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.

They first became known in 1675, when the naturalist Marcello Malpighi published his discovery in Anatome plantarum. However, he was unable to unravel their real purpose, which served as an impetus for the development of further hypotheses and research.

History of study

In the 19th century, the long-awaited progress in research came. Thanks to Hugo von Mol and Simon Schwendener, the basic principle of the stomata and their classification according to the type of structure became known.

These discoveries gave a powerful impetus to understanding the functioning of the pores, but some aspects of past research continue to be studied to this day.

leaf structure

Plant parts such as the epidermis and stomata are internal device sheet, but first you should study it external structure. So the sheet is:

• Leaf plate - a flat and flexible part responsible for photosynthesis, gas exchange, water evaporation and vegetative reproduction(for certain types).
• The base in which the plate and petiole serve for growth. Also, with its help, the leaf is attached to the stem.
• Stipule - a paired formation at the base that protects the axillary buds.
• The petiole is the tapering part of the leaf that connects the blade to the stem. It is responsible for vital functions: orientation to light and growth through the educational tissue.

The external structure of a leaf may vary somewhat depending on its shape and type (simple/complex), but all of 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, outside The leaf consists of living cells, different in size and shape. The most superficial of them have a transparency that allows sunlight to penetrate the inside of the leaf.

Smaller cells, located somewhat deeper, contain chloroplasts, which give the leaves green color. Due to their properties, they were called closing. Depending on the degree of moisture, they either shrink or form stomatal gaps between them.

Structure

The length of a plant's stomata varies depending on the species and the amount of light it receives. The largest pores can reach a size of 1 cm. They form the guard cells of the stomata, which 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.

The purpose of the stomatal opening

Probably, there is no need to dwell on such an aspect as the functions of the sheet. Even a student knows about it. But what are the stomata responsible for? Their task is to ensure transpiration (the process of movement of water through the plant and its evaporation through external organs such as leaves, stems and flowers), which is achieved through the work of guard 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 opening closes, maintaining the moisture content required to maintain life.

Conversely, its excess leads to increased 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 is reduced precisely through transpiration.

Also under the slot is an air cavity that serves for gas exchange. Air enters the plant through the pores to further enter into 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 by which photosynthesis, respiration, transpiration, guttation and vegetative reproduction are carried out. 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 medium, the excretion and circulation of fluid inside a tree or flower is equally important for its life. At the same time, the plant absorbs only 0.2% of all moisture passing through it, while the rest goes to transpiration and guttation, due to which the movement of dissolved mineral salts and cooling occurs.

Vegetative propagation often occurs by cutting and rooting the leaves of flowers. Many houseplants grown In a similar way, because this is the only way to preserve the purity of the variety.

As mentioned earlier, they help to adapt to various natural conditions. For example, transformation into thorns helps desert plants reduce moisture evaporation, tendrils enhance stem functions, and big sizes often serve to preserve liquid and nutrients where climatic conditions do not allow replenishing stocks regularly.

And this list is endless. 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 opening is characteristic of higher plants, it is present in all species, and it is a mistake 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 the stomata and their work in conifers, ferns, horsetails, and swimmers differ from those in flowering ones. In most of them, the slits are open during the day and actively participate in gas exchange and transpiration; the exceptions are cacti and succulents, in which the pores are open at night and close in the morning to conserve moisture in dry regions.

The stomata of a plant whose leaves float on the surface of the water are located only in top layer epidermis, and in "sessile" leaves - in the lower. In other varieties, these gaps are present on both sides of the plate.

Stomatal location

The stomatal gaps are located on both sides of the leaf plate, however, their number in the lower part is slightly larger than in the upper one. This difference is due to the need to reduce the evaporation of moisture from a well-lit leaf surface.

For monocot 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 plant leaves oriented vertically contains the same number of pores in both the upper and lower layers.

As mentioned earlier, floating leaves do not have stomatal openings on the underside, since they absorb moisture through the cuticle, as well as completely aquatic plants, which have no such pores at all.

stomata coniferous trees located deep under the endoderm, which contributes to a decrease in the ability to transpiration.

Also, the location of the pores differs relative to the surface of the epidermis. The gaps can be flush with the rest of the "skin" cells, go higher or lower, form regular rows, or be randomly scattered over the integumentary tissue.

In cacti, succulents and other plants, the leaves of which are absent or modified, transforming into needles, the stomata are located on the stems and fleshy parts.

Types

Stomata in 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 in the epidermis. As one of its simple modifications, the laterocyte type can be called.
• Paracytic - characterized by a parallel adjunction of accompanying cells relative to the stomatal gap.
• Diacite - has only two side particles.
• Anisocytic - a type inherent only in flowering plants, with three accompanying cells, one of which differs markedly in size.
• Tetracytic - characteristic of monocots, has four accompanying cells.
• Encyclocytic - in it, side particles close in a ring around the trailing ones.
• Pericytic - it is characterized by a stomata that is not connected to the accompanying cell.
• Desmocytic - differs from the previous type only in the presence of adhesion of the gap with the side particle.

