Curriculum 02

Plant Taxonomy and Field Identification

The universal language of plants — from scientific names to field identification

Section I — Foundation

Chapter 5: Using a Dichotomous Key

Why this matters: A dichotomous key is the standard tool of professional botanists for making definitive plant identifications. It asks a series of either/or questions that lead you step-by-step to a correct identification. Once you can use a key, you can identify any plant in a regional flora — even one you've never seen before.

5.1 What is a Dichotomous Key?

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DICHOTOMOUS = divided into two parts

From: 5.1 What is a Dichotomous Key?

Image file: ../../../images/s01-foundation/c02-plant-taxonomy/ch05/c02-plant-taxonomy_ch05_dichotomous_keys_fig01.png

Save image as ../../../images/s01-foundation/c02-plant-taxonomy/ch05/c02-plant-taxonomy_ch05_dichotomous_keys_fig01.png in this folder, then replace this block with:
<figure><img src="../../../images/s01-foundation/c02-plant-taxonomy/ch05/c02-plant-taxonomy_ch05_dichotomous_keys_fig01.png" alt="DICHOTOMOUS = divided into two parts"></figure>

5.2 Types of Keys

BRACKETED (INDENTED) KEY:
  Each couplet is indented, easier to follow visually
  
  1a. Petals 4 .......................................... 2
  1b. Petals 5 or more ................................... 6
    2a. Stamens 6 (4 long + 2 short) ....BRASSICACEAE
    2b. Stamens not 6 ................................... 3
      3a. Ovary inferior ................................. 4
      3b. Ovary superior ................................. 5

NUMBERED KEY:
  More common in technical floras
  Numbers in text rather than indentation
  
  1. Petals 4 ............. 2
  1. Petals 5 or more ...... 6
  2. Stamens 6 ............. BRASSICACEAE
  2. Stamens not 6 ......... 3
  3. Ovary inferior ......... 4
  3. Ovary superior ......... 5

DICHOTOMOUS PICTORIAL KEY:
  Uses drawings/photos for each choice
  Easier for beginners
  Less precise for edge cases
  Good for common species in a limited area
  Examples: most wildflower pocket guides

5.3 How to Use a Key Successfully

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BEFORE YOU START

From: 5.3 How to Use a Key Successfully

Image file: ../../../images/s01-foundation/c02-plant-taxonomy/ch05/c02-plant-taxonomy_ch05_dichotomous_keys_fig02.png

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<figure><img src="../../../images/s01-foundation/c02-plant-taxonomy/ch05/c02-plant-taxonomy_ch05_dichotomous_keys_fig02.png" alt="BEFORE YOU START"></figure>

5.4 A Working Example — Step by Step

Let's identify a common Texas wildflower using a simplified key.

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SPECIMEN in hand

From: 5.4 A Working Example — Step by Step

Image file: ../../../images/s01-foundation/c02-plant-taxonomy/ch05/c02-plant-taxonomy_ch05_dichotomous_keys_fig03.png

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<figure><img src="../../../images/s01-foundation/c02-plant-taxonomy/ch05/c02-plant-taxonomy_ch05_dichotomous_keys_fig03.png" alt="SPECIMEN in hand"></figure>

5.5 Common Mistakes and How to Avoid Them

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MISTAKE 1: Reading only the first choice of a couplet

From: 5.5 Common Mistakes and How to Avoid Them

Image file: ../../../images/s01-foundation/c02-plant-taxonomy/ch05/c02-plant-taxonomy_ch05_dichotomous_keys_fig04.png

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<figure><img src="../../../images/s01-foundation/c02-plant-taxonomy/ch05/c02-plant-taxonomy_ch05_dichotomous_keys_fig04.png" alt="MISTAKE 1: Reading only the first choice of a couplet"></figure>

5.6 Botanical Terminology in Keys

These terms appear constantly in dichotomous keys. Know them before opening a flora.

