Stoma

Stoma . They are the main participants in photosynthesis , since the mechanical gas exchange takes place through them, that is to say that oxygen (O2) leaves here and carbon dioxide (CO2) enters .

Table of Contents

Summary

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1 Location
2 Functions
3 Formation and cellular origin
4 Layout
5 Structure
6 Mechanism for opening and closing stomata
7 Classification
- 1 According to the number and arrangement of attached cells
- 2 According to its origin and development
8 Sequence of appearance of stomata on the leaf blade
9 Sources

Location

They are found in the green aerial parts of the plant, particularly in the leaves , where they can be found in one or both epidermis , more frequently in the lower part. Their number ranges from 22 to 2,230 per mm2.

The roots do not have stomata. Parasitic plants without chlorophyll like Monotropa and Neottia do not have stomata (Orobanche has them on stem). The aerial parts without chlorophyll (variegated leaves) can have stomata but are not functional ( Louguet 1990), like those of the petals .

Features

They are used for gas exchange with the medium. Oxygen and carbon dioxide are exchanged with the environment through these pores. The acquisition of carbon dioxide and the exchange of oxygen are essential for the photosynthesis and respiration processes of plants to develop. However, its opening also causes the loss of water from the plant in the form of steam through the process called transpiration.

For this reason, the opening or closing of the stomata is very finely regulated in the plant by environmental factors such as light, the concentration of carbon dioxide or the availability of water. According to research, it is known that some cations such as potassium and calcium and anions such as chloride are actively involved in the opening and closing of stomata. In cases of drought (water stress) the stomata close, preventing water losses in the plant, which, however, also makes it impossible to exchange gases and, consequently, the entry of atmospheric CO2 necessary for nutrition of plants through the photosynthesis process. That is why in xerophilic regions, stomata are often small or almost non-existent, and also contain appreciable amounts of waxes, hairs and trichomes, which make it difficult for water vapor to escape.

Formation and cellular origin

The origin of stomata begins when a meristematic stem cell undergoes an asymmetric division, giving rise to a meristemoid and a larger sister cell. In turn, the meristemoid can continue making asymmetric divisions or it can differentiate until it becomes a stem cell. Finally, the stem guard cell undergoes symmetrical division and differentiates into a pair of mature guard cells around a stomatal pore.

Provision

The parallel- lined leaves of monocotyledons , some dicotyledons, and the acicular ones of Coniferae have stomata arranged in parallel rows; in dicotyledons with reticulated venation leaves they are scattered. In mesophytes the stomata are arranged at the same level as the fundamental cells, but in many gymnosperms and in leaves of xerophytic plants , the stomata are sunken and as if suspended from the attached cells that form a vault , or hidden in crypts. In plants with humid environments the stomata are elevated.

Structure

Each stoma is made up of 2 specialized cells called occlusives that leave an opening between them called the ostiole or pore. In many plants there are 2 or more cells adjacent to the occlusives and functionally associated with them. These cells, morphologically different from the fundamental ones, are called adnexal, subsidiary or adjunct cells.

The ostiole leads to a wide intercellular space called the sub-stoma chamber, putting the intercellular space system in communication with the outside air. When the stomata are in rows, the stomatal chambers connect to each other.

Opening and closing mechanism of stomata

Mnium type , mosses and Pteridophyta : It is the most primitive. The walls that limit the opening are thin, and the rest are thick. As the turgor increases, the tangential walls separate and the wall that limits the opening straightens, enlarging the ostiole. The movement is perpendicular to the surface.
Gramineae type , Cyperaceae : As the turgidity increases, the bulbous portions swell and the middle portions separate. The movement is parallel to the surface.
Helleborus type , in dicotyledons and many monocots: It is the most common. With greater turgor, the dorsal wall expands into the adjoining, separating the walls that limit the opening. The movement is combined, parallel and perpendicular to the surface.

There are stomata so modified that they always remain open. In desert plants in late summer the occlusive cells are cutaneous and the walls thicken to the point that they probably remain closed during the critical drought period ( Fahn , 1985 ).

Classification

According to the number and disposition of attached cells

Anomocytic or Ranunculáceo : without attached cells; it is the most frequent in dicotyledons and also the oldest. In monocotyledons: Amaryllidaceae , Dioscoreaceae .
Paracytic or Rubiaceous : with 2 attached cells, arranged in parallel with respect to the stops.
Anisocytic or Cruciferous . With 3 attached cells, 1 smaller. Also in Solanaceae .
Tetracytic : 4 subsidiary cells. Common in several monocot families such as Araceae , Commelinaceae , Musaceae .
Diacítico or Cariofiláceo : 2 related cells perpendicular to plosives. Few families, Cariofiláceas, Acantáceas.
Cyclocytic : numerous subsidiary cells, arranged in one or two circles around the subsidiary cells.
Helicocytic : with several subsidiary cells spirally arranged around the stops.

According to its origin and development

The protodérmicas cells undergo asymmetric divisions, the smallest cell is the parent cell, which divides producing the guard cells. These acquire their characteristic shape with increasing size. The ostiole is formed by swelling and subsequent dissolution of the middle lamella. The substoma chamber by enlarging the intercellular spaces of the mesophyll. The position of the stomata above or below the epidermis takes place through spatial adjustments.

Three types of stomata are distinguished, taking into account the origin of all the cells that make up the stomatal apparatus.

Mesogenic stoma: the occlusive cells and the attached cells originate from the same cell by 3 successive divisions. This type of stoma is not found in monocots.
Perigenic stoma. The stem cell originates only the occlusive cells, the attached cells originate from other protodermal cells. They have been recorded in species of all vascular plant groups.
Mesoperigenic stoma: the stem cell originates the occlusive cells and an adjoining cell, while the rest originate from other protodermal cells. These stomata have been found in all vascular plant groups.

Sequence of appearance of stomata on the leaf blade

In leaves with parallel venation and in the internodes of articulated species of Chenopodiaceae , the sequence of differentiation of the stomata is basipetal, from the end towards the base. On the other hand, in leaves with reticulated venation, it does not follow a certain order, but occurs in mosaic.