According to statolith theory, a plant distinguishes up from down by means of

Options

A. Phytochrome
B. Amyloplasts
C. Auxin distribution
D. Ascent of sap.

Answer

**Step-by-Step Solution:**

1. **Understanding the Statolith Theory**: The statolith theory explains how plants perceive gravity and distinguish between up and down. This is crucial for their growth and orientation.

2. **Identifying Statoliths**: According to the theory, statoliths are specialized organelles that help in gravity perception. In plants, these statoliths are primarily dense amyloplasts.

3. **Role of Amyloplasts**: Amyloplasts are a type of plastid that stores starch. Their density allows them to sediment in response to gravity, which helps the plant to determine its orientation.

4. **Gravitropism**: The process by which plants grow in response to gravity is known as gravitropism (or geotropism). In this process, the sedimentation of amyloplasts in the root tips and shoot cells sends signals that influence growth direction.

5. **Auxin Distribution**: The sedimentation of amyloplasts leads to the redistribution of auxin, a plant hormone that regulates growth. In response to gravity, auxin accumulates on the lower side of the plant, promoting downward growth of roots (positive geotropism) and upward growth of stems (negative geotropism).

6. **Conclusion**: Therefore, according to the statolith theory, a plant distinguishes up from down primarily by means of amyloplasts.

**Final Answer**: Amyloplasts.

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