Water diffuses 1.56 times faster than CO 2 . When soil moisture is limiting, transpiration and CO 2 uptake is reduced because the stomates close. Uptake of water causes guard cells to swell and become turgid. This opens the stomates because of radially oriented cellulose microfibrils designed to do this. The guard cells then take up water by osmosis.
Opening involves a cascade of events.
Malic acid is produced in guard cells. The size of stomates proves to be nearly optimal for maximum gas or water diffusion. Ideally suited for CO 2 gain or H 2 O loss. Guard cells contain chloroplasts, neighboring epidermal cells do not. FICK'S LAW Diffusion is proportional to the driving force, and inversely proportional to the resistance.