The fungus which grows around the root surface is called :

Options

A. endomycorrhizae
B. ectomycorrhizae
C. rhizomorph
D. haustoria

Explanation

To solve the question "The fungus which grows around the root surface is called:", we will follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Mycorrhizae**: Mycorrhizae are symbiotic associations between fungi and plant roots. They play a crucial role in nutrient exchange and help plants absorb water and minerals.

2. **Identify the Types of Mycorrhizae**: There are two main types of mycorrhizae:
- **Ectomycorrhizae**: These fungi grow on the outer surface of the root and do not penetrate the root tissues.
- **Endomycorrhizae**: These fungi penetrate the root tissues and form structures inside the plant cells.

3. **Analyze the Question**: The question specifically asks for the fungus that grows around the root surface. This indicates that we are looking for a type of mycorrhiza that does not penetrate the root.

4. **Evaluate the Options**:
- **Option A: Endomycorrhiza** - Incorrect, as these fungi penetrate the root tissues.
- **Option B: Ectomycorrhiza** - Correct, as these fungi grow on the outer surface of the roots.
- **Option C: Rhizomor** - Incorrect, as this term does not refer to a type of mycorrhiza.
- **Option D: Hostoria** - Incorrect, as these are structures formed by parasitic plants, not mycorrhizal fungi.

5. **Conclusion**: Based on the analysis, the correct answer is **Option B: Ectomycorrhiza**.

### Final Answer:
The fungus which grows around the root surface is called **ectomycorrhiza**.