<P> The xylem transports water and soluble mineral nutrients from the roots throughout the plant . It is also used to replace water lost during transpiration and photosynthesis . Xylem sap consists mainly of water and inorganic ions, although it can contain a number of organic chemicals as well . The transport is passive, not powered by energy spent by the tracheary elements themselves, which are dead by maturity and no longer have living contents . Transporting sap upwards becomes more difficult as the height of a plant increases and upwards transport of water by xylem is considered to limit the maximum height of trees . Three phenomena cause xylem sap to flow: </P> <Ul> <Li> Pressure flow hypothesis: Sugars produced in the leaves and other green tissues are kept in the phloem system, creating a solute pressure differential versus the xylem system carrying a far lower load of solutes - water and minerals . The phloem pressure can rise to several MPa, far higher than atmospheric pressure . Selective inter-connection between these systems allows this high solute concentration in the phloem to draw xylem fluid upwards by negative pressure . </Li> <Li> Transpirational pull: Similarly, the evaporation of water from the surfaces of mesophyll cells to the atmosphere also creates a negative pressure at the top of a plant . This causes millions of minute menisci to form in the mesophyll cell wall . The resulting surface tension causes a negative pressure or tension in the xylem that pulls the water from the roots and soil . </Li> <Li> Root pressure: If the water potential of the root cells is more negative than that of the soil, usually due to high concentrations of solute, water can move by osmosis into the root from the soil . This causes a positive pressure that forces sap up the xylem towards the leaves . In some circumstances, the sap will be forced from the leaf through a hydathode in a phenomenon known as guttation . Root pressure is highest in the morning before the stomata open and allow transpiration to begin . Different plant species can have different root pressures even in a similar environment; examples include up to 145 kPa in Vitis riparia but around zero in Celastrus orbiculatus . </Li> </Ul> <Li> Pressure flow hypothesis: Sugars produced in the leaves and other green tissues are kept in the phloem system, creating a solute pressure differential versus the xylem system carrying a far lower load of solutes - water and minerals . The phloem pressure can rise to several MPa, far higher than atmospheric pressure . Selective inter-connection between these systems allows this high solute concentration in the phloem to draw xylem fluid upwards by negative pressure . </Li> <Li> Transpirational pull: Similarly, the evaporation of water from the surfaces of mesophyll cells to the atmosphere also creates a negative pressure at the top of a plant . This causes millions of minute menisci to form in the mesophyll cell wall . The resulting surface tension causes a negative pressure or tension in the xylem that pulls the water from the roots and soil . </Li>

Where would xylem cells that no longer conducts water be found in a plant