Stomata Function

There are two main Stomata functions that allow for the uptake of carbon dioxide and to limit the loss of water due to evaporation. In most of the plants, stomata remain open during the day time and closed during the night. Stomata remain open during the day because of photosynthesis typically occurs.


Stomata are the special pore-like structures in the epidermis of plants. They occur mostly in leaves.


Each stoma is surrounded by two kidney-shaped guard cells containing chloroplast, which control the opening and closing of the stomata.

The inner wall of the guard cell is thick and inelastic due to the presence of a secondary cellulose layer. The outer wall of the guard cell is thin elastic and permeable.


The opening and closing of stomata depends upon theĀ  turgidity of their guard cells,

Opening of stomata

Stomata are open when the guard cells are turgid. The inner margins curve due to outward stretch of outer walls resulting in the widening of stomata pore.

Closing of stomata

Stomata are close when the turgidity of guard cell decreases. With the loss of turgidity, the inner margins become straight and come close to each other which results in the closing of stomatal pore.


The evaporation of water from aerial parts of the plant is known as transpiration. Transpiration may be.

  1. Stomatal transpiration: more than 90% of water transpired through stomata.
  2. Lenticular transpiration: it occurs through lenticels of the stem.
  3. Cuticular transpiration: it occurs through epidermal cells through the cuticle.
  • Transpiration results in a lesser concentration of water and greater concentration salts in the leaf cells.
  • Osmotic pressure in leaf cells thus increases and draws more water from the xylem.
  • Water deficit in the xylem produces a pull or tension, called transpiration pull.
  • As a result of transpiration pull and cohesion of water molecules, water is pulled upward in the xylem vessel as a continuous column, called the transpiration stream.
  • Transpiration helps in the ascent of sap through the transpiration pull and transpiration stream.


The rate of transpiration is affected by the following environmental factors.

  1. Light
  • it is one of the most important factors because it regulates the opening and closing of stomata.
  • The stomata remain wide open during day time, as result transpiration take place through them but at night they are closed, hence transpiration is greatly checked.
  1. Temperature
  • The increase in temperature the rate of transpiration by increasing the rate of evaporation of water from cell surface.
  1. Humidity
  • Transpiration decreases with the increases in humidity (moisture of air) and increases with the deceases in humidity.
  • It means that transpiration can only take place when the atmosphere is partially saturated or dry.
  • The difference in water content of the plant level that of the atmosphere affects the rate of transpiration.
  1. Wind
  • The increase in wind velocity increases the rate of transpiration through a reduction in the density of the air.

Importance of transpiration

  1. By active transpiration, a suction force the transpiration pull is created which helps in the upward movement of water and minerals.
  2. It also increases the rate of absorption.
  • It helps the evaporating excess amount of water.
  1. It maintains a suitable temperature for the leaves and prevents overheating.
  2. It facilitates the ascent of sap.
  3. It brings about the opening and closing of stomata which indirectly the process of photosynthesis and respiration.
  • Excessive loss of water from aerial parts may also result in wilting and dehydration, leading to the death of plants in extreme conditions.


In response to the stimulus plant as a whole or its parts show some activity known as plant movements.

  1. Tactic movements
  2. Tropic movements
  • Nastic movements
  1. Tactic movements
  • These movements are locomotors in which the plant body as a whole move from one plan to another in response to external stimuli such as light chemicals, temperature, etc.
  • Tactic movements may be of the following types depending upon the nature of stimuli.
  1. photo taxis
  • tactic movement in response to the stimulation of sun-light is called phototaxis.
  • Motile plants swim towards light showing positive phototaxis and move away from strong light showing negative phototaxis.
  • Phototaxis can be observed in Chlamydomonas and euglena.
  1. Chemotaxis
  • Tactic movement in response to certain chemicals is described as chemotaxis.
  • The male gametes of bryophytes and pteridophytes are attracted by some chemicals secreted by female sex organs.
  1. Chemotaxis
  • This Is the movement in response to temperature.
  • When three is a different in temperature the unicellular algae are seen to move towards the warmer side.

These are movements of plants organs of stationary plants in response to unilateral stimuli or external stimuli such as light, gravity, water etc, tropic movements are of following three types

  1. phototropism
  2. geotropism
  3. hydrotropism
  4. phototropism
  • it is the movement of plant in response to the stimulus of unilateral light.
  • Stems grow directly towards light and are said to be positively phototropic leaves show positive response.
  • While the root grow in the opposite direction and are
  • Negatively phototropic
  1. Geotropism
  • It is the movement of plant organs to the stimulus of gravity.
  • Stems grow away from it, are said to be negatively geotropic.
  • The primary root grows, directly down towards the force of gravity is said to be positively geotropic.
  • It is evident that the geotropic response of the root and shoot systems places them in the most favorable position with respect to their functions.
  1. Hydrotropism
  • It is the movement of plant organs in response to
  • The stimulus of moisture.
  • Root apices respond positively towards the water while shoot apices respond negatively.
  • The influence of water on the root is stronger than that of gravity.

These are the movements of mainly dorsiventral organs like leaves and petals, induced by an external stimulus such as contact, light, and temperature, etc. the response of the plant is always the same whatever is the direction from which the stimulus is acting. The following are the main types of nastic movements.

  1. Nyctinasty or sleep-movement
  • This movement is caused by the alternation of day and night.
  • Flowers of oxtails and leaves of leguminous plants open in the morning and close at night.
  1. Photo nasty
  • This movement is caused by a variation in the intensity of light.
  • Flowers of primrose open during evening and night and remain closed during the day time.
  1. Thermonasty
  • This movement is caused due to variation in temperature.
  • Crocus and tulip flowers open increased temperatures.
  1. Seism nasty
  • It is the movement caused by mechanical stimuli such as contact touch.
  • The folding of leaves In mimosa is the example of this movement.
  • On touch, the lower cells of leaf lose water and due to fall in turgidity leaves become flaccid and open again with the regain of turgidity.

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