Essay on the Stomata | Aperture | Higher Plants | Botany

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Essay on the Stomata

Essay Contents:

1. Essay on the Meaning of Stomata
2. Essay on the Occurrence of Stomata
3. Essay on the Position of Stomata
4. Essay on the Structure of Stoma
5. Essay on the Origin of Stomata
6. Essay on the Types of Stomata
7. Essay on the Functions of Stomata

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Essay # 1. Meaning of Stomata:

Stomata are the microscopic pores on the epidermal surface of aerial parts of higher plants formed by a pair of specialised epidermal cell called guard cells. The special anatomy and diurnal changes of water potential value of these cells control the opening and closing of the pore thus regulating the rate of transpiration and gaseous exchange between the plant and envi­ronment.

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Essay # 2. Occurrence of Stomata:

The stomata are present in all higher plants, pteridophytes and in the capsules of Bryopsida and Anthocerotopsida. In higher plants stomata are present in the epidermis of all aerial parts including the floral parts. They are also loca­ted in the awns, paleas and lemma of grass spikelet and in the skin of banana, pods and other fruits. They are also recorded in the aerial rhi­zomes and on the surfaces of seeds inside the pod.

Non-functional stomata are present in petals, albino plants and in the stems of Orobanche — a root parasitic angiosperm. Stomata are absent in Monotropa, an achlorophyllous saprophytic angiosperm, in the orchid Neottia and in the sub­merged species of Ranunculaceae, Nymphaeaceae, Ceratophyllaceae etc.

Stomata occur most abundantly throughout the surface of lamina except usually the vein areas. When the lamina is very thick, stomata may occur along the veins. Amphistomatous leaf contains stomata on both surfaces with usually ewer stomata on the upper surface.

In Lumnitzera (Combretaceae), however, stomata are more numerous on the upper surface Hypostomatous leaves contain stomata only on the lower surface (e.g., Ficus) whereas the epistomatous leaves contain stomata on the upper surface only (e.g., plants with floating leaves).

In Psilopsida and Sphenopsida stomata are formed in grooves between the ridges of stem surface Stomata are present in the leaves and stems of Lycopsida. In Pteropsida they occur usu­ally on the lower surface of leaves. In Marsilea he aerial leaves are amphistomatous whereas the floating leaves are epistomatous.

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Essay # 3. Position of Stomata:

The stomata may be located in pits (Ammonophilia arenaria), below the epidermal leaf surface (Pinus) or may be sunken (Nerium, Xanthorrhoea) covered by trichomes. The pores of Taxus baccata are surrounded by large epider­mal cell protuberances. In Saxifraga stolonifera stomata are located on raised patches and pro­ject above the level of the leaf surface.

Frequency:

The number of stomata per unit leaf area is called stomatal frequency. In dicot leaves, stomata remain randomly distributed in the epidermis while in monocots and gymnosperms stomata are usually arranged in rows. Stomatal frequency varies depending on the plant types, age and the portion of the leaf.

Environmental factors like CO₂ concentration, temperature and light intensity also affect sto­matal frequency. Generally, in higher stomatal frequency, the stomata are smaller in size. The area occupied by the total stomatal pores may be as much as five percent of the total leaf area, but normally the value remains below two percent.

As the stomatal frequency varies, the term stomatal index was introduced by Salisbury in 1928.

The stomatal index is calculated as:

Stomatal index is found to be constant with­in the leaves of a plant.

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Essay # 4. Structure of Stoma:

The term stoma mean­ing mouth was first coined by deCandole in 1827. The stoma constitutes the pore and two guard cells. The surrounding epidermal cells in association with the guard cells are the sub­sidiary cells and they together constitute the stomatal complex or stomatal apparatus. The stomatal pore opens below to the stomatal cavity or substomatal chamber.

Subsidiary Cell:

These are specialised epi­dermal cells which are distinguished by their shape and size from the guard cells and the rest of the epidermal cells. They are also called accessory cells.

