Construction and Use of Dichotomous Keys
Typically, the first step of a dichotomous key looks something like this: 1a. Learning Intention: Students will understand how a dichotomous key is used to classify dichotomous key generator identify organisms in the living world. Taxonomists-scientists who classify things into groups-create and use dichotomous keys to help others identify. Key or a dichotomous key by consulting different sources and finding the characteristics of the above species. The best source that we have is the Bergey's Manual. Thus, we go to the index or table of contents of this manual, and finding the genera names, determine if they are cocci, rods or spirilla and also find their reaction to the Gram stain.
• Biological keys are sets of statements that act as clues leading to the identification of an organism.
• By following the keys we can be able to place an organism in its group.
• The most common key is the dichotomous key.
• This is a biological tool for identification of unknown organisms.
• The word dichotomous means branching into two.
• A single characteristic is considered at a time.
• Two contrasting statements are put forward to describe the characteristics in such a way as to separate the organisms.
• This continues until all the organisms have been identified.
Rules Used to Construct a Dichotomous Key
• Use morphological characteristics as far as possible e.g. type of leaf - simple or compound.
• Select a single characteristic at a time and identify it by number. 1. Type of leaf. .
• Use identical forms of words for two contrasting statements e.g.:
a) Flowers scented.
b) Flowers not scented.
• Start with a major characteristic that divide the organisms into two large groups then proceed to lesser variations that would separate the organisms further into smaller groups.
• Use positive statements especially the first one.
• Avoid generalizations e.g. short plants. Be specific in your description e.g.:
a) Plants above 1m tall.
b) Plants below 1m tall.
Some Common Features Used for Identification
In Plants
Leaves
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1. Type of leaf Leaf
Compound leaves.
• Simple leaf
• Trifoliate
• Pinnate
• Type ofleaf margin.
Type of venation.
• Type ofleaf arrangement on stem.
• The colour of leaf.
• The texture ofleaf; whether hairy or smooth.
• Shape of the leaf e.g. palmate.
Stem
• Type of stem - woody or herbaceous.
• Shape of stem - cylindrical or rectangular.
• Texture of stem smooth or spiny.
Infloresence
• Are flowers terminal or lateral
• For each flower:
• Is the flower regular or irregular?
• Number of floral parts for each whorl.
• Are floral parts free or fused?
Roots
• Type of root system- Taproot or fibrous?
• Function of the root.
In Animals
Features used to identify animals:
• Type of mouthparts.
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• Type of skeleton.
• Presence or absence of antennae.
• Body segmentation.
• Body covering: scales, fur, hair or feathers.
• Number of body parts.
• Locomotory structures: legs, wings and fins.
• Presence or absence of vertebral column.
• Presence and type of eves.
Dichotomous key
A dichotomous key is guide, sort of like a map of characteristics, to lead the user to the correct identification of the plant. The word dichotomous comes from two Greek words that mean divide in two parts. To make a dichotomous key you will choose physical characteristics that can be used to divide a collection into two parts. Possible physical characteristics to use include plant size, plant shape, leaf shape, stem type, color, and presence of seeds, fruits or flowers. You will continue to divide each group in two until all of the groups have only one member.
How to do it
Step 1: Begin by gathering your plants (or pictures of plants) to be identified. For example: a corn plant, hosta, oak tree and dandelion.
Step 2: Decide how to split your group in two parts. Carefully observe your specimens to pick a characteristic which can be used to decide which plant goes into which group. Record the chosen characteristic and the groups formed. For example, this group of corn, hosta, oak tree and dandelion could be divided by leaf color:
A. The leaves are solid green. See #2.
B. The leaves have a white margin. Your plant is a hosta.
Step 3: After the collection has been divided into two groups, divide the first group (group A) into two more groups based on one characteristic. Record your characteristic and groups. When you form a group with only one plant, name it. The next division in this example is leaf edge:
A. The leaves have a straight edge. Your plant is corn.
B. The leaves have a curvy edge. See #3.
Step 4: Continue to identify characteristics and divide groups until all of the plants are named. Be sure to record each division made. When the first group is complete, repeat the steps until all of the plants in the second group (group B) have been named. Your completed key should look something like this:
1)
A. The leaves are solid green. See #2.
B. The leaves have a white margin. Your plant is a hosta.
2)
A. The leaves have a straight edge. Your plant is corn.
B. The leaves have a curvy edge. See #3.
3)
A. The plant has a woody stem. Your plant is an oak tree.
B. The plant has soft stems. Your plant is a dandelion.
The sample key is fairly short and simple. The more items your beginning collection has, the longer your key will be. Have a friend try out your completed key to see that it leads users to the correct choices and identification.
A dichotomous key is a useful classification tool that notes the similarities and differences between different plant species. By observing similarities and differences in plants, scientists can classify plants into families and gain further understanding of their characteristics. Scientists use dichotomous keys to organize and classify large amounts of information so it can be easily shared with other scientists and studied.Dichotomous keys can be developed to identify anything.
Source: www.brighthubeducation.com
GoBotany.NewEnglandWild.org. Plant Dichotomous Key
Photo: commons.wikimedia.org by CNX OpenStax