A crustose lichen is

Options

A. Graphis
B. Parmelia
C. Cladonia
D. Usnea.

Answer

To answer the question "A crustose lichen is," we can break down the information provided in the video transcript into a step-by-step solution.

### Step-by-Step Solution:

1. **Understanding Lichens**:
- Lichens are a symbiotic association between two organisms: fungi and algae. The fungal partner is called the mycobiont, while the algal partner is called the photobiont.

2. **Function of Each Partner**:
- The algae (photobiont) are photosynthetic and provide food to the fungus.
- In return, the fungus (mycobiont) provides shelter and protection to the algae.

3. **Defining Crustose Lichens**:
- Crustose lichens are a specific type of lichen that form a thin crust-like structure.
- These lichens grow tightly adhered to their substratum (the surface they grow on) and cannot be easily detached.

4. **Characteristics of Crustose Lichens**:
- They often have an upper cortex that protects the algal layer located in the medulla.
- Crustose lichens can display differently colored rings of actively growing fungal hyphae around their margins.

5. **Example of Crustose Lichen**:
- An example of a crustose lichen is "Graphis." This genus is known for its crustose form.

6. **Elimination of Other Options**:
- Other options mentioned, such as "Pomelia," "Cladonia," and "Yersinia," refer to different types of lichens (foliose and fruticose) and are not crustose.

7. **Conclusion**:
- Therefore, the correct answer to the question is "Graphis."

### Final Answer:
A crustose lichen is "Graphis."

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