Subsidiary cells are generally smaller in size than the normal epidermal cells. In Equisetum the subsidiary cells overarch the guard cells. In grasses no plasmodesmata or pit were observed between the subsidiary cells and guard cells, but they exist in the leaves of Vicia faba and Nicotiana.

Guard Cell:

Typical stomata of dicots consist of two kidney-shaped or semilunar guard cells. In grasses the guard cells tend to be more elon­gated or dumbbell shaped. Guard cells contain a few chloroplasts, whereas their neighbouring epidermal cells seldom do. There is report that the stoma of Azolla pinnata (Sen, 1983) consists of a unicelled and binucleate guard cell with a pore, and the stoma is associated with one or more subsidiary cell(s) on its proximal side.

Each kidney-shaped guard cell has four sides – thick ventral side facing the pore, thin dorsal side towards the subsidiary cell, upper lateral side facing the atmosphere and the lower lateral side facing the stomatal cavity. The ventral walls of the two guard cells are heavily thickened and are not fused in contrast to dorsal wall.

A pair of heavily cuticularised lips protrudes from the upper edge of the ventral wall to guard the pore. The dumb-bell shaped guard cells of grasses are attached to each other by their thick-walled mid­dle portion and connected by their bulbous thin- walled ends.

The guard cells of Zea mays have thick upper and lower lateral walls while the dor­sal and ventral walls are thin. In most cases the guard cells remain fused with the other epider­mal cells by their walls only. But in Pinus and Equisetum the guard cells are enveloped by sub­sidiary cells.

Usually the guard cell wall is composed of cellulose microfibril and pectin excepting some ferns and gymnosperm where the guard eel walls contain lignin. The guard cell walls of Ophioglossum contain β-1, 3-glucan. The guard cell and subsidiary cell walls of grasses and Equisetum contain silicon.

The cellulose micro­fibrils remain oriented radially in semilunar guard cell called radial micellation. When a guard cell expands by taking up water, it cannot increase much in diameter as the microfibrils do not stretch much along their length.

Therefore because two guard cells are attached to each other at both ends, they bend outward when they swell, which opens the stomata. In the dumb-bell shaped guard cell walls the cellulose microfibrils have the axial arrangement in the middle of the guard cell while the bulbous ends have radial micellation.

The guard cells usually contain chloroplastids The number of chloroplasts per guard cell ranges from two (e.g., Anthoceros) to as many as hundred or more (e.g., Polypodium vulgare). The guard cell chloroplasts are poorly developed with rudimentary grana compared to the mesophyll cell chloroplasts. Generally the plastids of guard cells are laden with starch grains but they lack in many species of Liliaceae, Amaryllidaceae etc.

The guard cells contain dense cytoplasm with single nucleus and all other cell organelles. The guard cell vacuoles are smaller in compari­son to other cells. There are no plasmodesmatal connections between mature guard cell and the surrounding cells. The endomembrane system along with the subcellular particles involved in the synthesis of proteins and cell wall materials — are found to be present in the early stages of development of guard cells.

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Essay # 5. Origin of Stomata:

The stomata originate from the stomatal meristemoid cells of protoderm. These isodiametric cells possess a single conspicuous nucleus with dense cytoplasm. During develop­ment the stomatal meristemoids originate acropetally (e.g., stem of Psilotum), basipetally (e.g., Lycopodiales, Convolvulaceae, Rubiaceae etc.) or simultaneously (e.g., Erythrina).

The diffe­rentiation of a stomatal meristemoids leading to the formation of stomata varies in different plant groups. In Allium cepa an elongated meriste­moid cell of the protoderm divides unequally to produce a large and a small cell. The latter is the guard cell mother cell, which, by longitudinal division, forms two small sister guard cells.

With maturity, the middle lamella between the two sister guard cells are lost enzymatically. As a result, the pore is formed. Gradually, the ventral surfaces of the guard cells become thick and non-elastic.

