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Xylem (at least in dicots) is composed of vessel elements and tracheids. Vessel elements are similar in structure to the sieve-tube members of phloem, but they lack companion cells, and have perforated sides as well as pores at the ends. Tracheids are much narrower cells, with tapered and perforated ends, constituting most of the volume of the xylem tissue. Both tracheids and vessel elements are dead at maturity.

Xylem sap always moves from the roots to the leaves. It travels by bulk flow, like water in a series of pipes, rather than by diffusion through cells. Three phenomena cause xylem sap to flow:

• The soil solution (see soilSoil is the layer of minerals and organic matter, in thickness from centimetres to a metre or more, on the land surface. Its main components are mineral matter, organic matter, moisture, and air. Soils differ in the ratio of these components. Modern soil) is more dilute than the cytosolThe cytosol (as opposed to cytoplasm which also includes the organelles) is the internal fluid of the cell, and a large part of cell metabolism occurs here. Proteins within the cytosol play an important role in signal transduction pathways, glycolysis, an of the root cells. Thus, water moves osmoticallyOsmosis is the diffusion of a solvent through a semipermeable membrane from a region of low solute concentration to a region of high solute concentration. The semipermeable membrane is permeable to the solvent, but not to the solute, resulting in a chemic into the cells, creating root pressureRoot pressure is one of the phenomena used by vascular plants to move water into the leaves. The water in the soil tends to be poorer in solutes than the water in the plant's cells, due to the plant's active absorption of dissolved nutrients. The resultin. Even under optimal conditions, root pressure can only lift water a couple of feet.
• Capillary actionCapillary action or capillarity is the ability of a narrow tube to draw a liquid upwards against the force of gravity. It occurs when the adhesive intermolecular forces between the liquid and a solid are stronger than the cohesive intermolecular forces wi helps sap to flow up the narrow tracheids.
• By far the most important cause of xylem sap flow is transpirational pullTranspirational pull is the main phenomenon driving the flow of sap in the xylem tissues of large plants. Transpirational pull results ultimately from the evaporation of water from the surfaces of cells in the interior of the leaves. This evaporation caus. This is the reverse of root pressure, caused by the transpirationTranspiration is a continuous process caused by the evaporation of water from leaves of plants and its corresponding uptake from roots in the soil. Transpiration cools plants down and enables mass flow of minerals to where it is needed in the plant. Mass of waterDrinking water This article focuses on water as we experience it every day. The water (molecule) article describes water from a scientific and technical perspective. Water is an abundant substance on Earth. It exists in many forms, such as sea, rain, and from leaves.

In perennial plants, xylem is laid down in multiple phases. Primary xylem is one of the tissues left behind by the apical meristem. Secondary xylem is laid down by vascular cambium on the outside of the xylem column.