How do you construct a dichotomous key from flower photographs?

A dichotomous key identifies a specimen through a series of choices between two contrasting descriptions. To make one from flower photographs, first list the features you can see. Then write pairs of statements so that each specimen fits one choice, not both.

Use characters actually shown in the images. A clear central trumpet, the number of visible floral segments or the arrangement of flowers on a stem can work. Root structure, scent or flowering season cannot be judged from a close-up photograph unless additional information is supplied.

Suppose you have clear photographs of four selected examples: daffodil, sunflower, hibiscus and primrose. The hypothetical key below assumes that those photographs show the usual single-flowered forms and the distinguishing features described. No specimen photographs are supplied here.

Test it in both directions. The daffodil stops at 1a. The sunflower follows 1b then 2a. Hibiscus follows 1b, 2b and 3a; primrose follows 1b, 2b and 3b. Each specimen reaches one endpoint, and every step offers two choices.

The wording matters. A daffodil's corona is its central trumpet-like structure, not simply a group of fused petals. A sunflower's apparent single flower is a head made of many small flowers, called florets. In hibiscus, the staminal column is the projecting column formed by joined stamen filaments. Using these terms accurately helps you describe what you see without misidentifying the structures.

Colour can help when it is clear in the supplied photographs. It is less reliable as the only identifying feature because cultivated varieties, flower age, lighting and camera processing can change how it appears. "Light" versus "dark" also leaves the reader unsure where to draw the line unless you define the difference.

This key identifies the four selected examples; it does not cover all flowering plants or every cultivated variety of these species. If you cannot see a feature clearly, get a better view or revise the pair of choices, called a couplet. A key helps you identify organisms; it is not a diagram of their evolutionary ancestry.

Build your own key using four of the flower photographs in the Biology Unity and Diversity questionbank. Follow every route afterwards to check that each flower reaches one answer and that every choice uses a feature you can actually see.