One important difference between the anatomy and physiology of roots and leaves is that:

Options

A. unlike roots, the xylem vessels in leaves normally transport sap in both directions
B. a waxy cuticle covers most leaves but is absent in roots
C. leaves, but not roots, have ground tissue consisting mainly of parenchyma cells
D. phloem always conducts sap away from leaves and toward roots

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for an important difference between the anatomy and physiology of roots and leaves. This means we need to focus on structural and functional differences.

2. **Analyzing the Options**:
- **Option 1**: "Unlike roots, the xylem vessels in leaves normally transport sap in both directions."
- This is incorrect. Xylem primarily transports water and minerals upwards from the roots to the leaves.
- **Option 2**: "A waxy cuticle covers most leaves but is absent in roots."
- This is correct. The waxy cuticle on leaves helps prevent water loss through transpiration, which is not necessary for roots as they are underground and do not lose water in the same way.
- **Option 3**: "Leaves but not roots have ground tissue consisting mainly of parenchyma cells."
- This is incorrect. While leaves do have parenchyma cells, they are primarily composed of mesophyll, which is a specific type of parenchyma.
- **Option 4**: "Phloem always conducts sap away from leaves and towards fruit."
- This is incorrect. Phloem transports nutrients and organic compounds from the leaves to other parts of the plant, including roots.

3. **Identifying the Correct Answer**: Based on the analysis, the correct answer is that a waxy cuticle covers most leaves but is absent in roots.

4. **Explaining the Reasoning**: The waxy cuticle on leaves serves to minimize water loss due to transpiration, which is critical for leaves exposed to air. In contrast, roots are typically underground and do not require such a protective layer as they absorb water directly from the soil.

### Final Answer:
The important difference between the anatomy and physiology of roots and leaves is that a waxy cuticle covers most leaves but is absent in roots.

---