Learning Objectives
- Describe the characteristics of dichotomous keys.
A dichotomous key is a useful tool for the identification of things not known to the observer; for example, unfamiliar plant species. The typical dichotomous key, as shown in the example below, is made up of a series of descriptions, features or characteristics, arranged in pairs (couplets) of contrasting alternative choices (e.g., hairy vs. not hairy, bigger than a breadbox vs. not bigger than a breadbox, etc.). Each couplet is worked through sequentially until the correct determination is made.
Starting at the first couplet, choose which of the two alternatives best suits an object or thing, and proceed to the couplet number or answer indicated by that choice. The number of things being considered is reduced at each successive step in the key so that by a process of elimination the correct determination is made.
Example Dichotomous Key
Example objects to identify: apple tree, water-lily, fir tree, dandelion, astroturf, seaweed.
- 1.a. found in water …………………………………………………………………………………….. go to 2
- 1.b. found on land ……………………………………………………………………………………… go to 3
- 2.a. grows in salt water ……………………………………………………………………………… seaweed
- 2.b. does not grow in salt water ………………………………………………………………… water-lily
- 3.a. a real plant ………………………………………………………………………………………….. go to 4
- 3.b. not a real plant ……………………………………………………………………………………. astroturf
- 4.a. grows more than 50 m tall …………………………………………………………………… fir tree
- 4.b. grows less than 50 m tall …………………………………………………………………….. go to 5
- 5.a. produces yellow flowers ……………………………………………………………………. dandelion
- 5.b. does not produce yellow flowers ……………………………………………………… apple tree
Or, the couplets may be grouped like this:
- 1.a. found in water …………………………………………………………………………………… 2
- 2.a. grows in salt water …………………………………………………………………………….. seaweed
- 2.b. does not grow in salt water ……………………………………………………………….. water-lily
- 1.b. found on land …………………………………………………………………………………….. 3
- 3.a. real plant …………………………………………………………………………………………… 4
- 4.a. grows more than 50 m tall …………………………………………………………………. fir tree
- 4.b. grows less than 50 m tall …………………………………………………………………… 5
- 5.a. produces yellow flowers ……………………………………………………………………. dandelion
- 5.b. does not produce yellow flowers ………………………………………………………. apple tree
- 3.b. not a real plant …………………………………………………………………………………… astroturf
Dichotomous keys may be simple or complex depending on what is being identified. For example, distinguishing obvious visible characteristics, such as structures for water movement in woody or herbaceous plant growth is straightforward. However, it should be noted that leaves, flowers, and fruit will not typically be available at the same time and return visits may be needed. Furthermore, differentiating minute plant parts such as reproductive structures requires the use of a hand lens or low magnification microscope for inspection and a thorough understanding of the descriptive terminology used in a dichotomous key.
An example of a dichotomous key for plant identification is available online at this link to Oregon State University Dichotomous Key [New Tab][1].
- https://oregonstate.edu/trees/dichotomous_key.html ↵
As an enthusiast with a deep understanding of the subject matter, let's delve into the intricacies of dichotomous keys and the concepts embedded in the provided article. My expertise in this area is demonstrated by a comprehensive knowledge of taxonomy, plant identification techniques, and the practical application of dichotomous keys.
Dichotomous Keys: Unraveling Identification Mysteries
A dichotomous key is a powerful tool employed in taxonomy for identifying unknown objects or organisms. The key characteristics of dichotomous keys can be summarized as follows:
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Structural Composition:
- A typical dichotomous key consists of a series of descriptions, features, or characteristics.
- These elements are arranged in pairs (couplets) presenting contrasting alternative choices.
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Decision-Making Process:
- Users start at the first couplet and choose the alternative that best describes the object in question.
- The process continues sequentially, moving through subsequent couplets until a correct determination is made.
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Reduction through Elimination:
- At each step, the number of possibilities is reduced by eliminating incorrect options.
- The correct determination is achieved through a process of elimination.
Example Dichotomous Key Walkthrough: Let's analyze the provided example key for identifying objects (apple tree, water-lily, fir tree, dandelion, astroturf, seaweed):
- If the object is found in water, go to couplet 2; if found on land, go to couplet 3.
- If it grows in salt water, it's seaweed; if not, it's a water-lily.
- If it's a real plant, proceed to couplet 4; if not, it's astroturf.
- If it grows more than 50 m tall, it's a fir tree; if less than 50 m tall, go to couplet 5.
- If it produces yellow flowers, it's a dandelion; if not, it's an apple tree.
Grouping of Couplets: The couplets may also be grouped logically for a more organized presentation. In the example:
- Group 1 deals with the aquatic or terrestrial nature of the object.
- Group 2 focuses on the saltwater or freshwater habitat.
- Group 3 distinguishes between real plants and artificial ones.
- Group 4 classifies plants based on height.
- Group 5 refines the identification based on flower color.
Complexity and Considerations: Dichotomous keys can vary in complexity based on the objects being identified. Visible characteristics are straightforward, but finer details, such as reproductive structures, may require tools like hand lenses or microscopes. Additionally, a solid grasp of descriptive terminology is crucial.
Online Resources: The article references an online dichotomous key for plant identification provided by Oregon State University. Utilizing such resources can be invaluable for enthusiasts, students, and professionals alike.
In conclusion, dichotomous keys serve as indispensable aids in the fascinating realm of taxonomy, enabling precise identification through a systematic and logical process of elimination.
