Stomata in Plants

introduction image

Stomata in plants appear as minute pores primarily in the epidermis layer of the leaf surface and also in some of the herbaceous stems. It is originated from the Greek word “Stoma” which means mouth to relate it with the stomatal pore. A stoma is a singular form, whereas more than single stoma termed as stomata, a plural form. A stoma or stomatal pore is flanked by the two guard cells which can relate with the human lips.

Apart from stoma and guard cell, there are some accessory cells which surround the guard cells and controls the movement of the guard cell. Stomata are generally found in the vascular plants.

Content: Stomata in Plants

  1. Definition of stoma
  2. Structure of Stoma
  3. Number of Stomata
  4. Types of Stomata in Plants
  5. Opening and Closing of Stomata
  6. Functions of Stomata in Plants

Definition of stoma

A stoma can define as a tiny aperture generally found in the epidermis layer of the leaves. The number of stomata varies with the plants of different species. In the adaxial leaf surface, the number of stomata are usually less in quantity, and more confined to the abaxial surface of the leaf. Stoma resembles the mouth which opens and close by the movement of guard cell surrounds it that resembles the lips. There is one more specialized or modified epidermal cells adjacent to the guard cell and refers as Subsidiary or Accessory cells.

In plants, stomata are present majorly in the leaves and sometimes in stems, fruits, stamens, petals and gynoecia. A root system lacks the presence of stomata.

Structure of Stoma

Stomata contribute to 1-2% of the leaf area when it is open. It includes the following structural properties:

structure of stoma

Shape: The shape of the stoma is generally “Elliptical” but can vary from plant to plant.

Size: When the stoma is open, it measures a width of 3-12mm and a length of 10-40mm.

Guard cell: There are two specialized guard cells. In dorsiventral leaves, a guard cell possesses a kidney-shape, while in isobilateral leaves possesses a dumb-bell shape. An inner layer of guard cell surrounding the stomata is generally thick-walled and inelastic. While the rest of the outer layer that surrounds a guard cell is thin-walled, flexible and semi-permeable, it consists of a central vacuole, cytoplasmic lining, single nucleus and few chloroplasts. The chloroplasts in guard cell are non-functional or result in reduced photosynthesis as these lacks an enzyme “Rubisco”.

Subsidiary cells: Apart from stoma and guard cells, there are some modified epidermal cells refers to Accessory cells and helps in the movement of the guard cell. It also consists of a cytoplasmic layer, a large central vacuole, single nucleus but lacks chloroplast.

Number of Stomata

The number of stomata can range from 1000-60,000 of stomata in per square centimetre and refers as “Stomatal frequency”. In isobilateral leaves, the number of stomata is approximately the same on both adaxial and abaxial epidermis. While in dorsiventral leaves the stomata are more confined to the adaxial epidermis than the abaxial epidermis.

Types of Stomata in Plants

Stomata in plants can classify into different types based on its location, structure and development.

Based on Location

There are four types of stomata based on its location in the leaves.

types of stamoata based on location

Epistomatic: In this type, stomata are found only in the upper leaf surface.
Example: Waterlily

Heterostamatic: In this type, stomata found in large number on the lower surface of the leaf.
Example: Potato, cabbage etc.

Isostomatic: In this type, stomata found equally in both the lower and upper surface of the leaf.
Example: Oats and other grasses.

Astomatic: In this type, a stoma is absent on both the upper and lower leaf surface.
Example: Potamogeton and submerged aquatic plants.

Based on Development

There are three types of Stamata based on the kind of development.

types of stomata based on development

Mesogynous: It is a type of stomatal development, where the guard cells and the accessory cells develop from the identical or similar mother cell.
Example: Members of the Brassicaceae family.

Perigynous: It is another type of stomatal development, where both the guard cells and the accessory cells develop from the non-identical or different mother cell. A guard cell develops from a mother cell, and the accessory cells develop from the neighbouring cells.
Example: Members of Cucurbitiaceae family.

Mesoperigynous: It is a type of stomatal development, which correlates with both misogynous and Perigynous type. In mesoperigynous, the guard cells and one accessory cell develop from the single mother cell while the other accessory cells may develop independently from the neighbouring cell.
Example: Members of the Brassicaceae family.

Based on the Structure

There are seven types of Stamata based on its structure.

types of stomata based on structure

Anomocytic: This type of stomata also refers as “Ranunculaceous stomata”. Anomocytic stomata are surrounded by the subsidiary cells generally in an irregular fashion and are less in number.
Examples: Members of Ranunculaceae, Malvaceae etc.

Anisocytic: This type of stomata also refers as “Cruciferous stomata”. Anisocytic stomata are surrounded by the subsidiary cells generally unequal in size and are three in number.
Examples: Solanum, Nicotiana species etc.

Paracytic: This type of stomata also refers as “Rubiaceous stomata”. Paracytic stomata are surrounded by the subsidiary cells generally two in number and parallel to each other along the axis of pore and guard cell.
Examples: Solanum, Nicotiana species etc.

Diacytic: This type of stomata also refers as “Caryophyllaceous stomata”. Diacytic stomata are surrounded generally by the pairs of subsidiary cells and to the 90Degrees of guard cell.
Examples: Acanthacea, Mucaceae species etc.

Actinocytic: This type of stomata are surrounded by the four or more subsidiary cells which form a radial arrangement towards the centre of a stoma.
Examples: Araceacea, Mucaceae species etc.

Cyclocytic: This type of stomata comprises of four or more subsidiary cells surrounds the guard cell, as radially arranged narrow rings.
Examples: Palmae, Pandanus etc.

Graminaceous: In this type, the guard cells are dumbbell-shaped and the accessory cells surrounding it, lie parallel to each other along the longitudinal axis of stomatal pore.
Example: Grameneaceae, Cyperaceae etc.

Opening and Closing of Stomata

The opening and closing activity of stomata are mediated by the two guard cells, to maintain the plant’s water balance and to access CO2. This event of opening and closing depends upon the solute concentration of the guard cells.

opening and closing of stomata

A stoma is opened: It occurs when the stomata have a high water potential. There must be an osmotic movement of solute from the area of high concentration (Surrounding) to the area of low concentration (Guard cell). This osmotic movement makes the guard cell turgid, where it swells to open the stomatal pore to influx CO2 into and water vapours and oxygen out of it.

A stoma is closed: It occurs when the stomata have low water potential. There must be an osmotic movement of solute from the area of low concentration (Surrounding) to the area of high concentration (Guard cell). This osmotic movement makes the guard cell flaccid, where it shrinks to close the stomatal pore to retain the water.

Factors like low and water concentration, low CO2 content, high temperature cause an opening of stomata. While, factors like mechanical stress, low temperature, insufficient light cause closing of stomata.

Functions of Stomata in Plants

Stomata perform two significant roles in a plant like:

  • An influx of carbon dioxide for the photosynthesis in plants.
  • To maintain the water balance in a plant cell.

Stoma opens during the day time when the photosynthesis occurs in the presence of sunlight. As we need fuel for the cooking of food, plants also need carbon dioxide as fuel to prepare food for itself. Therefore, stomata play an essential role in the process of photosynthesis.

Stoma opens or closes as per its water need. On dehydration of the plant cell, a stoma closes to retain the water, and when there is an excess of water, it releases out in the form of water vapours and oxygen. Thus, in this way, a stoma functions to maintain the cell turgidity by maintaining the water potential. Therefore, the stomata provide strength to the plant cell by taking part in photosynthesis.

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