Boundary layer resistance - still air at surface of leaf. It can be reduced by wind. Stomatal resistance - diffusion through the stomatal pore. It becomes the major resistance when the pore is closed or nearly closed. Intercellular air space resistance - diffusion from the pore to the mesophyll cell wall.
Leaves have many air spaces between the mesophyll cells.
Allows faster diffusion, as CO 2 diffuses 10,000 times faster in air than in water. If the stomates are open this is the major resistance to CO2 movement, even though it is a short distance. Water diffuses in the opposite direction to CO 2 . Partial stomatal closure slows transpiration more than CO 2 uptake by photosynthesis. Rise in Atmospheric CO 2 .