It is an opening/ pore/ intercellular space in the epidermis, through which gaseous exchange takes place in plants. It is surrounded by two specific cells known as guard cells. Stomatal pores together with guard cells are known as stoma. Generally guard cells are bean or crescent shaped in dicots which have dorsiventral or bifacial leaves, and dumbell shaped in monocots which have isobilateral or unifacial leaves. Further, these guard cells are surrounded by various number of cells known as subsidiary cells. This epidermal intercellular space or pore together with guard cells and subsidiary cells is known as stomatal complex (stoma + subsidiary cells). Below the stomata there is intercellular space in mesophyll cells known as stomatal chamber. Stomata are found on young stem, leaves, flower, and fruit. Exceptionally they are also found on roots of Pea (Pisum arvense) and Ceratonia siliqua. Stomata which are found on non-foliar part (flower, fruit, seed) are non functional. Distribution of stomata on leaf surfaces varies from species to species. They may be found on both the surfaces of leaves (adaxial and abaxial) or may be restricted to only one surface. When stomata are found only on upper surface, the leaf is called epistomatic and when found exclusively on lower or abaxial surface, the leaf is called hypostomatic. Frequency of stomata differs from species to species eg 402 in Quercus calliprinos, 1198 in Q. lyrata, 176 in Pistacia palaestina and 255 in P. lentiscus. It is found that with decreasing light intensity stomatal frequency decreases. Stomata are often not found on solid masses of cells where there are no intercellular spaces like sclerenchyma of leaf margins and fibre bundles of veins, as these tissues don’t provide space for diffusion of gases. There is an unusual distribution of stomata in some plants like in Saxifraga, they are present only near the leaf tip; in Daphne petraea there is a longitudinal band of stomata on both sides of midrib. A very unusual distribution is found in Mimosa cruenta, where on upper surface there is uniform distribution of stomata, however on the lower surface only one of the longitudinal halves bear stomata and the other half completely lacks them.
Non-green total parasitic (holoparasite) plants lack stomata e.g. Rafflesia (a total root parasite) and Neottia and members of family Balanophoraceae and Monotropaceae. These holoparasites lack cholorophyll, thus they are completely dependant on their host for food. Orobanchae, a achlorophyllous holoparasite possess only few stomata on the stem. Semi parasites like mistletoes (Loranthaceae) depend on host for water synthesizes their food and minerals only, have stomata but lesser in number.
Position of stomata is also governed by climatic conditions. In xerophytic conditions stomata are sunken (at lower level than other epidermal cells) to reduce the water loss. They may be found in cavities, stomatal crypts (Nerium oleander), or in grooves (Ericales) lined with epidermal hair. In some hydrophytes like Nymphaea and Nelumbo with floating leaves, stomata are found only on upper surface. However, there are no stomata in some submerged hydrophytes belonging to families Ceratophyllaceae, Nymphaeaceae, Podostemonaceae and some Ranunculaceae member. Stomata bars are found in Eucalyptus, these are protuberances of cuticle over the guard cells. Guard cells may differ from other epidermal cells in their origin and morphology but chemically they are similar to other epidermal cells. They are rich in mitochondria, endoplasmic reticulum, dictyosomes and vacuoles and fewer grana in plastids but are sufficient for the sustenance of guard cells. There is no plastid in Paphiapedilum an orchid.
Types of stomata
Depending upon the number and arrangement of subsidiary cells around the guard cells stomatal complex has been classified into following types (Tomlinson, 1961, 1974; Metcalfe and Chalk, 1979;). A species can have more than one of the following stomatal complex types, even a single leaf can have more than one type.
- Paracytic type– One or more subsidiary cells are arranged parallel to the guard cells. e.g. Convolvulaceae, Leguminoseae,