Stomata are openings in the epidermal layer of leaves of plants. Two guard cells surround each stoma. The guard cells are bean-shaped. The inner cellwalls of the guard cells are elastic and thicker than the outer cell walls. This enables the guard cells to change in shape thus allowing opening and closing of the stoma. Stomata are frequently found on leaves and aerial parts of stems and branches in herbaceous plants. Guard cells contain chloroplasts and carry out photosynthesis for the plants.


Mechanism of opening and closing of stomatal pore

The opening and closing of the stomatal pore (stoma) depends on the turgidity of the guard cells. Variations in turgor pressure change the shapes of the guard cells.


Opening of the stomatal pore (stoma)

The chloroplasts of the guard cells carry out photosynthesis during the day. Sugar manufactured during photosynthesis increases the concentration of the cell sap. Water moves from the adjacent epidermal cells into the guard cells by osmosis. The guard cells become turgid. The pressure resulting from the turgidity causes the bulging of the thinner cell walls to press into the adjacent epidermal cells. This causes the thicker inner cell walls to assume crescent shape and thus opening the stomatal pore.


Closing of the stomatal pore

At night, when photosynthesis has ceased, the sugar in the guard cells is converted into starch. The concentration of the cell sap of the guard cells is now lower than the adjacent epidermal cells. Water moves by osmosis from the guard cells to the adjacent epidermal cells. This makes the guard cells to shrink and becomes flaccid due to loss of water causing the inner cell walls to straighten and close the stomatal pore.



Factors that lead to the opening and closing of stomata

  1. Concentration of Carbon (IV) oxide (CO2): When the concentration of carbon (IV) oxide is more than 0.04%, stomata close and open when less than 0.04%.
  2. Dry weather: When the weather is dry and hot, the rate of transpiration is high. Stomatal pore has to close to conserve water for the plant.
  3. Acidity: High acidity in the guard cells prevents or inhibits the action of enzymes that digest starch to sugar. However, starch in the guard cells makes the guard cells to lose water and become flaccid. Stomata therefore close up. Low acidity stimulates the action of enzymes to convert starch to sugar. Increase in sugar content increases water volume of the guard cells by osmosis. Guard cells swell and stomatal pores open.


Lenticels (breathing pores)

Lenticels are small openings found in the epidermis or bark of woody stem through which gases are exchanged between the internal tissues and the atmosphere. Under each lenticel, are cork cells that are loosely arranged with many intercellular air spaces between them.

Cork cambium gives rise to cork cells. The cork cells are dead and lined with a waxy material called suberin so that the loss of water from the plant is greatly reduced. Lenticels remain open throughout the life of the plant.

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