Transpiration Stream

• The flow of water through the xylem from the roots to the leaf, against gravity, is called the transpiration stream
• What properties of water allow it to rise against gravity through the xylem vessels?

  cohesion and adhesion
• Cohesion is the force of attraction between two particles of the same substance
• Water molecules are polar and can form a type of intermolecular association called a hydrogen bond
• The cohesive property causes water molecules to be dragged up the xylem towards the leaves in a continuous stream
• Adhesion is the force of attraction between two particles of different substances
• The xylem wall is also polar and hence can form intermolecular associations with water molecules
• As water molecules move up the xylem via capillary action, they pull inward on the xylem walls to generate further tension
• The xylem is a specialised structure that functions to facilitate the movement of water throughout the plant
• The xylem is a tube composed of dead cells that are hollow to allow for the free movement of water
• What substance do xylem cells lack that make them hollow?

  protoplasm
• Because the cells are dead, the movement of water is an entirely passive process and occurs in one direction only
• The cell wall contains numerous pores, which enables water to be transferred between cells
• What are the pores in the cell wall of the xylem called?

  pits
• The walls of the xylem have thickened cellulose and are reinforced by lignin, so as to provide strength as water is transported under tension
• Xylems can be composed of tracheids and vessel elements
• All vascular plants have xylems composed of tracheids
• Vessel elements are only present in some vascular plants
• Tracheids are tapered cells that exchange water solely via pits, leading to a slower rate of water transfer
• In vessel elements, the end walls have become fused to form a continuous tube, resulting in a faster rate of water transfer
• All xylem vessels are reinforced by lignin, which may be deposited in different ways
• In annular vessels, the lignin forms a pattern of circular rings at equal distances from each other
• In spiral vessels, the lignin is present in the form of a helix or coil
• Tracheids


  long cells in the xylem of vascular plants that transport water and mineral salts