Step 1: Define the term "bioindicator".
Bioindicators are organisms or substances used to monitor the health of an environment. They respond to environmental changes, especially pollution, and can help us assess the quality of air, water, and soil.
Step 2: Role of Lichens in Pollution Monitoring
Lichens are excellent bioindicators of air quality. They are sensitive to sulfur dioxide, nitrogen oxides, and other pollutants, making them good indicators of industrial pollution.
Step 3: Why Lichens are Ideal Bioindicators
Lichens do not have a protective cuticle, so pollutants can directly affect them. Their presence or absence, as well as their species diversity, can reveal the level of air pollution in an area.
Final Answer:
\[
\boxed{\text{Lichens}}
\]