Transpiration stream

Transpiration is the evaporation of water at the surfaces of the spongy mesophyll cells in leaves, followed by loss of water vapour through the stomata.

Cross-section of a leaf showing water travelling through xylem cells to the spongy mesophyll cells, where it evaporates and exits through the stoma as water vaperTranspiration in the leaf

Water moves through the xylem vessels in a continuous transpiration stream:

root → stem → leaf

Transpiration produces a tension or ‘pull’ on the water in the xylem vessels by the leaves. Water molecules are cohesive so water is pulled up through the plant.

The transpiration stream has several functions. These include:

  • transporting mineral ions
  • providing water to keep cells turgid in order to support the plant
  • providing water to leaf cells for photosynthesis
  • keeping the leaves cool by evaporation

Root hair cells

The root hairs are where most water absorption happens. They are long and thin so they can penetrate between soil particles, and they have a large surface area for absorption of water.

Cross-section of root hair cell: a roughly rectangular shape with a long, thin tail extending to the right and a nucleus at the top left.

Water passes from the soil water to the root hair cell’s cytoplasm by osmosis. This happens because the soil water has a higher water potential than the root hair cell cytoplasm:

SolutionWater potentialConcentration of dissolved solutes
Soil waterHighLow
Root hair cell cytoplasmLowHigh
Cross-section of a plant root, showing water passing into root hair cells, through root cortex cells and into xylem cells.Osmosis causes water to pass into the root hair cells, through the root cortex and into the xylem vessels