• Made from xylem vessels - joined end to end - forms long hollow tube structure
• No end walls - allows water + ions to pass through uninterrupted
• Vessel elements - dead cells
• Lignin is deposited in the xylem walls - in patterns - flexibility and support - stops it collapsing in on itself
• Pits in the xylem wall - non-lignified - water+ ions can move in and out

Companion cells

• Carry out living functions for themselves and STE
• Provide energy (ATP) for active transport of solutes - many mitochondria
• Connected to STE - Plasmodesmata

Symplast Pathway

• Water moves through continuous cytoplasm - plant cells - living parts via osmosis
• Cytoplasm of adjacent cells - connected - Plasmodesmata
• RHC - higher water potential than adjacent cell - water moved in from soil - cytoplasm - dilute
• Process continues till water reaches - xylem
  
  

Apoplast Pathway

• Water moves through - intercellular space between cell walls - non-living parts
• water can carry solutes - move from areas of high hydrostatic pressure to low hydrostatic pressures
• When water in Apoplast pathway - reaches casparian strip - joins the water - Symplast pathway
• Must through partially permeable cell-surface membrane- stops toxic solutes from reaching living tissues

Endodermal cells

• Solute Conc of cytoplasm of endodermal cell - dilute compared to cells in xylem
• Endodermal cells move mineral ions into xylem - active transport - increasingly lowers water potential of xylem cells
• Increases rate of water moving into xylem via osmosis through Symplast pathway
• Once inside vascular bundle - water returns to Apoplast pathway to enter the xylem itself
• Active transport of mineral ions into xylem - root pressure
• Gives water push up the xylem - most circumstances - not the major factor

Evidence of the role of active transport in root pressure

• If poisons such as cyanide are applied to RHC - affect mitochondria preventing the prod of ATP - no energy supply - no root pressure
• Root pressure increases with a rise in temp and falls with a fall in temp - suggesting chemical reactions -involved
• If level of oxygen and respiratory substrates fall - root pressure falls
• sap may exude from the cut ends of stems

Transpiration stream

• Water evaporates from - surface of mesophyll cells into air spaces in the leaf
• Diffuses from stomata into external air down a conc gradient
• Loss of water lowers water potential of mesophyll cells - water moves in from adjacent cells via osmosis - both Apoplast and Symplast pathway - repeated across - leaf to the xylem where water moves into cells of leaf - osmosis
• Water forms hydrogen bonds with the carbs in the xylem vessel walls - adhesion
• Also forms hydrogen bonds with themselves and tend to stick to each other- cohesion
• Water rises up in a continuous stream against gravity to replace water lost - evaporation
• Transpiration pull - tension in xylem - helps to move water across the root from soil

Factors affecting transpiration rate

• Light - required for photosynthesis - for gas exchange - increasing light intensity increases the width and number of stomata open - increasing the amount of water vapour diffusing out

  
  

• Humidity - high humidity - lowers transpiration rate - reduced water vapour potential gradient - inside and outside air in the leaf

  
  

• Increase in temp - increases KE water molecules have - increases evaporation from mesophyll cells

• Also increases the conc of water vapour that the external leaf can hold - lowers the humidity

  
  

• Windier it is - lot of water molecules blown away from around the stomata - increases water potential gradient - increases transpiration rate

  
  

• Soil water availability - if soil is dry - plant will be under water stress - rate of transpiration rate lowers

Process of transpiration

• Plants open their stomata to exchange co2 and oxygen with the air via diffusion outside and inside the leaf - water vapour also diffuses out of the leaf

Adaptations in xerophytic plants

Cacti

• Thick waxy cuticle - layer is waterproof
• Spines instead of leaves - reduces the SA for water loss
• Close their stomata at the hottest time of the day- when transpiration is at its highest
  
  

Marram grass

• Stomata sunk in pits - sheltered from the winds - traps moist air in - pits - lowering water potential
• Layer of hair on the epidermis - traps moist air round the stomata - reduces water potential gradient between leaf and air
• In hot or windy conditions - roll their leaves - traps moist air + reduces exposed SA for losing water + protect the stomata- wind
• Thick waxy cuticle

Adaptations in hydrophytic plants

• Need adaptations - cope in low oxygen levels

  
  

• Air space in tissues - helps plant float + store of oxygen - air space in stem + leaves allow oxygen to move from leaves to parts underwater

  
  

• Stomata only present on the upper surface of floating leaves - maximise gas exchange

  
  

• Flexible leaves + stem - helps to prevent damage by water currents

• Don't need rigid stems for support - they are supported by water

  
  

• Aerenchyma - found in leaves, stems and roots- have many large air sacs - leaves + stems - buoyant

• Forms low-resistance internal pathways for the movement of substances to tissues under water - cope with anoxic conditions