Due to osmotic entrance of water the guard cells swell in length only caused by radial micellation. The elastic dorsal surface will stretch more than the thick non-elastic ventral side. As a result, the two guard cells gradually separate from each other at the pore region and become reniform.

The number of subsidiary cells in the stom­atal complex varies in different plants. Subsidiary cells are absent in the stomata of Allium. A single meristemoid gives rise to the stoma.

Following are different ontogenetic types of stomata in plants (Pant, 1965):

1. Mesogenous Type:

In this ontogenetic type of stomata the subsidiary cells and guard cells of a stomatal complex are derived from a single meristemoid cell. In Lactuca viscosa the stomatal meristemoid cell have four cutting faces.

There are three anticlinal cutting faces in Beta vulgaris, Datura stramonium, Nicotiana etc. and two parallel cutting faces in Rubiaceae, Casuarina, Gnetum gnemon, Ricinus communis etc. In Schizaea a single annular subsidiary cell originates by a curved periclinal division of the stomatal meristemoid.

2. Perigenous Type:

In this type all the subsidiary cells of a stomatal complex are derived from protoderm cell. The subsidiary cell initials lie adjacent to guard cell mother cell. The guard cell mother cell divides once to produce two guard cells. Lycopodium, Ephedra, Trapa, Cuscuta etc. are the examples of this type.

3. Mesoperigenous Type:

In this develop­mental type the subsidiary cells are derived from different types of mother cells. The guard cell mother cell and one subsidiary cell are formed from a single stomatal meristemoid whereas the other subsidiary cells are derived from the neigh­bouring cells adjacent to guard cell mother cell.

The first division of the meristemoid and the partition of the guard cell mother cell may be:

(i) Parallel (e.g., Tetracentron sinensis), or

(ii) At right angles (e.g, some members of Onagraceae and CaryophyIlaceae), or

(iii) At other angles to each other (e.g., Ranunculus).

There are two ontogenetic stomatal patterns in gymnosperm called haplocheilic and syndetocheilic (Florin, 1933).

Haplocheilic Stomata:

In this type the pro­toderm or stomatal meristemoid cell directly gives rise to guard cells and the neighbouring protoderm cells differentiate into subsidiary cells. Therefore, the subsidiary cells and the guard cells of the stomatal complex are not derived from the same stomatal meristemoid and are termed as perigene (e.g., Cycadales, Pteridospermopsida, Coniferopsida, Ephedra etc.).

Syndetocheilic Stomata:

In syndetocheilic type the protoderm or stomatal meristemoid cells divide to form guard cell and subsidiary mother cells. Thus subsidiary cells are derived from the same stomatal meristemoid that also produces guard cell mother cell and are termed as meso- gene (e.g., Cycadeoidales, Gnetum, Welwitschia etc.).

Later on, it has become evident that the pro­toderm cells cannot form trichomes or stomata directly. They are committed to form the meriste­moid of either trichomes or stomata. So neither a trichome nor a stoma can be differentiated directly from protodermal cells by-passing the meristemoid stage. Thus there are no perigenous stomata instead they are either mesogenous or mesoperigenous.

Payne (1979) classified stomatal complex on the basis of mode of division of guard cell mother cell with respect to the orientation of the walls formed by it. The orientation of the wall that separates the two guard cells may be per­pendicular, parallel or have no special orienta­tion with respect to the wall that cuts the GMC from stomatal meristemoid.

Following three types are recognised:

1. Diameristic (Dia means across and Meros means Part):

In this particular type the in of ontogenetic types of stomata stomatal meristemoid cuts off the GMC at the distal end, which again divides by perpendi­cular wall to the previous wall, i.e. at right angles to the wall that forms the GMC. The sub­sidiary cells may be mesogene (e.g., Lamiaceae) or mesoperegene (e.g., Allium, Caryophyllaceae).

