The stomata of a plant are pores located in the layers of the epidermis. They serve to evaporate excess water and gas exchange of the flower with the environment.
The first time they became known in 1675, when the naturalist Marcello Malpigi published his discovery in the work of Anatome plantarum. However, he could not unravel their real purpose, which served as an impetus for the development of further hypotheses and research.
Study history
Further, the baton was taken by Marcello's contemporary - Nehemiah Gru. He suggested that the value of stomata in plant respiration is similar to the role of trachea in insects, and in some ways his assumption was close to the truth.
In the XIX century, the long-awaited progress in research came. Thanks to Hugo von Molle and Simon Schwendener, the basic principle of the operation of stomata and their classification by type of structure became known.
These discoveries gave a powerful impetus to understanding the functioning of pores, however, some aspects of past studies continue to be studied until now.
Leaf structure
Parts of plants, such as the epidermis and stomata, belong to the internal structure of the leaf, but first examine its external structure. So, the sheet consists of:
- A leaf plate is a flat and flexible part responsible for photosynthesis, gas exchange, water evaporation and vegetative propagation (for certain species).
- The base, in which the educational tissue is located , which serves for the growth of the plate and petiole. Also, with its help, the leaf is attached to the stem.
- Stipules is a paired formation at the base that protects the axillary kidneys.
- Petiole is the tapering part of the leaf connecting the plate to the stem. He is responsible for vital functions: orientation to light and growth through educational tissue.
The external structure of the sheet may vary slightly depending on its shape and type (simple / complex), but all the parts listed above are always present.
The internal structure includes the epidermis and stomata, as well as various forming tissues and veins. Each of the elements has its own design.
For example, the integumentary tissue on the outside of the leaf consists of living cells that are different in size and shape. The most superficial of them have transparency, allowing sunlight to penetrate into the sheet.
Smaller cells, located somewhat deeper, contain chloroplasts, which give the leaves a green color. Due to their properties, they were called closing. Depending on the degree of hydration, they either shrink or form stomatal gaps between themselves.
Structure
The stomata length of a plant varies depending on the type and degree of illumination it receives. The largest pores can reach a size of 1 cm. The stomata form closing cells that regulate the level of its opening.
The mechanism of their movement is quite complicated and varies for different plant species. In most of them - depending on the water supply and the level of chloroplasts - the cell tissue turgor can both decrease and increase, thereby regulating the opening of the stomata.
The purpose of the stomatal fissure
Probably, there is no need to dwell on such an aspect as the functions of the sheet. Even a schoolboy knows about this. But what are stomata responsible for? Their task is to ensure transpiration (the process of water moving through a plant and its evaporation through external organs, such as leaves, stems and flowers), which is achieved through the work of trailing cells. This mechanism protects the plant from drying out in hot weather and does not allow the process of decay to begin in conditions of excessive humidity. The principle of its operation is extremely simple: if the amount of fluid in the cells is not high enough, the pressure on the walls drops, and the stomatal gap closes, while maintaining the moisture content required to maintain vital activity.
On the contrary, its oversupply leads to increased pressure and the opening of pores through which excess moisture evaporates. Due to this, the role of stomata in plant cooling is also great, since the air temperature around it decreases precisely through transpiration.
Also under the slit is an air cavity serving for gas exchange. Air enters the plant through the pores in order to subsequently enter the process of photosynthesis and respiration. Excess oxygen is then released into the atmosphere through the same stomatal gap. Moreover, its presence or absence is often used to classify plants.
Sheet functions
The leaf is an external organ through which photosynthesis, respiration, transpiration, gutting and vegetative propagation are performed. Moreover, it is able to accumulate moisture and organic matter through stomata, as well as provide the plant with greater adaptability to difficult environmental conditions.
Since water is the main intracellular environment, the excretion and circulation of fluid inside a tree or flower is equally important for its life. In this case, the plant absorbs only 0.2% of all moisture passing through it, the rest goes to transpiration and gutation, due to which the movement of dissolved mineral salts and cooling takes place.
Vegetative propagation often occurs through cutting and rooting of flower leaves. Many houseplants are grown in this way, since this is the only way to maintain the purity of the variety.
As mentioned earlier, modified leaves help to adapt to various environmental conditions. For example, transformation into thorns helps desert plants reduce moisture evaporation, tendrils enhance the function of the stem, and large sizes often serve to preserve fluid and nutrients where climatic conditions do not allow to replenish stocks regularly.
And the list goes on and on. It is difficult not to notice that these functions are the same for the leaves of flowers and trees.
Which plants do not have stomata?
Since the stomatal gap is characteristic of higher plants, it is present in all species, and it is erroneous to consider it absent, even if the tree or flower has no leaves. The only exception to the rule is kelp and other algae.
The structure of stomata and their work in conifers, ferns, horsetails, floaters and bryophytes are different from those in flowering plants. Most of them have openings during the day and are actively involved in gas exchange and transpiration; the exception is cacti and succulents, in which the pores are wide open at night and close with the onset of morning in order to save moisture in arid regions.
The stomata of a plant whose leaves float on the surface of the water are located only in the upper layer of the epidermis, and for "sessile" leaves in the lower. In other varieties, these gaps are present on both sides of the plate.
Location stomata
In dicotyledonous plants, stomatal fissures are located on both sides of the leaf plate, however, their number in the lower part is somewhat larger than in the upper. This difference is due to the need to reduce moisture evaporation from a well-lit sheet surface.
For monocotyledonous plants, there is no specificity regarding the location of stomata, since it depends on the direction of growth of the plates. For example, the epidermis of leaves of plants oriented vertically contains the same number of pores in both the upper and lower layers.
As mentioned earlier, stomatal fissures are absent on floating leaves on the underside, since they absorb moisture through the cuticle, as are completely aquatic plants, which do not have such pores at all.
The stomata of conifers are located deep beneath the endoderm, which reduces the ability to transpire.
Also, the location of the pores varies relative to the surface of the epidermis. The gaps can be flush with other “skin” cells, go higher or lower, form regular rows or be scattered randomly on the integumentary tissue.
In cacti, succulents and other plants, the leaves of which are absent or altered, transformed into needles, the stomata are located on the stems and fleshy parts.
Types
The stomata of a plant are divided into many types depending on the location of the accompanying cells:
- Anomocytic - is considered as the most common, where side particles do not differ from others located in the epidermis. As one of its simple modifications, the laterocyte type can be called.
- Paracitic - characterized by a parallel abutment of the accompanying cells relative to the stomatal gap.
- Diacytic - has only two side particles.
- Anisocytic is a type inherent only in flowering plants, with three accompanying cells, one of which is noticeably different in size.
- Tetracitic - characteristic of monocotyledons, has four accompanying cells.
- Encyclocytic - in it, secondary particles are closed by a ring around the closers.
- Pericidal - it is characterized by stomata, not connected to the accompanying cell.
- Desmocytic - differs from the previous type only in the presence of adhesion of the gap with the secondary particle.
Here are only the most popular species.
The influence of environmental factors on the external structure of the sheet
For the survival of the plant, the degree of its adaptability is extremely important. For example, wet places are characterized by large leaf plates and a large number of stomata, while in arid regions this mechanism acts differently. Neither flowers nor trees differ in size, and the number of pores is markedly reduced to prevent excessive evaporation.
Thus, it is possible to trace how parts of plants under the influence of the environment change over time, which affects the number of stomata.