Which one of the following is an example of crustose lichen?
A. Parmelia
B. Cladonia
C. Usnea
D. Graphis

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Hint: Lichens are the symbiotic organisms that are formed due to the association of algae and fungi. Lichens are the plants –like but they are not the plants. They are of many colours, sizes and forms.

Complete Answer:
- Crustose lichens are the lichens that form a crust which strongly adhere to the substrate (soil, rock, tree bark ).
- In some lichens the crust may be thick and lumpy.
- Making separation of lichen from the substrate impossible without destruction.

 The crustose lichen is consist of three parts –
1. Cortex layer – Cortex layer is the upper layer of the lichen. This layer is differentiated and pigmented.
2. The algal layer –The algal layer beneath the cortex layer. The algal layer consists of algal members.
3. The medulla – The medulla fastens the lichen to the substrate and is made up of fungal hyphae.

- The surface of the crustose lichen is characterized by the presence of branching cracks.
- These cracks are periodically close in response to climatic variations such as alternate wetting and drying regimes.
- Examples of crustose lichens include Graphis, Lepraria, Lecidae, etc.

Hence, option D is the correct option for this question.

Note: The lichens are widely used as the bio indicators because they cannot grow in a polluted area. If the air of a certain place is badly polluted with sulphur-dioxide, there may be no lichens present in that particular area. If the air is clean then shrubby, hairy and leafy lichens may be present in that particular area. Lichens are very sensitive to the atmospheric pollutants.
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