Understanding Transpiration in Biology: Factors Affecting the Rate

[ritwik06]

Explain this:
This is what may book says:

1) Transpiration is reduced if air outside is humid. High humidity in the air reduces the rate of outward diffusionof the internal water vapour across stomata, thereby reducing the rate of transpiration.

I say if the humidity increases, atmospheric pressure will decrease. Transpiration rate should be more.
again in my book:

2)Rate of transpiration increases with the decrease in atmospheric pressure, it enhances diffusion of water vapour.

Isnt this point gets biased with the first one.

Please explain these.

 

[ritwik06]

So no one (like me) knows about transpiration it seems...

 

[jim mcnamara]

Air pressure is the pressure from ALL of the molecules, including water vapor.

So, if humidity goes up transpiration rate goes down. This is why plants in arid environment often have small leaves or no leaves at all - like cactus. The humidity is so low that transpiration has to be restricted or the plant will wilt.
Plants also have other mechanisms to control opening and closing stomata to control trspirational water loss.

 

[Moonbear]

I say if the humidity increases, atmospheric pressure will decrease.

This statement is the incorrect assumption that is leading to the confusion.

Also, as Jim alluded to, if you have lower air pressure, water more easily vaporizes from liquid to gas. Have you taken any chemistry classes yet? This is easiest to understand if you have had a chemistry class and recall those diagrams of how temperature and pressure affect the phase transitions from solid to liquid to gas of molecules.