2. Parameristic (Para means beside and Meros means Part):

In this type the stomatal meristemoid cuts off the GMC which then divides by a wall parallel to the preceding one. The subsidiary cells may be mesogene (e.g., Cruciferae, Rabiaceae) or mesoperegene (e.g., Liriodendron).

3. Anomomeristic (Anomalos means Irre­gular and Meros means Part):

In this type the stomatal meristemoid cuts off the GMC that further divides by a wall without any special orientation i.e., at any angle with respect to the previous wall and, thus, the stoma lies at any angle to the wall that forms the GMC. The sub­sidiary cell is usually single and mesogene (e.g., Ranunculaceae).

The stomatal cavity or substomatal chamber underneath each stomatal complex is formed by the natural extension of existing air spaces and dissolution of some of the mesophyll cells.

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Essay # 6. Types of stomata:

The following types of stomata are commonly found in different plant groups:

1. Anomocytic (Irregular) or Ranunculaceous Type:

In this type of stomatal complex the subsidiary cells are indistinguishable in shape and size from the neighbouring epidermal cells and, at the same time, the number and arrange­ment of subsidiary cells may not be definite (e.g., Cucurbita, Capparidaceae, Malvaceae, Papaveraceae etc.).

2. Anisocytic (Unequal) or Cruciferous Type:

These stomata remain encircled by three subsidiary cells of which one is distinctly smaller or larger in size than the other two. The sub­sidiary cells may not be very distinct from ordinary epidermal cells (e.g., Petunia, Solanum, Sedum, Nicotiana etc.).

3. Paracytic (Parallel) or Rubiaceous Type:

This type of stoma usually has two subsidiary cells parallelly oriented to the long axis of guard cells and the stomatal pore (e.g., Phaseolus, Arachis, Psoralea etc.).

4. Hemiparacytic:

There is a single sub­sidiary cell parallely placed to the long axis of the pore in the stomatal complex. This subsidiary cell may be long or short in length in contrast to the guard cells (e.g., Tetracentron).

5. Diacytic (Cross-Walled) or Labiatous or Caryophyllaceous Type:

In this type of stomatal complex there are two large subsidiary cells. The common walls of them remain perpendicular to the long axis of the guard cells as well as that of the stomatal aperture (e.g., Hygrophila, Dianthus etc.).

6. Actinocytic:

In this stomatal complex the two guard cells remain surrounded by four or more radially arranged elongated subsidiary cells (e.g., Ancistrocladus).

7. Cyclocytic:

In this type the stoma remains surrounded by four or more subsidiary cells arranged in a narrow ring around the guard cells (e.g., Lumnitzera, Laguncularia etc.).

8. Tetracytic:

Tetracytic type shows four subsidiary cells surrounding the guard cells. Of them, two subsidiary cells remain on the polar sides and the other two on the lateral sides of the guard cells (e.g., Rhoeo).

9. Hexacytic:

In hexacytic type six sub­sidiary cells surround the guard cells. Two of them are situated on the two polar sides and rest four subsidiary cells are found to occur on the two lateral sides in parallel to the long axis of the guard cells and the aperture.

10. Graminaceous:

Metcalfe and Chalk (1950) described this type of stomata found in monocot leaves — it is the characteristic of the families Gramineae and Cyperaceae. In this type the guard cells are dumb-bell-shaped or osteate whose two attached ends are bulbous and the middle parts are much narrower and straight resembling dumb-bells.

In the narrow portion the cell wall is highly thickened whereas the bulbous ends are relatively thin-walled. The subsidiary cells occur parallel to the long axis of the pore. The subsidiary cells of Zea mays are triangular-shaped.

Stebbins and Khush (1961), however, have reported four types of stomatal complex in monocotyledons:

1. First Type:

In this type the guard cells remain surrounded by 4 subsidiary cells — two at the polar sides and two at the lateral sides giving a square like appearance in surface view (e.g., Tradescantia, Rhoeo, and Zebrina). In Commelina there are six subsidiary cells of which four are situated on the two lateral sides and the rest two at the polar sides of guard cells.

