Loss of water in the vapor state from the aerial parts of plants is known as transpiration. 98-99% of water absorbed by a plant is lost through transpiration. Most of the transpiration occurs through leaves and is called foliar transpiration. Stomata are minute pore complexes, which occur on the soft aerial parts of the plants, especially the
leaves. They hardly occupy 1-2% of leaf surface but due to perimeter diffusion, the exchange capacity of stomata is very high – almost equal to the whole surface area of the leaf.
Each stomata is surrounded by two small specialized and differentially thickened guard cells. Guard cells contain chloroplasts while the same are generally absent from other epidermal cells.

Amount of Transpiration:

Due to transpiration 98 – 99% of absorbed water is lost. Only 0.2% is employed in photosynthesis. Rest is used for growth.


Instrument for measuring atmospheric humidity as well as amount of water transpired.

Percentage of various modes of Transpiration :

(i) Stomatal Transpiration: 50 – 97% of total.
(ii) Cuticular/Epidermal Transpiration: Ordinarily
3 – 10% but in herbaceous mesephytic plants it may be upto 50%.
(iii) Lenticular Transpiration: 0.1%.
(iv) Bark Transpiration: 0.5%

Relative Humidity and Stomata:

Stomata remain open at R.H. above 70% and close below R.H. of 50%.

Transpiration on Hills:

High due to lower atmospheric pressure but low due to lesser hours of light and lower temperature. Transpiration is, therefore, near normal but the plants show xeromorphy due to lesser water availability.

Turgor Changes in Guard Cells:

Schwendener (1881) was the first to point out that stomatal opening and closing is due to turgor changes in guard cells.

CO2 and Stomata:

Low CO2 concentration induces stomata to open. High CO2 concentration causes stomatal closure.Photoactive Stomata: Stomata open in response to light.

Photoactive Stomata

Stomata open in response to light. The action spectrum consists of red and blue light (blue light is more effective in stomatal opening). Since, most of the transpiration is stomatal. The action spectrum of transpiration is red and blue light.


It is loss of turgidity and drooping of leaves and other soft aerial parts of the plant due to rate of water absorption being less than the rate of water loss in transpiration. Rapid absorption of water during day time for meeting requirement of transpiration produces water deficit around the rootlets.Factors Influencing Transpiration:

Factors Influencing Transpiration:

(i) Light: increases transpiration through opening of

(i) Light: increases transpiration through opening of stomata and increased protoplasmic permeability. Solar radiations also increase temperature which simulates more loss of water.

(ii) Temperature: Increase in temperature increases transpiration.

(iii) Humidity: Increase in humidity decreases transpiration and vice versa.

(iv) Wind (Air movement): Air movement increases rate of transpiration by bringing dry air and removing moist air around the transpiring material. high velocity wind, however, closes stomata.

(v) Atmospheric Pressure: Low atmospheric pressure increases transpiration.

(vi) Availability of Water: The water availability influenced the transpiration. Reduced availability of soil water causes Wilting or loss of turgidity causing their drooping and rolling.

(vii) Leaf or Transpiring Area: Reduction in leaf area reduces transpiration as in succulents, cacti and euphorbia.

(viii) Root/ Shoot Ratio: Higher root/shoot ratio has higher transpiration.

(ix) Stomata: Sunken stomata reduce transpiration. Stomatal opening and number have direct effect on transpiration.

(x) Hair: They reduce transpiration.

(xi) Cuticle: Transpiration is lower with the increase in thickness of cuticle.


Loss of water in the liquid state from uninjured parts of plants. It usually occurs from tips and margins of leaves during night or early
morning when there is high atmospheric humidity as during wet seasons. Guttation occurs in some plants only (345 genera) e.g., Cucurbits, Potato, Tomato, Garden Nasturtium, Colocasia and other aroids, many grasses.
Photometer: It is an instrument for measuring the rate of transpiration by shoots through measuring the rate of their water absorption.
Pyrometer: An instrument that gives a rough idea about the degree of stomatal opening

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