Question

At which stage of a dichotomous key are individuals or items most similar?

Answer 1

Answer:

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

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.

• 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.