2. Second Type:

In this type also there are 4-6 subsidiary cells. Of them two are round and smaller in size and each lies at each pole. The remaining four are arranged on the lateral sides (e.g., Pandanus haerbachii, members of Palmae and Cyclanthaceae). Caryota and Calamus of Plamae, however, have two subsidiary cells only on the two lateral sides of the guard cells.

3. Third Type:

The two guard cells of a stoma are surrounded by two subsidiary cells each remaining on the lateral sides of guard cells (e.g., Juncus effuses). This is the most predomi­nant type of stomatal complex found in 24 monocot families so far investigated (Gramineae, Cyperaceae, Juncaginaceae, Typhaceae, Liliaceae etc.)

4. Fourth Type:

In this type the stomatal complex is formed without any subsidiary cell. This is also very common type of stomatal com­plex found in the families Amaryllidaceae, Iridaceae, Orchidaceae, Agavaceae etc.

Stomata are usually described as Ranunculaceous, Cruciferous, Rubiaceous and Labiatous or Caryophyllaceous (Vesque, 1889) but it does not literally mean that these types are confined to respective families only rather identical types also occur in very distantly related families.

Therefore, Metcalfe and Chalk (1950) proposed the terms anomocytic, anisocytic, paracytic and diacytic instead of Ranunculaceous, Cruciferous, Rubiaceous and Labiatous or Caryophyllaceous respectively. Usually a particular stomatal type characterises a taxonomic group. But in case of Anopyxis, the same leaf bears paracytic, aniso­cytic and anomocytic stomatal complexes.

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Essay # 7. Functions of Stomata:

Stomata are the gateway through which gaseous exchange takes place between the plants’ interior and the atmosphere. When the stomata remain open CO₂ and O₂ are exchanged with the atmosphere and at the same time water is lost in the form of vapour from internal tissues of the leaf called transpiration.

Carbon dioxide is required during photosynthesis and oxygen is required for respiration. If the photosynthetic by-product oxygen concentration becomes greater than atmosphere it diffuses out from the stomatal cavity to the atmosphere through the stomata.

Pathogens may infect the foliage through stomata. The stomata are very much sensitive to environmental changes. The stomatal size and frequency are reduced in dry and polluted air and these changes can be criteria for determining air pollution. Thus stomatal characterisctics of certain fern taxa can be used as biomonitors of air-pollution.

Salt Glands:

Salt glands store salts which may be exuded by guttation flow. These glands play an important role in the regulation of mineral contents of plants. They occur in the epidermis of several angiospermic families like Plumbaginaceae, Tamariaceae, Acanthaceae, Chenopodiaceae, Primulaceae, and Gramineae etc. They predominantly occur in mangrove plants (e.g., Avicennia, Aegiceras etc.).

In con­trast to epithem hydathodes these glands have no direct connection with the vascular elements and do not possess water pores. Salt glands may be two-celled (e.g., Spartina) or many-celled (e.g., Acanthus, Aegiceras etc.). In Artiplex the salt glands have unicellular or multicellular hairs with head bladder cell.

Function of Epidermis:

1. The thickly cuticularised epidermis gives protection to the inner tissues from different external forces and gives resistance to insect and pathogen attack.

2. Thick cuticle prevents plants from desiccation.

3. The specialised bulliform cells of the epider­mis help in water storage, unrolling of deve­loping leaves and the opening and closing movements of mature leaves.

4. Velamen of orchid root and epiblema with root hairs help in absorption.

5. The epidermis, in some cases, store water and unwanted materials (salt glands) and food.

6. The wax deposition, suberisation, cutinisation etc. are the processes of secretion done by the epidermis.

7. The epidermal cells perform the function of perception of external stimuli and respond accordingly.