The two sentences contradictory statements in the key are called

Couplet

Explanation:

Taxonomic Key

A taxonomic is a tool used by scientists to identify and organism. It consists of a series of contradictory statements or questions that eventually lead to the classification of the organism. Choosing the appropriate option in each of the statements gives a detailed description of an organism's characteristics that help in its identification.

This pair of contradictory statements is called a couplet and each statement in a couplet is called a lead.

For a plant, the follow contradictory statements will be called a couplet:

1. Seed colour: Black

1. Seed colour: white

2. Plant height: Tall

2. Plant height: short

oxygen = 21% of the atmosphere

nitrogen = 78%

carbon dioxide = 0.1%

argon =0.934%