The movement of water up the stem is through cohesion-tension. Water molecules are bonded together by hydrogen bonds. This is called cohesion.When one water molecule evaporates from a mesophyll cell it pulls the chain of water molecules up the xylem. The next molecule evaporates and pulls the chain of water molecules up further. As a result there is a continuous pull of the column of water molecules up the xylem.The continuous pull is called the transpiration pull. When water evaporates from the top of the xylem, the pressure at the top of the xylem is less than the pressure at the bottom of the xylem and the xylem walls are put under tension. Without the lignin they would collapse inwards.The pull of water up the xylem by a combination of evaporation and the cohesion of water molecules is called the cohesion -tension theory. Transpiration is a passive process requiring no metabolic energy. The energy is provided by the Sun.