Transpiration as diffusion of water vapor down a concentration gradient

Steps:

• Transpiration is the process where water evaporates from mesophyll cells into leaf air spaces, creating high water vapor concentration inside.
• This high internal concentration exceeds the lower concentration in external air, driving diffusion outward.
• Options must describe loss of vapor (not liquid water) from higher internal to lower external concentration.
• D accurately matches this by specifying vapor loss from higher internal air spaces.

Why D is correct:

• It aligns with Fick's law of diffusion, where net water vapor movement occurs from higher concentration in leaf air spaces to lower concentration outside.

Why the others are wrong:

• A: Incorrectly describes "evaporation of water" instead of vapor loss; transpiration emphasizes the gaseous diffusion.
• B: Reverses the gradient; internal air spaces have higher, not lower, vapor concentration.
• C: Specifies "lower in air spaces," inverting the actual high-to-low diffusion direction.

Final answer: D