial uses, practical uses etc.
So, it is a need for a well-thought-out and systematic approach to make a complete process of identifying plants.

Plant identification is a process of matching specimen(plant) to a known taxon. It uses various methods. The most common methods of identifying plants are as follows:

  • Expert’s determination
  • Recognition
  • Matching with known herbarium specimens or illustration/ photographs/ literature.
  • Using molecular techniques and DNA Bar Coding (the process of identifying plant species through the amplification and sequencing of specific and conserved regions of plant DNA).
  • Using keys.

In this article, the basic study and procedure of plant identification using keys are discussed.

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Keys are devices consisting of series of contrasting statements requiring the identifier to make comparison and decision based on statements of the key as related to the material to be identified.

A key is device which when properly constructed and used enables a user to identify an organism.

There are two types of taxonomic Keys:

  1. Dichotomous keys
  2. Polyclave or multiaccess or synoptic key

Dichotomous keys

keys in which the choices allow only two (mutually exclusive) alternative couplets 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 i.e. the statements typically begin with broad characteristics and become narrower as more choices are required.

Fig: Dichotomous key; Source: cobblearning.net

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 ranger reduces to a single species and the identity of the unknown plant is revealed Dichotomous comes from the Greek root dich’ meaning “two” and ‘temnein’ meaning “to cut”.
Couplets can be organized in several forms. The couplets can be presented using numbers (numeric) or using letters
(alphabetical). The couplets can be presented together or grouped by relationships. There is no apparent uniformity in presentation of dichotomous keys.

Types of Dichotomous keys

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 user goes to the next indented couplet following the lead that was selected.

Example of an indented key of Rhododendron

1a. Flowers in shades of red

    2a. Flowers blood-red, leaves oblong  ovate, lethery and thick matty texture……….. R sikkimense                               2b. Flowers crimson-ted, leaves broad, oval to elliptic oblong.shiny green above…………. R fulgens

1b. Flowers in shades of rose pink

3a. Calyx 3-5 mm long. leaf under surface covered with tufts of brown hair…………….A wallichii
3b. Calyx obscure, 1-2 mm long, leaf under surface covered with continuous indumentum……..go to 4

4a. Corolla in shades of deep rose-pirk flushed externally with red-purple, young leaves aeruginose, leaf margins inrolled………….R. aeruginosum
4b. Corolla pale lavender blue, mauve or rose-purple, rarely white, young leaves not aeruginose, leaf margins not inrolled…………..A. campanulatum

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. This key has exactly the same choices as the first example. The choices are separated. But it is easy to see the relationships. While this key might be more difficult to construct, it gives more information to the user.

Example of a bracketed key of Rhododendron

1a. Flowers in shades of red………………go to 2
1b. Flowers in shades of rose-pink………go to 3

  2a. Flowers blood-red leaves oblong-ovate, leathery and thick matty texture……..R.sikkimense                                 2b. Flowers crimson red, leaves broad, oval to elliptic oblong. shiny green above……….R. fulgens

3a. Calyx 3-5 mm long, teal under surface covered with tuits of brown hair…………R. wallichii
3b. Calyx obscure, 1-2 mm long, leal under surface covered with continuous indumentum……..go to 4

4a. Corolla in shades of deep rose-pink flushed externally with red-purple, young leaves aeruginose, leaf margins inrolled…………R. aeruginosum
4b. Corolla pale lavender blue, mauve or rose-purple, rarely white, young leaves not aeruginose, leaf margins not inrolled………….R. campanulatum


Problems of using Dichotomous Keys

A key may be difficult to use at times because:

I. The key may not include all potential variations in the species;

II. The key may rely on features not present in that season;

III. The key may not include “all” species of interest;

IV. One may misinterpret a feature or make a mistake.

Poly Clave Keys

Another type of key, which is relatively a new alternative to dichotomous keys and becoming increasingly popular especially because of the case of computerizing them is termed multiple access or poly clave or synoptic key. The
advantage of these keys is that they allow the user to enter the key at any point.
This key is based on the identification of organisms by a process of elimination. In a written poly clave key there is a series of characters and character states. Each state is followed by a number or code for the species that possess that feature.
The user needs to select any character and then copy down the list of species that possess the feature. Then the user has to select another character and eliminate any species that is not common to both lists. This process has to be continued until the specimen is identified.

Identification is a basic activity and one of the primary objectives of systematics. Although identification is a separate activity or process, in practice it involves both classification and nomenclature. Identification is simply the determination of the similarities or differences between two elements, i.e., two elements are the same or they are different.


This Article is completely based on by the lecture of Dr. Mohammed Zasim Uddin, Professor, Department of Botany, University of Dhaka.

Author has added Some info and pictures.

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About Syeda Nusrat Jahan Mili

A girl with a lot of dreams to accomplish and make her parents proud. Currently studying in 1st year of B.Sc in University of Dhaka. Want to walk with Plantlet a long way. Interested with debate, music, photography and poem recitation. Email: syedanusratbd@gmail.com Minimum Monthly resolution: Publish (1), Revise(1), Share (1)

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