Bulk flow of substances over the longer distances through the vascular tissue is called translocation. This process involves the movement of organic and inorganic solutes, such as water, minerals, and nutrients, from the source to the sink. Translocation primarily occurs in the phloem, which is a specialized tissue responsible for transporting the products of photosynthesis from the leaves to other parts of the plant.

Mechanism of Translocation
Translocation occurs through the following mechanism:
1. Source and Sink: The source is a region of the plant that produces the solutes, such as the leaves. The sink is a region that consumes the solutes, such as the roots, fruits, and flowers.
2. Loading of Solutes: The solutes are loaded into the phloem sieve tubes by the companion cells. This process requires energy, and it can occur through active transport or facilitated diffusion.
3. Pressure Gradient: The loading of solutes creates a pressure gradient between the source and sink. The pressure is higher at the source than at the sink.
4. Mass Flow: The pressure gradient causes the solutes to flow in the phloem from the source to the sink. This process is called mass flow.
5. Unloading of Solutes: The solutes are unloaded from the phloem sieve tubes by the companion cells at the sink. The unloaded solutes are then used by the sink for growth and metabolism.

Conclusion
Translocation is a vital process in plants that allows them to transport solutes over long distances. The process involves the loading of solutes into the phloem sieve tubes, the creation of a pressure gradient, and the mass flow of solutes from the source to the sink. Translocation is a complex process that requires energy and the coordination of various plant tissues.

11th ncert page no 175 ,2nd para last line