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DICHOTOMOUS KEYS

Introduction • A dichotomous key is a tool that allows the user to determine the identity of items in the natural world, such as trees, wildflowers, mammals, reptiles, rocks, and fish. • "Dichotomous" means "divided into two parts". Therefore, dichotomous keys always give two choices in each step. • A dichotomous key is a listing of characteristics, such as structure and behavior, organized in such a way that an organism can be identified or classified. • Think of a dichotomous key as a type of scavenger hunt.

Using a Dichotomous key At each step of the process of using the key, the user is given two choices; each alternative leads to another question until the item is identified. 1a. If the leaves are flat….go to question 4. 1b. If the leaves are needle-like….go to question 2. 2a. Are the needles in a bunch? Go to question 5 2b. Are they spread along the branch?“…pine tree

Eventually, when enough questions have been answered, the identity of the tree is revealed.

Dichotomous Key Example 1. a. wings covered by an exoskeleton – go to step 2 b. wings freely observed – Go to step 3 2. a. body has a round shape ……….ladybug a red beetle with black spots b. body has an elongated shape ……….grasshopper a green insect that hops 3. a.. wings point out from the side of the body ……….dragonfly an insect that is 10- 15 cm long and lives in marshes b. wings point to the posterior of the body ……….housefly a flying insect with red eyes and an annoying buzz

Dichotomous Key Example of a dichotomous key… 1 With flower. Without flower 2 Seedless. Seed bearing 3 Plant body do not differentiated into root, stem and leaves. Plant body differentiated into root, stem and leaves 4 Without photosynthetic pigment. With chlorophyll or other photosynthetic pigment 5 No vascular tissues. With vascular tissues

Angiosperms Go to 2 Go to 3 Gymnosperms Go to 4 Go to 5 Algae Fungi Mosses Fern

Dichotomous key • the dichotomous key can also be expressed in a diagrammatic Pla nts form With flower

Without flower

Flo we ring Pla nts (Ang io sp erm )

Non-flo wering p la nts

Se ed -b e a ring

Se ed le ss

Pla nt b o d y no t d iffe re ntia te d into roo t, ste m a nd le a ves

Without p ho tsynthetic p ig m e nt

Alg a e

Pla nt b o d y d iffe re ntia te d into stem a nd le a ves

With c hloro p hyll or o the r p ho to synthe tic p ig m ent

Fung i

No va sc ula r tissues

With va sc ula r tissues

Mo sse s

Ferns

Gym no sp e rm s

Making a Dichotomous Key Hints: Use constant characteristics rather than variable ones. (Flowers change with the seasons) Use measurements rather than terms like "large" and "small". Make the choice a positive one - something "is" instead of "is not". If possible, start both choices of a pair with the same word.

Finish the dichotomous key with a description of the organism