LEAF TERMS:
  Glabrous:          smooth, no hairs
  Pubescent:         hairy (general term)
  Villous:           long soft hairs
  Tomentose:         densely matted woolly hairs
  Hirsute:           stiff, rough hairs
  Glandular:         with gland-tipped hairs (sticky)
  Scabrous:          rough to the touch
  Ciliate:           marginal fringe of hairs
  Glaucous:          covered in waxy bloom, bluish
  Punctate:          with dots/glands (visible with lens)
  Decurrent:         leaf base running down the stem
  Amplexicaul:       leaf base clasping the stem
  Petiolate:         with a petiole (leaf stalk)
  Sessile:           no petiole (leaf attached directly)
  Stipulate:         with stipules

FLOWER TERMS:
  Actinomorphic:     radially symmetrical
  Zygomorphic:       bilaterally symmetrical
  Gamopetalous:      petals fused
  Polypetalous:      petals free
  Hypogynous:        superior ovary (petals below ovary)
  Epigynous:         inferior ovary (petals above ovary)
  Perigynous:        petals around ovary (intermediate)
  Staminate:         male flower (stamens only)
  Pistillate:        female flower (pistil only)
  Perfect:           has both stamens and pistil
  Pedicel:           stalk of an individual flower
  Peduncle:          stalk of an inflorescence
  Bract:             modified leaf below flower
  Involucre:         whorl of bracts below flower cluster
  Receptacle:        enlarged end of stem bearing floral parts
  Calyx:             all sepals together
  Corolla:           all petals together
  Perianth:          calyx + corolla together
  Tepal:             when petals and sepals look the same

FRUIT/SEED TERMS:
  Dehiscent:         opens at maturity
  Indehiscent:       does not open
  Locule:            chamber inside ovary/fruit
  Endosperm:         food tissue in seed
  Hilum:             scar on seed where attached to pod
  Raphe:             ridge on seed from hilum to chalaza

SIZE ABBREVIATIONS in keys:
  mm = millimeter (1/10 of a cm)
  cm = centimeter
  dm = decimeter (10 cm)
  m  = meter
  
  Small wildflower = typically measured in mm
  Leaf: "3-7 cm long" (measure precisely)

5.7 Practical Key Exercises — Texas Plants

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EXERCISE 1: Prickly Pear (Opuntia)

From: 5.7 Practical Key Exercises — Texas Plants

Image file: ../../../images/s01-foundation/c02-plant-taxonomy/ch05/c02-plant-taxonomy_ch05_dichotomous_keys_fig05.png

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<figure><img src="../../../images/s01-foundation/c02-plant-taxonomy/ch05/c02-plant-taxonomy_ch05_dichotomous_keys_fig05.png" alt="EXERCISE 1: Prickly Pear (Opuntia)"></figure>

5.8 Texas-Specific Key Resources

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BOOKS

From: 5.8 Texas-Specific Key Resources

Image file: ../../../images/s01-foundation/c02-plant-taxonomy/ch05/c02-plant-taxonomy_ch05_dichotomous_keys_fig06.png

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<figure><img src="../../../images/s01-foundation/c02-plant-taxonomy/ch05/c02-plant-taxonomy_ch05_dichotomous_keys_fig06.png" alt="BOOKS"></figure>

Chapter Summary

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DICHOTOMOUS KEY: series of either/or choices

From: Chapter Summary

Image file: ../../../images/s01-foundation/c02-plant-taxonomy/ch05/c02-plant-taxonomy_ch05_dichotomous_keys_fig07.png

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<figure><img src="../../../images/s01-foundation/c02-plant-taxonomy/ch05/c02-plant-taxonomy_ch05_dichotomous_keys_fig07.png" alt="DICHOTOMOUS KEY: series of either/or choices"></figure>

📝 Interactive Quiz
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Part A — True or False

