Explain how long distance transport of water is caused ?

Explanation

Experiment of Transport:
Place a twig bearing white flowers in coloured water and after some time the colour of flower will be changed.
The cut end of the twig after a few hours kept in coloured solution then coloured water is transported in it.
That experiment very easily demonstrates that the path of water movement is through vascular bundles, more specifically, the xylem.
Long distance transport of substance within a plant cannot be by diffusion alone. Diffusion is a slow process. It can account for only short distance movement of molecules.
For example, the movement of a molecule across a typical plant cell (about 50 um) takes approximately 2.5 s. At this rate, can you calculate how many years it would take for the movement of molecules over a distance of 1 m within a plant by diffusion alone ?
Requirement of transport system :
In large and complex organisms, often substances have to be moved across very large distances.
Sometimes the sites of production or absorption and sites of storage are too far from each other. (leaf -root)
So diffusion of active transport would not suffice. Special long distance transport systems become necessary so as to move substances across long distances and at a much faster rate.
Mass or bulk flow : Water and minerals and food are generally moved by a mass or bulk flow system.
Mass flow is the movement of substances in bulk or en mass from one point to another as a result of pressure differences between two points.
It is a characteristic of mass flow that substances whether in solution or in suspension, are swept along at the same pace, as in a flowing river. This is unlike diffusion.
Where different substances move independently depending on their concentration gradients.
Bulk flow can be achieved either through a positive hydrostatic pressure gradient or a negative hydrostatic pressure gradient.
Transport is done through what ? :
The bulk movement of substances through the conducting or vascular tissues of plants is called translocation.
The higher plants have highly specialised vascular tissues xylem and phloem.
Xylem is associated with translocation of mainly water, mineral salts, some organic nitrogen and hormones, from roots to the aerial parts of the plants.
Phloem translocates a variety of organic and inorganic solutes, mainly from the leaves to other parts of the plants.