Food is transported by the vascular tissue phloem from a source to a sink. The source and sink may be reversed depending on the

Options

A. season
B. plant's needs
C. geographical distribution
D. both (1) and (2)

Answer

### Step-by-Step Solution:

1. **Understanding the Concept of Source and Sink**:
- In plants, food (mainly in the form of sugars) is produced in the leaves (source) through photosynthesis.
- The sugars are then transported to other parts of the plant (sinks) where they are utilized or stored, such as in roots, fruits, or seeds.

2. **Defining Source and Sink**:
- **Source**: This is typically a photosynthetic organ (like leaves) where sugars are synthesized.
- **Sink**: This is any part of the plant that consumes or stores the sugars, such as roots, fruits, or developing leaves.

3. **Reversal of Source and Sink**:
- The roles of source and sink can change based on various factors:
- **Seasonal Changes**: In different seasons, the demand for sugars may vary. For example, during the growing season, leaves may act as sources, while during dormancy, roots may store sugars and act as sources.
- **Plant's Needs**: Depending on the developmental stage of the plant (e.g., flowering, fruiting), different parts may require more sugars, thus changing their roles.

4. **Transport Mechanism**:
- The transport of sugars through the phloem is explained by the **Mass Flow Hypothesis** (or Pressure Flow Hypothesis). This theory describes how sugars move from areas of high concentration (source) to areas of low concentration (sink) due to pressure differences.

5. **Sugar Conversion**:
- Sugars produced in the leaves are often converted from glucose to sucrose for transport. This is because sucrose is a non-reducing sugar, making it less reactive and more stable during transport.

6. **Conclusion**:
- Therefore, the source and sink can indeed be reversed depending on the season and the plant's needs, confirming that both factors play a role in determining the direction of food transport in plants.