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Science 7–10 · Year 7

Constructing a dichotomous key for ten school-ground plants

Science understanding: Biological sciences

PracticalLow risk

The idea

A useful key is built from observable, unambiguous structural characters, and separating n groups always takes n minus 1 couplets.

What you need

  • Ten labelled specimens (a leafy twig each) from ten different plant species on the school grounds, collected by the teacher with permission
  • x10 hand lenses, 30 cm rulers, index cards and string or a large sheet for drawing the tree
  • Access to the Atlas of Living Australia species search to confirm the ten names
  • A second set of the same ten specimens, unlabelled, for testing

How to do it

  1. Examine each specimen and record structural characters that do not change with age or season: leaf arrangement (opposite or alternate), leaf margin (entire or toothed), simple or compound leaves, venation pattern, presence of hairs, and presence of oil dots against the light.
  2. Reject colour and size as characters; both vary within a species.
  3. Choose one character that divides the ten specimens into two groups, write it as couplet 1 with an a and b lead, and repeat within each group until every branch ends at one species.
  4. Count the couplets. A correct key for ten species has nine.
  5. Swap keys with another group. Using the unlabelled second set, each group identifies all ten specimens with the other group's key and records the number correct.
  6. For every wrong identification, find the couplet that failed and rewrite it with a clearer character.
  7. Confirm the species names against Atlas of Living Australia records for your local area.

What you should see

A finished key has exactly nine couplets for ten species, because each couplet splits one group into two. When a second group uses it on the unlabelled set, every wrong identification can be traced to the couplet where it went wrong; couplets written on colour or size are the likely failure points, because those characters vary within a species. The learner knows the key works when the second group names every specimen correctly after the failed couplets have been rewritten.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • A key can be written on any difference, including colour or size.
  • Classification is fixed and cannot be revised when a couplet fails.
  • Two plants with similar-looking leaves must be closely related.

Safety card

Low riskLearners carry it out

Hazards

  • Irritant sap from some garden plants (for example Euphorbia species) and pollen allergies

Controls

  • The teacher selects the ten species and excludes plants with irritant sap
  • Wash hands after handling specimens

Note

No hazardous chemicals; record a RiskAssess risk assessment for the activity as school procedure requires.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-4/fa3acda8f7
  2. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-4/faa7a5c228
  3. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
  4. australian.museum/learn/teachers/learning/invertebrate-dichotomous-key
  5. australian.museum/learn/teachers/learning/animal-adaptations
  6. bie.ala.org.au
  7. www.riskassess.com.au

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