stomata

stomata
An important feature of leaf epidermis is the presence of stomata which occur either on both sides of the leaf or only on one side of the leaf. When stomata occur on both sides, the leaf is known as amphistomatic, when they are confined to the upper side, the leaf is known as epistomatic and when to the lower side, the leaf is called hypostomatic.


Structure of Stomata
Stomata occupy roughly 1-12% of the total leaf area. Each stomata consists of an opening (pore) bounded by two specialized, usually kidney shaped, epidermal cells known as guard cells. The guard cells have unevenly thickened walls. The inner wall facing the aperture is highly thickened while the one away from the aperture is thin and extensible. The guard cells are also covered with cuticle which extends to the inner wall forming the boundary of the pore and the sub-stomatal chamber.


Guard cells are surrounded by a variable number of epidermal cells which are called subsidiary or accessory cells. These cells may be morphologically similar to the other epidermal cells or very different from them.

Position of Stomata



Stomata may occupy three different positions in relation to epidermal cells. They may be at the same level as the adjoining epidermal cells as in most of the mesophytic plants. In xerophytic plants, they are sunken as they are located in a cup shaped depression. In some plants (as in cucurbita), the stomata are slightly raised above the surface of epidermis.






In a dicot leaf, the stomata are scattered on the surface. The number of stomata in dicots on the lower epidermis is more than the upper epidermis. On the other hand, the monocots have equal number of stomata on both epidermises.






Types of Stomata


On the basis of number and orientation of subsidiary cells, there are four types of stomata in dicotyledons.






1.Anomocytic (irregular celled) type – In this type, the stomata are surrounded by a limited number of epidermal cells which are indistinguishable from other epidermal cells.


2.Anisocytis (unequal celled) type – This type is characterized by the presence of three subsidiary cells of which one is distinctly smaller than the other two.


3.Paracytis (parallel celled) type – In this type, the stomata is accompanied on either side by one or more subsidiary cells which lie parallel to the long axis of the pore and the guard cells.


4.Diacytic ( cross celled) type – In this type, the stomata is enclosed by a pair of subsidiary cells whose common wall is at right angle to the long axis of the guard cells.