Class 11 Biology Transport In Plants Transpiration


  • Transpiration is the evaporative loss of water by plants, occurs mainly through the stomata in the leaves.
  • Exchange of oxygen and carbon dioxide in the leaf also occurs through pores called stomata.
  • The immediate cause of the opening or closing of the stomata is a change in the turgidity of the cells called guard cells.
  • The inner wall of each guard cell, towards the pore or stomatal aperture, is thick and elastic.
  • The opening of the stoma is also aided due to the orientation of the microfibrils in the cell walls of the guard cells.
  • When turgidity increases within the two guard cells flanking each stomatal aperture or pore, the thin outer walls bulge out and force the inner walls into a crescent shape.
  • When the guard cells lose turgor, due to water loss, the elastic inner walls regain their original shape, the guard cells become flaccid and the stoma closes.
  • Transpiration is affected by several external factors: temperature, light, humidity, wind speed.
  • Plant factors that affect transpiration include number and distribution of stomata, number of stomata open, per cent, water status of the plant, canopy structure etc
  • The transpiration driven ascent of xylem sap depends mainly on the following physical properties of water:
  • Cohesion – mutual attraction between water molecules.
  • Adhesion – attraction of water molecules to polar surfaces.
  • Surface Tension – water molecules are attracted to each other in the liquid phase more than to water in the gas phase.
  • The physical properties give water high tensile strength and high capillarity.
  • High tensile strength is the ability to resist a pulling force.
  • High capillarity is the ability to rise in thin tubes.
  • In plants, capillarity is aided by the small diameter of the tracheary elements – the tracheids and vessel elements.
  • Because of lower concentration of water vapour in the atmosphere as compared to the substomatal cavity and intercellular spaces, water diffuses into the surrounding air, which creates a ‘pull’.


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