Stomata (Stoma; singular) are tiny pore openings present in the epidermis of stems, leaves, and other organs of plants.
Location of Stomata
Stomata are mainly present in leaves where they control the exchange rate of gases and water. They are present in aerial parts but absent in roots. Non-functional stomata are present in filaments of stamen, petals, carpels, and seeds of plants.
Isobilateral leaves have stomata on both the upper and lower surfaces of leaves. Bifacial leaves only have stomata on the lower surface of leaves. In xerophytic plants, stomata are sunken.
Structure of Stomata
Each stoma is bound by a pair of guard cells. These guard cells are bean-shaped. These cells are rich in chloroplast and starch grain. There is a stomatal aperture – a space between two guard cells. The guard cells, subsidiary cells, and stomatal aperture collectively form the stomatal apparatus.
The vertical division of the stomatal mother cell produces guard cells. When the walls of guard cells become fully turgid, stomata open. The stomata close on the deflation of guard cells.
Guard cells possess one or more subsidiary cells. Plants have the characteristic number of subsidiary cells according to plant type. These cells are distinct from other epidermal cells.
Structure of Guard Cells
They are mostly kidney-shaped. They may be dumbbell-shaped or elongated in some plants. The ends of guard cells are expanded and thin-walled. In middle, the cells are elongated and thick-walled. They have a narrow lumen. The expanded ends swell and push the middle elongated portion apart. This forms an opening between them.
Guard cells have cytoplasmic lining and a large central vacuole filled with cell sap. The nucleus is in the cytoplasm. These cells also contain chloroplasts. Thus, photosynthesis occurs in guard cells.
This increases the osmotic concentration of guard cells for the opening of stomata. Guard cells are covered with cuticles.
Development of Stomata
Protoderm gives rise to stomata. The protodermal cells divide unequally and produce two cells. Smaller cell is guard the mother cell and it further divides to form two cells. These cells differentiate into guard cells. These guard cells enlarge and assume characteristic shapes.
The middle lamella between two guard cells swells and finally disintegrates to form the stomatal pore. Positions of guard cells are adjusted. They may be raised or lowered. Subsidiary cells arise from the cells related to the mother cells of guard cells.
Types of Stomata
There are five main types of stomata:
- Anomocytic stomata:
In this type of stomata, the guard cells are surrounded by a certain number of cells. These cells have similar shapes.
- Anisocytic Stomata:
In this type of stomata, the guard cells are surrounded by three subsidiary cells. These cells have different sizes.
- Paracytic Stomata:
In this type, each guard cell is surrounded by one or more cells. The longitudinal axis of these cells is parallel to the guard cells and apertures.
- Diacytic Stomata:
Each guard cell is surrounded by two subsidiary cells. The common wall of these cells is at the right angle to the longitudinal axis.
- Actioncytic Stomata:
In this type of stomata, the stomata are surrounded by a circle of radiating cells.
Functions of Stomata
- Exchange of gases by opening and closing of stomata.
- Aids in the removal of excess water from leaves.
- In dry conditions, control water loss.
- Regulate water movement by the process of transpiration.
- During photosynthesis, takes in carbon dioxide and gives out oxygen.