• Water is transported through hollow, thick walled tubes called xylem vessels. The main force that pulls water through the xylem vessels in the stem of the plant is through transpiration.
  • Transpiration is a passive process powered by the evaporation of water at the leaves by the sun


Movement of water out through Stomata


  • The atmosphere that surrounds the stomata has a lower humidity compared to inside of the stomata.
  • There is a water potential gradient from the air spaces through the stomata to the air.
  • If the stomata are open, water vapour molecules diffuse out of the air in the plant out to the atmosphere
  • Water lost by diffusion from the air space is replaced by water evaporating from the cell wall of the surrounding mesophyll cells.
  • Changing the sizes of the stomatal pores, the plant is able to control the rate of transpiration


Movement of water across the cells of a leaf


  • Water is lost through the mesophyll cells through evaporation from their cell walls
  • This water loss creates an air space which is then replaced by water reaching the mesophyll cells from the xylem from both the cell wall and the cytoplasm
  • The cytoplasmic route:


  1. Mesophyll cells lose water to the air spaces by evaporation from heat energy produced by the sun
  2. A lower water potential is formed and therefore water enters by osmosis from neighbouring cells
  3. The loss of water from these neighbouring cells lower their water potential which results in water being taken in through from their neighbour’s cell


  • Water potential gradient is established that pull water from the xylem, across the leaf mesophyll and finally out into the atmosphere


Movement of water up the stem in the xylem


  • Cohesion tension is the main driving force for the movement of water up a plant
  • Water evaporation off of the mesophyll cells leading to transpiration
  • Water molecules form hydrogen bonds between one another and hence tend to stick together; known as cohesion
  • Water forms a continuous, unbroken column across the mesophyll cells and down the xylem; known as the transpiration pull


Evidence for cohesion tension theory


  • The diameter of a tree trunk reduce during the night due to transpiration being at its lowest
  • When the xylem vessels are broken, the air enters and the tree can no longer draw water up the tree, as the continuous water column has been broken
  • Water does not leak out of a broken xylem vessel