Epidermis of a leaf is not continuous at some places due to the presence of small pores, called stomata.
Each stomata is bounded by a pair of specialised epidermal cells called guard cells.
The stomata allows gaseous exchange to occur during photosynthesis and respiration. Structure of Stomata : Opening & Closing of Stomata :
Stomata opens in presence of light, at high temperature & at low CO2
When guard cells becomes turgid
stomatal pore opens
, while when they becomes flaccid
stomatal pore closes
Due to endo-osmosis
guard cells becomes turgid
while due to exo-osmosis
guard cells becomes flaccid
Due to increase in the amount of osmotically active sugars in guard cells, their osmotic pressure increases and water enters inside the cells increasing the turgidity of cells and hence stomata opens.
When amount of sugar decreases, stomata closes. Several theories have been proposed by the scientist to explain the opening and closing of stomata.