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Flower Morphology Dichotomous Keys and a Crime to Solve

Flower Morphology

This Forensics for the Biology Laboratory activity, based on our popular “Flowers for Freddy” Forensics Kit, introduces students to flower morphology and gives them an opportunity to identify floral species using a dichotomous key. Correct identification of the floral species will help students determine the location of a recent murder, giving police valuable information as to the identity of the suspect.

Solve the case

Fredrick A. “Fat Freddy” Garbonza, a local crime boss, was found shot dead in a locked service area behind a restaurant. When the sheriff’s deputy arrived, the victim was found on his back, hands placed over his abdomen in a position of repose, like a body in a casket. The lack of blood and the statements of witnesses who heard no gunshots suggest that the crime was committed elsewhere. Several local business owners had keys to the service area because they sold supplies to the restaurant, and several also owed Fat Freddy money.

The suspects

John Chapman owns an orchard and supplies the restaurant with fresh fruits. Thomas Ato grows and supplies fresh vegetables. Carl VonLinne runs a greenhouse/flower shop and supplies centerpieces.

The evidence

A crime scene investigator found a number of flowers under the victim and in his clothing, collected them, and placed them in a liquid preservative. Your forensics lab has been asked to examine and identify the flowers and to determine the likely location of Fat Freddy’s shooting death, in the hopes that this information will implicate one of the suspects.

Flower anatomy

Before students can work with the dichotomous key, they need to know the names of the parts of the flower (see Fig. 1). Basic flower parts include:

  • Petals. The colorful and usually bright part of the flower. Helps attract pollinators.
  • Sepals. The cover outside of a flower bud. Protects flower before it opens.
  • Pistil. The female part of the flower, comprised of stigma, style, and ovary.
  • Stigma. Located in the center of the flower. It receives pollen grains for germination.
  • Style. Long stalk that holds the stigma.
  • Ovary. Contains ovules. Has seeds inside.
  • Stamen. The male part of the flower, comprised of filament and anther.
  • Anther. Part of the stamen that produces pollen.
  • Filament. The fine, hair-like stalk that holds the anther.

Teaching about dichotomous keys

Many students have difficulty understanding how to use a dichotomous key. If your students have not previously used keys, consider beginning with the following exercise:

  1. Explain to students that the purpose of a key is to identify an individual by eliminating groups to which it does not belong and narrowing down the group to which it does belong.
  2. Provide students with the Key to Writing Implements (see Fig. 2).
  3. Hold up a regular (wooden) pencil and take your class step-by-step through the key to identify it. Be sure students understand that at each stage they should consider all of the options before making a choice. Point out that by deciding that the pencil writes with graphite, they have eliminated all the pens and markers.
  4. You may want students to create their own classification keys for one of the following:
    • Shoes
    • Watches
    • Backpacks
  5. Explain to students that they will follow this process of elimination to identify the flowers from the crime scene.

Flower dissection

Flower Dissection Handout
  1. Remove the sepals, count them, and set them aside. If the sepals are fused into a calyx tube, slit the tube and try to remove it in as few pieces as possible. Count the number of lobes at the open end of the tube.
  2. Continue with the petals, treating them as you did the sepals. If the stamens are fused to the petals, remove them together.
  3. Remove and count the stamens. Counting the anthers is sometimes the easiest way, but remember that most anthers are made of 2 distinct halves.
  4. The remaining parts in the center should be the pistil(s). Often there is only 1. The stigma(s) are usually surrounded by the stamens, but the stigma lobes generally look clearly different from the anthers. The ovary can usually be identified by following the style towards the stem. The ovary is swelling at the bottom of the style.
  5. Note whether the ovary or the petals are closer to the stem. This will enable you to determine whether the ovary is superior or inferior. This may require looking at the whole flower you saved for reference.

Flower identification key

This key is designed for use by people with little botanical training. It was developed to help students identify the type of preserved flowers provided in the "Flowers for Freddy" Forensics Kit. Here is an excerpt:

1. The “flower” is actually a tight cluster of many small flowers (a head or spadix) go to 2
1. The flowers are single, or loosely associated together (i.e., umbels or cymes) go to 5
2. The flowers are surrounded by 4 large, showy bracts Cornus
2. The flowers are not surrounded by 4 large, showy bracts 3
3. The flowers form a finger-shaped spadix Arisaema
3. The flowers form a tight disk (a head) go to 4
4. All the flowers look the same, although some may be more mature than others; Taraxacum
4. There are 2 distinct types of flowers, the outer ones with a much larger single petal Chrysanthemum
5. The flowers lack petals (wind pollinated) 6
5. The flowers have petals (animal pollinated) 8
6. The flowers are perfect, containing both stamens and pistils Avena
6. The flowers are not perfect; each contains either stamens or pistils 7
7. The bract of each flower is spiky Carya
7. The bract of each flower is rounded Quercus