SL Content Statements

• B3.1.9

  Transpiration as a consequence of gas exchange in a leaf
  

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  Students should be aware of the factors affecting the rate of transpiration.
  

• B3.1.10

  Stomatal density
  

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  AOS: Students should use micrographs or perform leaf casts to determine stomatal density.
  NOS: Reliability of quantitative data is increased by repeating measurements. In this case, repeated counts of the number of stomata visible in the field of view at high power illustrate the variability of biological material and the need for replicate trials.
  

• B3.2.7

  Transport of water from roots to leaves during transpiration
  

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  Students should understand that loss of water by transpiration from cell walls in leaf cells causes water to be drawn out of xylem vessels and through cell walls by capillary action, generating tension (negative pressure potentials). It is this tension that draws water up in the xylem. Cohesion ensures a continuous column of water.
  

• B3.2.8

  Adaptations of xylem vessels for transport of water

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  Include the lack of cell contents and incomplete or absent end walls for unimpeded flow, lignified walls to withstand tensions, and pits for entry and exit of water.