What is the bulk or mass flow hypothesis ?

Explanation

### Step-by-Step Solution:

1. **Definition of Bulk or Mass Flow Hypothesis**: The bulk or mass flow hypothesis refers to the movement of molecules, particularly in plants, from one area to another in large quantities or groups, rather than individually. This movement is primarily driven by pressure differences within the plant's vascular system.

2. **Mechanism of Movement**: In plants, this movement occurs due to the pressure gradient created between different regions, such as the source (where substances are produced or stored) and the sink (where substances are utilized or stored).

3. **Example of Glucose and Sucrose**: A common example involves the conversion of glucose, produced during photosynthesis, into sucrose. The source cells, where sucrose is concentrated, create a high-pressure area.

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