In this article we will discuss about the occurrence and position of stomata in epidermis.
Stoma occurs in the epidermises of any of the aerial parts of plants. It occurs in Bryophytes, Pteridophytes and Spermatophytes. In Bryophyta stoma is found on the capsules of Anthocerotopsida and Bryopsida. In Marcnantiopsida stomata are absent; instead pores are present on the surface of thalli. The pores remain open permanently.
The phylum Pteridophyta has stomata in all of its four divisions. Large stoma occurs in Psilopsida, Lycopsida and Pteropsida. Psilopsida (ex. Psilotum) and Sphenopsida (ex. Equisetum) possess stomata that are confined to longitudinal grooves of the aerial stem ridges. In Lycopsida stomata are present on leaves and stems.
In Equisetum each scale leaf has two rows of stomata. In Pteropsida stomata are present in leaves, predominantly on abaxial surface. In the phylum Spermatophyta the epidermal layers of most aerial parts have stomata. They are found in sepals, petals, stamens and carpel of flowers. Awn, palea and lemma of cereal inflorescences also possess stomata.
Stomata are also present in the inner and outer surfaces of fruits, in the surface of seeds, inside pods and in the skins of banana. The presence of stoma is also recorded from rhizomes. The chlorophyll-less parasitic plants like Monotropa and Neottia lack stomata. Another chlorophyll-free plant Orobanche possesses stomata on the stems.
Plants belonging to Balanophoraceae, Monotropaceae and Rafflesiaceae are chlorophyll-free and there are reports that these plants lack stomata. The submerged aquatic plants do not possess stomata.
Usually stomata are absent in roots. But in seedling roots of Ceratonia siliqua and Pisum arvense stoma is observed. Nonfunctional stoma is found in albino plants, petals, staminal filaments (ex. Colchicum), carpel and seeds.
Stoma predominantly occurs in young stems and leaves.-Stoma is found in both adaxial (upper) and abaxial (lower) surfaces of a leaf. Stomata may be present on both surfaces, on abaxial surface and on adaxial surface only of a leaf.
Accordingly the following terminologies are proposed to describe a leaf on the basis of surface(s) where stoma occurs:
i. Amphistomatous (= amphistomatic leaf):
A leaf is said to be amphistomatous where stomata are present on both adaxial and abaxial surfaces. The abaxial surface contains more number of stoma in comparison to adaxial surface. Most plants of herbaceous dicotyledons possess such leaves. Ex. Basella.
ii. Hypostomatous (= hypostomatic leaf):
A leaf is said to be hypostomatous where stomata are present on abaxial surface only. Most tree species of dicotyledons possess such leaves. Ex. Ficus, Nerium etc.
iii. Epistomatous (= epistomatic leaf):
A leaf is said to be epistomatous where stomata are present on the adaxial surface only. Such type of leaf is found in aquatic plants where the leaves float on the surface of water, e.g. Nymphaea. The aquatic fern Marsilea can also grow on land. On the floating leaves of Marsilea stomata are restricted to the adaxial surface while the aerial leaves possess stomata on both adaxial and abaxial surfaces.
Stomata, in leaves of most mesophytic plants those occur in temperate climate with adequate supply of water, are more frequent on the abaxial surface. Stomata are situated more or less at the same level with other epidermal cells. In xerophytes where the plants are adapted to grow in restricted supply of water, stomata may be located in pits (ex. Ammophila arenaria) or in stomatal crypts (ex. Nerium).
Stomata may be sunken (ex. Pinus), that is, stomata occur below the level of other epidermal cells. The abaxial epidermises of Nerium, Ficus etc. are indented at several places thus forming stomatal crypts or grooves or cavities. Stoma occurs in these cavities. This is a means to reduce transpiration. The cavities may have hairs (ex. Nerium, Fig. 12.4A) that create a pocket of water vapour and this is means to reduce further water loss.
Raised stoma is present on the peduncle of Cucurbita (Fig. 12.4B). Stomata occur in clusters in Begonia and Nothofagus etc. The clustering of stomata is a means to reduce water loss. Stomata are close to each other. The water that they transpire coalesces and forms a small humid zone that reduces water loss. The stomata of Saxifraga stolonifera project above the level of leaf surface. This is an anatomical adaptation to increase the rate of transpiration.