Introduction to Classification of Organisms
Senior 1 · Biology
Introduction to Classification of Organisms
Lesson 5 of 5
25 min
Your class collects one leaf from each of six plants growing around the school: maize, banana, bean, cassava, eucalyptus and coffee. The task: make a key that a Primary 6 pupil could use to name any of the six leaves, without your help. Using a key is one skill; building one is harder, because you must decide which features to use and in what order.
Figure 1: The six leaf specimens (not drawn to the same scale: the banana leaf is really more than a metre long).
Definition
Feature (character): an observable property of a specimen, such as leaf venation or number of legs, that can be used to tell organisms apart.
Specimen: one organism, or a part of one, collected for study.
Look carefully at every specimen (use a hand lens for the veins) and record the same features for each in a table.
| Leaf | Veins | Leaflets | Lobes | Shape of blade |
|---|---|---|---|---|
| A: maize | parallel | one blade | none | long and narrow, under 10 cm wide |
| B: banana | parallel | one blade | none | very broad, over 30 cm wide |
| C: bean | net | three leaflets | none | each leaflet oval and pointed |
| D: cassava | net | one blade | deep, finger-like | divided like a hand |
| E: eucalyptus | net | one blade | none | narrow and curved like a sickle; smells when crushed |
| F: coffee | net | one blade | none | oval, glossy, with a wavy edge |
Worked example 1: building the leaf key
Figure 2: The finished leaf key as a branching diagram.
Warning
Rules for good couplets
Worked example 2: a key for four small animals
Construct a key for an earthworm, a garden snail, a spider and a housefly.
Check: four animals, three couplets. Every statement is observable with a hand lens, and each couplet contrasts one feature.
In 2017 a new pest, the fall armyworm, reached Rwanda's maize fields. Many caterpillars feed on maize, and most do far less damage, so extension officers taught farmers a short key. A fall armyworm caterpillar has a pale upside-down "Y" on its head and four large dark spots arranged in a square near the end of its body. A farmer who checks those two features can decide whether to act before the crop is lost.
How to make a Dichotomous Key (MooMooMath and Science). Watch how the list of features is turned, one split at a time, into numbered couplets.
Tip
Memory hook: build keys with OSTC: Observe and tabulate, Split into two, Two statements about one feature, Check by testing.
Practice: constructing keys
Key points
Further reading for this unit
Sources and credits
Written by Ganzaa: the lesson text, the leaf drawings (Figure 1), the feature table, the keys (Figure 2), the worked examples and the practice questions.