1. A dichotomous key always presents exactly two choices at each step.

2. Dichotomous keys require the user to make observations about the specimen.

3. You should always start at step 1a when using a dichotomous key.

4. If you cannot observe a flower, you cannot use any dichotomous key.

5. Brackets in a key indicate which couplets go together.

6. A bracketed key shows both choices of a couplet adjacent to each other.

7. An indented key shows one choice immediately and the other choice later.

8. Keys work equally well for all life stages of a plant.

9. You should measure, smell, and carefully examine specimens before using a key.

10. A lead is one of the two choices in a couplet.

11. Keys can lead you to a wrong identification if specimens are atypical.

12. You should always verify your key result with a photograph or description.

13. Vegetative keys can identify plants in winter without flowers or fruit.

14. Electronic keys (apps like iNaturalist) replace the need to understand traditional keys.

15. A polyclave allows choosing multiple characters simultaneously.

16. Reading an entire couplet before choosing is important to avoid errors.

17. Keys work only for identifying species, not genera or families.

18. If you take a wrong path in a key, you must start over from the beginning.

19. Using a fresh specimen gives better results than a dried pressed one.

20. Practice with known specimens before attempting unknowns.

Part B — Short Answer

1. What is a dichotomous key and how does it work?

2. What is a couplet in a botanical key?

3. What is the difference between a bracketed key and an indented key?

4. What observations should you make before beginning to key a plant?

5. What should you do if you reach an answer in a key that does not match your specimen?

6. What is a polyclave and how does it differ from a dichotomous key?

7. Why is it important to read both choices in a couplet before deciding?

8. Name three resources for dichotomous keys specific to Texas plants.

9. What are the limitations of using smartphone plant ID apps?

10. How do you verify a key identification?

Part C — Fill in the Blank

1. A key that presents both choices of each choice pair next to each other is a key.

2. A key that indents the second lead of a pair further down the key is an key.

3. Each pair of choices in a dichotomous key is called a .

4. Each individual option within a couplet is called a .

5. A key that allows multiple characters to be entered at once is a .

6. The Texas comprehensive botanical reference used with dichotomous keys is .

7. Observation of , fruit, leaves, stems, and bark are typical first steps before keying.

8. If you suspect a wrong key result, you should your answer against a photograph or description.

9. Keys designed for non-flowering specimens are called keys.

Part D — Practical Exercises

Complete in the field. Check each off as you go.

1. Practice Keying: Print a simple dichotomous key for Texas trees (Texas A&M Forestry). Collect 10 tree specimens with leaves or twigs. Key each to species.

2. Build a Mini-Key: Choose 10 plants from your property. Create a simple dichotomous key that distinguishes them based on 4-5 characters. Have someone else test it.

3. Backtracking Exercise: Deliberately take a wrong path in a published key. When the result is clearly wrong, backtrack to find where you diverged. What character did you misread?

4. App Comparison: Photograph the same 5 plants with iNaturalist, PictureThis, and Seek. Compare results. Then manually key the same plants and compare accuracy.

5. Known Specimen Test: Select 5 plants you can positively identify. Use a dichotomous key to identify them without using your knowledge. How accurate is the key? Where does it fail?

0 / 39 answered correctly

Practice Exercises

  1. What is the first step you should take before beginning to use a dichotomous key on an unknown plant?
  2. In a key couplet, choice 1a says "leaves opposite" and choice 1b says "leaves alternate." Your specimen has opposite leaves. Why should you still fully read choice 1b before moving on?
  3. You key a plant to "Gaillardia pulchella" but when you look up the description it mentions reddish-orange ray tips, and your flowers are pure yellow. What should you do?
  4. What tool is considered essential for using a technical botanical key, and what features require it?
  5. You are trying to count stamens in a small tubular flower. The anthers seem to be inside the tube. How do you count them accurately?
  6. What is the difference between a "pedicel" and a "peduncle"?

Next Chapter → Reading Botanical Descriptions

Connections to Other Topics

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→ C02 Ch03: Vegetative Identification — characters used in keys

From: Connections to Other Topics

Image file: ../../../images/s01-foundation/c02-plant-taxonomy/ch05/c02-plant-taxonomy_ch05_dichotomous_keys_fig08.png

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