When the site of the production of food or absorption of water is quite far from the plant’s storage organs, diffusion and cytoplasmic streaming supplemented by active transport do not suffice. They can only account for short-distance movements in plants. Therefore, a special long-distance transport system called a mass or bulk flow system exists in plants to transport water, minerals and food, at a faster rate than diffusion.
The mass flow system moves substances in bulk, which arises out of a pressure difference between the two points. Bulk flow is achieved either through a positive hydrostatic gradient like the one caused by a garden hose or through a negative hydrostatic pressure like the one caused by suction through a straw.
This bulk flow system conducted through the plant’s vascular tissues, xylem and phloem, is referred to as translocation. Xylem transports water, minerals and hormones upwards from the plant’s roots while phloem transfers organic or inorganic solutes from the leaves to the other parts of the plant.
The bulk flow system differs from diffusion in transporting substances across long distances at the same pace and is achieved through a positive or negative hydrostatic gradient.