What is stomatal apparatus? Explain the structure of stomata with a labelled diagram.

• Stomatal apparatus refers to the complete structure involved in the regulation of gaseous exchange and transpiration in plants. It includes the stomatal pore, guard cells, and associated subsidiary (accessory) cells.

• Importance of stomatal apparatus

  • Regulates gaseous exchange (intake of CO₂ and release of O₂).

  • Controls transpiration, thus influencing water regulation in plants.

  • Plays a critical role in photosynthesis and respiration.

  • Responds to environmental stimuli such as light, CO₂ concentration, and humidity.

Structure of Stomata

• Stomatal pore

  • A small opening found on the epidermis of leaves, young stems, and floral parts.

  • Surrounded by two kidney-shaped (in dicots) or dumbbell-shaped (in monocots) guard cells.

• Guard cells

  • Two specialized epidermal cells flanking the stomatal pore.

  • Possess chloroplasts, allowing them to photosynthesize and respond to osmotic changes.

  • Change shape by regulating turgor pressure to open or close the stomatal pore.

• Subsidiary (accessory) cells

  • Surround guard cells and provide mechanical support and assistance in stomatal movement.

  • Their number and arrangement vary by plant species.

Types of Stomata (Based on Subsidiary Cell Arrangement)

• Anomocytic: No distinct subsidiary cells (e.g., Ranunculaceae).

• Anisocytic: Three unequal subsidiary cells (e.g., Brassicaceae).

• Paracytic: Subsidiary cells parallel to guard cells (e.g., Rubiaceae).

• Diacytic: Subsidiary cells perpendicular to guard cells (e.g., Caryophyllaceae).

• Actinocytic: Radiating subsidiary cells around guard cells.

Diagram of Stomatal Apparatus

Here is a labelled diagram of a typical dicot stomatal apparatus:

In monocots, the guard cells are dumbbell-shaped, and subsidiary cells are often more distinct and uniform.