Stomata are minute pores which occur on epidermal surface of leaves and also some herbaceous stems. Each stoma is guarded by two specialised epidermal cells, called guard cells. These guard cells are also surrounded by other specialised epidermal cells called subsidiary cells or accessory cells. These cells also plays an important role during opening and closing of stomata.
Metcalfe and Chalk (1950) classified stomata on the basis of number and arrangement of the subsidiary cells in to the following types:
Example: Ranunculaceae, Malvaceae, Papaveraceae2. Anisocytic (Unequal celled) or Cruciferous: In this stomata remains surrounded by three subsidiary cells of which one is distinctly smaller than the other two.
Example: Cruciferacea, Solanum, Nicotiana etc.
3. Paracytic (Parallel celled) or Rubiaceous: In this type, the stomata surrounded by two subsidiary cells which are parallel to the longitudinal axis of pore and guard cells.
4. Diacytic (Cross celled) or Caryophyllaceous: In this type, the stomata remains surrounded by a pair of subsidiary cells whose common wall is at right angles to the guard cells.
Example: Acanthacea, Caryophyllaceae
5. Actinocytic: These stomata are surrounded by four or more subsidiary cells, elongated radially to the stomata.
Example: Araceae, Musaceae, Commelinaceae
6. Cyclocytic: The stomata are surrounded by four or more subsidiary cells arranged in a narrow ring around the stoma
Example: Palmae, Pandanus, Cyclanthaceae
7. Graminaceous type: The stomatal guard cells are dumb bell shaped. They are surrounded by subsidiary cells which are lying parallel to the long axis of the pore.
Example: In the members of Poaceae and cyperaceae