Let us make in-depth study of the structure, number, distribution and types of stomata.
Stomata was discovered by Pfeffer & name ‘stomata’ was given by Malphigii. Stomata cover 1-2% of leaf area. It is minute pore present in soft aerial parts of the plant. Algae, fungi and submerged plants do not possess stomata.
(a) Stomata are minute pores of eliptical shape, consists of two specialized epidermal cell called guard cells.
(b) The guard cells are kidney shape in dicotyledon and dumbell shape in monocotyledon.
(c) The wall of the guard cell surrounding the pore is thicken and inelastic due to rest of the walls are thin, elastic and semi-permeable.
(d) Each guard cell has a cytoplasmic lining, central vacuole. It cytoplasm contains single nucleus and number of chloroplast. The chloroplast of guard cell are capable of very poor photosynthesis, because the absence of RUBISCO enzyme.
(e) Guard cells are surrounded by modified epidermal cells, known as subsidiary cells or accessory cells, which supports in the movement of guard cells.
(f) The Size and shape of stoma and guard cell vary from plant to plant. When fully open, the stomatal pore measures 3-12 in width and 10-40 in length.
(g) In many gymnosperms and xerophytic plants {plants growing in desert), the stomata are present embedded deeply in the leaves, so that they are not exposed to sunlight directly. Such deeply embedded stomata are called sunken stomata. This is an adaptation to check excessive transpiration in these plants.
In many gymnosperms and xerophytic plants (plants growing in desert), the stomata are present embedded deeply in the leaves, so that they are not exposed to sunlight directly. Such deeply embedded stomata are called sunken stomata. This is an adaptation to check excessive transpiration in these plants.
Number of Stomata (Stomatal Frequency):
The number of stomata in a definite area of leaf varies from plant to plant. Xerophytes possess larger number of stomata than mesophytes. Number of stomata/sq cm. is 1000 — 60,000 in different plant species. The number of stomta per unit area of leaf is called Stomatal Frequency.
Stomata frequency of trees and shrubs is higher than herbs. Stomata nearly occupy one to two percent of total leaf area when fully open. In isobilateral leaves (in monocots). approximately the same number of stomata are found on upper surface (adaxial) and lower (abaxial) surface. But in dorsiventral leaves (indicots) the number of stomata on the upper surface is much less in comparison to those found on the lower surface.
Distribution and Types of Stomata:
Depending upon the distribution and arrangement of stomata in the leaves five categories of stomatal distribution have been recognized in plants (Fig. 4.4).
1. Apple or mulberry (hypostomatic) type:
Stomata are found distributed only on the lower surface of leaves, e.g., apple, peach, mulberry, walnut, etc.
2. Potato type:
Stomata are found distributed more on the lower surface and less on its upper surface, e.g., potato, cabbage, bean, tomato, pea, etc.
3. Oat (amphistomatic) type:
Stomata are found distributed equally upon the two surfaces, e.g. maize, oats, grasses, etc.
4. Water lily (epistomatic) type:
Stomata are found distributed only on the upper surface of leaf, e.g., water lily, Nymphaea and many aquatic plants.
5. Potamogeton (astomatic) type:
Stomata are altogether absent or if present they are vestigeal. e.g., Potamogeton and submerged aquatics.
Metacalf and Chalk recognized four types of stomata on the basis of their structure-
a. Anomocytic type:
In these stomata, accessory cells are absent. The guard cells are surrounded by ordinary epidermal cells, e.g., families Ranunculaceae, Cucurbitaceae, Papaveraceae and Malvaceae.
b. Anisocytic type:
In these stomata the guard cells are surrounded by three accessory cells. Of these two are larger whereas one is smaller in size.g., family Brassicaceae.
c. Diacytic type:
In these stomata the guard cells are surrounded by two accessory cells. Their common walls are at right angle to the walls of guard cells, families Caryophyllaceae, Acanthaceae.
d. Paracytic type:
In these stomata the guard cells are also surrounded by two accessory cells, but their common walls are parallel to guard cells, e.g., families Rubiaceae, Fabaceae etc.
On the basis of development (Pant, 1965):
There are three types of stomata:
Mesogynous type:
In this type of stomata guard cells as well as subsidiary or Accessory cells both are developed from one mother cell. e.g. Rubiaceace &. Brassicaceae family.
Perigynous type:
In this type guard cells are formed from mother cell while subsidiary cells from nearby mother cells, eg.: Cucurbitaceae family.
Mesoperigynous type:
In this type guard cells & one subsidiary cells is formed from mother cell while other subsidiary cells develop Independently. e.g.: Ranunculaceae, Caryophyllaceae family.