Transpiration:- The loss of water through stomata in the leaves is called transpiration.

- It is evaporative loss of water through stomata.

- Stomata are generally open only day time and close at night.

- Opening and closing of stomata are guarded by guard cells.

- Stomatal Aperture at the inner side of each guarded is thick and elastic.

- The flanking of stomatal aperture takes place when turgidity b/w two guard cell increase, the thin outer walls   bulge out and force the inner walls into crescent shape.

- Due to orientation of microfibrils, opening of stoma is aided in cell walls of guard cells.

- Elastic inner walls regain their original shape when guard cells lose their turgor due to loss of water, closing of stoma takes place and guard cells become flaccid.

- In dicotyledonous, lower surface of dorsoventral leaf has a greater no. of stomata.

- In an isobilateral leaf, stomata are equal on both surfaces

- Factors that effects transpiration are

External Factors	Internal Factors
Temperature	No. and distribution stomata
Speed	No. of stomata open
Light	Water status of plant
Wind speed	Canopy structure
Humidity

Physical properties of water on which transpiration driven ascent of xylem are:-

• Cohesion: Mutual attraction of b/w water molecules
• Surface tension: In liquid phase, water molecules, are more attracted to each other as compare to gaseous phase
• Adhesion: Attraction of water molecular to polar surface.

These property give high capillarity and tensile strength to water.

Tracheids and vessel elements aid the capillarity of plants

The water is pulled from xylem of root into the leaf since the thin film of water over the cells is continuous.

Water diffuses into the surrounding air because of lower conc. of water vapour in atmosphere as compound to Substomatal cavity and intercellular spaces.

A pull is created for movement of water by the forces generate by transpiration can create pressures sufficient to lift xylem sized column of water over 130 m high.

Transpiration and Photosynthesis:

Significance of Transpiration:

• Supplies water for photosynthesis.
• Transports minerals from soil to parts of plant.
• Maintains turgidity of cell thus maintaining shape and structure of plants.
• Creates transpiration pull for absorption and transport of plants.
• Cools leaf surface by waporation.

• Photosynthesis require water which is limited by available water after depleted by transpiration.
• Due to vast water cycling from root to leaf atmosphere and then back to soil, rainforests are humidify.
• For minimizing water loss, C₄ photosynthetic system is evolved which maximize the use of Co₂.