The stomata in the plant are the pores that are in the layers of the epidermis. They serve for evaporation of excess water and gas exchange of the flower with the environment.
The first time about them became known in 1675, when naturalist Marcello Malpighi 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.
Then the baton was adopted by a contemporary of Marcello - Nehemiah Grew. He suggested that the significance of stomata in the breathing of plants is similar to the role of trachea in insects, and in some ways his assumption was close to the truth.
In the XIX century there was a long-awaited progress in research. Thanks to Hugo von Mole and Simon Schwendener, the basic principle of stomata work and their classification by type of structure became known.
These discoveries gave a powerful impetus to understanding the functioning of pores, but some aspects of past research are still being studied.
The structure of the sheet
Such parts of plants as the epidermis and stomata refer to the internal arrangement of the leaf, but first we must study its external structure. So, the sheet consists of:
- Sheet plate - a flat and flexible part responsible for photosynthesis, gas exchange, evaporation of water and vegetative reproduction (for certain species).
- The base, in which there is an educational tissue serving for the growth of the plate and petiole. Also with it, the sheet is attached to the stem.
- Stipules - paired formation in the base, which protects the axillary buds.
- The petiolus is a tapering part of the leaf that connects 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 differ slightly depending on its shape and type (simple / complex), but all the above parts are always present.
The internal device includes the epidermis and stomata, as well as various forming tissues and veins. Each of the elements has its own design.
For example, the coverslip of the outer side of the leaf consists of living cells that are different in size and shape. The most superficial of them have a transparency that allows sunlight to penetrate the leaf.
Smaller cells, located somewhat deeper, contain chloroplasts, giving the leaves a green color. Due to their properties, they were called closing ones. Depending on the degree of moistening, they are compressed, then form stomatal gaps between them.
The length of the stomata in the plant varies depending on the type and degree of illumination it receives. The largest pores can reach a size of 1 cm. The stomata are formed by the closing cells that regulate the level of its opening.
The mechanism of their movement is quite complex and varies for different plant species. In most of them - depending on the water supply and the level of chloroplasts - the turgor of cell tissues can both decrease and increase, thereby regulating the opening of the stomata.
Intended use of stomatal cavity
Probably, there is no need to dwell in detail on such an aspect as the function of the sheet. Even a schoolboy knows this. But for what do the stomata answer? Their task is to ensure transpiration (the process of water movement through the plant and its evaporation through external organs such as leaves, stems and flowers), which is achieved by the operation of the closing cells. This mechanism protects the plant from desiccation in hot weather and does not allow the process of decay to commence in conditions of excessive moisture. The principle of its operation is extremely simple: if the amount of liquid in the cells is not high enough, the pressure on the walls decreases, and the stomatal slit closes, keeping the moisture content required for maintaining vital activity.
And on the contrary, its overabundance leads to an increase in pressure and the opening of pores, through which excess moisture evaporates. Due to this, the role of stomata in cooling plants is also great, since the air temperature around it is reduced by transpiration.
Also under the slit there is an air cavity serving for gas exchange. Air penetrates into the plant through the pores, in order to later enter into the process of photosynthesis and respiration. Excess oxygen then exits into the atmosphere through the same stomatal gap. Her presence or absence is often used to classify plants.
The leaf is an external organ by which photosynthesis, respiration, transpiration, guttation and vegetative reproduction are carried out. Moreover, it is able to accumulate moisture and organic matter through stomata, and also to provide the plant with greater adaptability to complex environmental conditions.
Since water is the main intracellular medium, the excretion and circulation of a liquid inside a tree or flower is equally important for its vital activity. In this case, the plant absorbs only 0.2% of all moisture passing through it, the rest is spent on transpiration and gutation, due to which the dissolved mineral salts move and cool down.
Vegetative reproduction often occurs by cutting and rooting the leaves of flowers. Many houseplants are grown in this way, because this is the only way to preserve the purity of the variety.
As it was said earlier, the modified leaves help to adapt to different natural conditions. For example, transformation into spines helps desert plants to reduce evaporation of moisture, tendrils strengthen the functions of the stem, and large sizes often serve to preserve fluid and nutrients where climatic conditions do not allow fuel to be fed regularly.
And this list can be continued indefinitely. It is difficult not to notice that these functions are the same for leaves of flowers and trees.
Which plants do not have stomata?
Since the stomatal slit 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 laminaria and other algae.
The structure of stomata and their work in conifers, ferns, horsetails, flukes and moss-like plants differ from those of flowering plants. Most of them have openings in the daytime and actively participate in gas exchange and transpiration; Except for cacti and succulents, in which the pores are open at night and close with the onset of the morning in order to save moisture in arid regions.
Stem in a plant whose leaves float on the water surface are located only in the upper layer of the epidermis, and in the "sessile" leaves - in the lower layer. In the remaining species, these gaps are present on both sides of the plate.
Location of the stomata
In dicotyledonous plants, stomatal gaps are located on both sides of the leaf plate, but their number in the lower part is somewhat larger than in the upper part. This difference is due to the need to reduce the evaporation of moisture from a well-lit sheet surface.
For monocotyledonous plants there is no specificity regarding the location of the 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 it was said earlier, the stomatal slots are absent from the lower side of the floating leaves, as they absorb moisture through the cuticle, as well as completely water plants, which have no such pores at all.
The stomata of conifers are found deep under the endoderm, which reduces the ability to transpiration.
Also, the location of the pores differs from the surface of the epidermis. The slits can be flush with the rest of the skin cells, go higher or lower, form the right rows or be scattered over the cover tissue chaotically.
In cacti, succulents and other plants, leaves that are absent or altered, transformed into needles, the stomata are located on stems and fleshy parts.
Stem in a plant is divided into many types depending on the location of the accompanying cells:
- Anomocytic - is considered as the most common, where the secondary particles do not differ from others in the epidermis. As one of its simple modifications it is possible to name the latotrocyte type.
- Paracitic - characterized by a parallel abutment of the accompanying cells relative to the stomatal slit.
- Diatite - has only two secondary particles.
- Anisocyte is a type peculiar only to flowering plants, with three accompanying cells, one of which differs markedly in size.
- Tetracitic - is characteristic of monocots, has four accompanying cells.
- Encyclopotent - in it, the secondary particles are joined by a ring around the closing ones.
- Pericyte - for it is characteristic stomata, not connected with the accompanying cell.
- Desmocitic - differs from the previous type only by the presence of cohesion of the gap with the secondary particle.
Here are only the most popular species.
Influence of environmental factors on the external structure of the leaf
For the survival of plants, the degree of adaptability is extremely important. For example, large leaf plates and a large number of stomata are characteristic of wet places, while in dry regions this mechanism acts differently. Neither flowers nor trees differ in size, and the amount of pores is markedly reduced in order 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.