Assertion: Crustose lichens, in a lithosere, secrete organic acids causing the weathring of rocks so that minerals essential for proper growth of lichens are released.
Reason: Early colonists in a primary suceession are usually lichens, which suggests that colonisation is easier when an organism has a mutualistic association.

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that crustose lichens in a lithosere secrete organic acids that cause the weathering of rocks, which releases essential minerals for their growth.
- Crustose lichens are indeed the pioneer species in a lithosere, which is a type of ecological succession that occurs on bare rock.

2. **Analyzing the Role of Crustose Lichens**:
- Crustose lichens can withstand harsh conditions, including desiccation (drying out).
- They secrete organic acids that chemically weather the rock surface, breaking it down and releasing minerals like calcium, potassium, and magnesium that are necessary for their growth.

3. **Understanding the Reason**:
- The reason states that early colonists in primary succession are usually lichens, suggesting that colonization is easier when there is a mutualistic association.
- Lichens are indeed a mutualistic association between fungi and algae or cyanobacteria, where both partners benefit: the fungus provides a protective environment and absorbs moisture, while the algae/cyanobacteria perform photosynthesis to produce food.

4. **Connecting the Assertion and Reason**:
- Both the assertion and reason are correct statements. The assertion correctly describes the role of crustose lichens in weathering rocks and facilitating their own growth.
- The reason correctly identifies lichens as pioneer species in primary succession and highlights their mutualistic nature.

5. **Evaluating the Relationship**:
- Although both statements are true, the reason does not directly explain the assertion. The assertion focuses on the chemical weathering caused by lichens, while the reason discusses the mutualistic relationship that allows lichens to colonize effectively.

6. **Conclusion**:
- Therefore, both the assertion and reason are correct, but the reason is not the correct explanation for the assertion.

### Final Answer:
Both the assertion and reason are correct, but the reason is not the correct explanation of the assertion.

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