Water enters through the root hair cells and then moves across into the xylem tissue in the centre of the root. Water moves in this direction because the soil water has higher water potential, than the solution inside the root hair cells. This is because the cell sap has organic and inorganic molecules dissolved in it. The root hairs provide a large surface area over which water can be absorbed. Minerals are also absorbed but, their absorption requires energy in the form of ATP because they are absorbed by active transport. They have to be pumped against the concentration gradient.
Water is removed from the top of xylem vessels into the mesophyll cells down the water potential gradient. This removal of water from the xylem reduces the hydrostatic pressure exerted by the liquid so the pressure at the top is less than at the bottom. This pushes the water up the tube. The surface tension of the water molecules, the thin lumen of the xylem vessels and the attraction of the water molecules for the xylem vessel wall (adhesion), helps to keep the water flowing all the time and to keep the water column intact.
Pressure to push water up can also be increased from the bottom. By actively pumping minerals from cells surrounding the xylem into the xylem itself, more water is drawn into the xylem by osmosis.
This increase in water pressure, called root pressure, certainly helps in the process but is less important than the simple movement of water down the water potential gradient, ultimately from the soil at the bottom, to the air at the top. This is because moving water this way does not require energy (it is passive).
Phloem tissue transports photosynthetic products, other organic molecules (e.g., plant hormones and waste products), and soluble nutrients throughout the plant. Unlike xylem, phloem is alive at maturity, but usually with a much reduced cell contents and no nucleus. This is logical because movement of material through phloem tissue relies on solute gradients and some active transport that require the activity of living cells. In non-angiosperm seed plants phloem elements consist mostly of sieve cells, while angiosperms have sieve tube cells in association with parenchymatous companion cells. Phloem fibers also provide some mechanical support. Phloem cells are commonly unlignified so they do not preserve as readily as xylem.
Phloem cells are usually located outside the xylem. The two most common cells in the phloem are the companion cells and sieve cells. Companion cells retain their nucleus and control the adjacent sieve cells. Dissolved food, as sucrose, flows through the sieve cells. Sieve tube elements, living cells which contain obstructions to flow of solution (i.e. sieve plates and, to a lesser extent, the cytoplasm) they contain no nucleus. The cell walls at ends develop into sieve plates.
Sieve tube elements are living, tubular cells that are connected end to end. The end cell walls have perforations in them to make sieve plates, formed from plasmodesmata, to allow transport of organic solutes through the plant. The cytoplasm is present but in small amounts and in a layer next to the cell wall. It lacks a nucleus and most organelles so there is more space for solutes to move. The cell walls are made of cellulose so solutes can move laterally a well as vertically. Next to each sieve tube element is a companion cell.
Companion Cells, are closely associated with each sieve element. Unlike sieve cells, they have a dense cytoplasm with all the usual cell organelles. Metabolically very active deduced from large numbers of mitochondria which is present. Very close link with sieve tube elements. If a companion cell dies, so do some elements. In leaves they function as transfer cells, absorbing sugars and transferring them to sieve tube elements
Companion cell: Since the sieve tube element lacks organelles, the companion cell with its nucleus, mitochondria, ribosomes, enzymes etc., controls the movement of solutes and provides ATP for active transport in the sieve tube element.