A stoma is a minute pore on the epidermis of aerial parts of plants through which exchange of gases and transpiration take place. Each stoma is surrounded by a pair of kidney-shaped guards’ cells (in dicot) and a dumbbell-shaped guard cell (in monocot). Each guard cell is a modified cell showing a prominent nucleus, cytoplasm, and plastids. The wall of the guard cell is differentially thickened. The inner wall of each cell facing the stoma is concave and rigid. The outer wall is convex and is thin and elastic. The guard cells are surrounded by a variable number of epidermal cells called subsidiary cells or accessory cells. The average length of stomata is 20to 28µ and breadth 5µm.
Mechanism of Opening and closing stomata: Opening and closing of stomata occur due to turgor changes in guard cells. Due to endosmosis, an increase in the turgor of the guard cells takes place which finally results in stretching and bulging out of their outer thin walls. This results in the pulling apart of the opposed inner thicker walls creating an opening or pore in guard cells of stomata. When the turgor pressure of guard cells decreases inner walls sag, leading to the closure of space between them. This is due to the loss of water from guard cells, resulting in thicker walls to move closer and finally shut the opening.
Function of stomata
- Evaporation of excess amount of water during transpiration.
- Absorption of carbon dioxide from the atmosphere.
- Release of oxygen in the atmosphere.