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A Handbook of Biology
During transpiration, exchange of O2 and CO2 in the leaf also occurs.
Stomata are open in the day time and close during night except CAM
plants
(NEET 2016)
Opening or closing of the stomata is due turgidity change in the guard
cells.
(AIPMT 2009)
Guard cells and subsidiary cells forms stomatal aperture.
Nucleus
Water
Potassium
Epidermal cells
Vacuole
STOMA
Chloroplast
STOMATA OPENING
Guard cells (Swollen)
STOMATA CLOSING
Guard cells (shrunken)
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
Cell walls
The inner wall of guard cell, lining the stomatal aperture is thick and
elastic and the outer wall is thin.
When turgidity of guard cells increases, the outer walls bulge out and
pull the inner walls into a crescent shape.
Cellulose microfibrils in the guard cells are oriented radially rather than
longitudinally making it easier for the stoma to open.
(NEET 2017)
The guard cells lose turgidity due to water loss (or water stress) and the
inner walls regain their original shape. As a result, the stoma closes.
Usually the lower surface of a dicot leaf has a greater number of stomata.
In monocot leaf, they are about equal on both surfaces.
FÅÇTØRS ÅFFËÇTÏÑG TRÅÑSPÏRÅTÏØÑ:
External factors: Temperature, light, humidity, wind etc.
(NEET 2018)
Plant factors:
Number and distribution of stomata
Number of stomata open.
Water status of the plant.
Canopy structure etc.