Here are just the most popular types.

Influence of environmental factors on the external structure of the leaf

For the survival of a plant, its degree of adaptability is extremely important. For example, for wet places large sheet plates and a large number of stomata, while in arid regions this mechanism operates differently. Neither flowers nor trees differ in size, and the number of pores is noticeably reduced to prevent excessive evaporation.

Thus, it is possible to trace how parts of plants change over time under the influence of the environment, which also affects the number of stomata.

There are three types of reactions of the stomatal apparatus to environmental conditions:

1. hydropassive reaction- this is the closing of the stomatal fissures, caused by the fact that the surrounding parenchymal cells are overflowing with water and mechanically compress the guard cells. As a result of compression, the stomata cannot open and the stomatal gap does not form. Hydropassive movements are usually observed after heavy irrigation and can cause inhibition of the photosynthesis process.

2. Hydroactive reaction opening and closing are movements caused by a change in the water content of the guard cells of the stomata. The mechanism of these movements is discussed above.

3. photoactive reaction. Photoactive movements are manifested in the opening of stomata in the light and closing in the dark. Of particular importance are red and blue rays, which are most effective in the process of photosynthesis. This is of great adaptive importance, because due to the opening of stomata in the light, CO 2 diffuses to the chloroplasts, which is necessary for photosynthesis.

The mechanism of photoactive movements of stomata is not entirely clear. Light has an indirect effect through a change in the concentration of CO 2 in the guard cells of the stomata. If the concentration of CO 2 in the intercellular spaces falls below a certain value (this value depends on the plant species), the stomata open. When the concentration of CO 2 increases, the stomata close. In the guard cells of the stomata there are always chloroplasts and photosynthesis occurs. In the light, CO 2 is assimilated in the process of photosynthesis, its content decreases. According to the hypothesis of the Canadian physiologist W. Skars, CO 2 affects the degree of stomata openness through a change in pH in guard cells. A decrease in the content of CO 2 leads to an increase in the pH value (a shift to the alkaline side). On the contrary, darkness causes an increase in CO 2 (due to the fact that CO 2 is released during respiration and is not used in the process of photosynthesis) and a decrease in pH (shift to the acid side). Changing the pH value leads to a change in the activity of enzyme systems. In particular, a shift in the pH value to the alkaline side increases the activity of enzymes involved in the breakdown of starch, while a shift to the acid side increases the activity of enzymes involved in the synthesis of starch. The breakdown of starch into sugars causes an increase in the concentration of dissolved substances, in connection with this, the osmotic potential and, as a result, the water potential become more negative. In the guard cells, water begins to flow intensively from the surrounding parenchymal cells. The stomata open. Opposite Changes occur when processes shift towards starch synthesis. However, this is not the only explanation. It was shown that the guard cells of the stomata contain significantly more potassium in the light compared to the dark. It has been established that the amount of potassium in the guard cells increases by 4-20 times when the stomata open, while this indicator decreases in the accompanying cells. There is a redistribution of potassium. When the stomata open, there is a significant gradient membrane potential between trailing and accompanying cells (I.I. Gunar, L.A. Panichkin). The addition of ATP to the epidermis floating on the KC1 solution increases the rate of stomata opening in the light. An increase in the ATP content in the guard cells of the stomata during their opening was also shown (S.A. Kubichik). It can be assumed that ATP, formed during photosynthetic phosphorylation in guard cells, is used to enhance the intake of potassium. This is due to the activity of H + -ATPase. Activation of the H + -pump promotes the release of H + from guard cells. This leads to transport along the K+ electrical gradient into the cytoplasm and then into the vacuole. Increased intake of K +, in turn, promotes the transport of C1 - along the electrochemical gradient. Osmotic concentration increases. In other cases, the intake of K + is balanced not by C1 -, but by malic acid salts (malates), which are formed in the cell in response to a decrease in pH as a result of the release of H +. The accumulation of osmotically active substances in the vacuole (K + , C1 - , malates) reduces the osmotic, and then the water potential of the guard cells of the stomata. Water enters the vacuole and the stomata open. In the dark, K + is transported from a certain value (this value depends on the type of plant), the stomata open. When the concentration of CO 2 increases, the stomata close. In the guard cells of the stomata there are always chloroplasts and photosynthesis occurs. In the light, CO 2 is assimilated in the process of photosynthesis, its content decreases. According to the hypothesis of the Canadian physiologist W. Skars, CO 2 affects the degree of stomata openness through a change in pH in guard cells. A decrease in the content of CO 2 leads to an increase in the pH value (a shift to the alkaline side). On the contrary, darkness causes an increase in CO 2 (due to the fact that CO 2 is released during respiration and is not used in the process of photosynthesis) and a decrease in pH (shift to the acid side). Changing the pH value leads to a change in the activity of enzyme systems. In particular, a shift in the pH value to the alkaline side increases the activity of enzymes involved in the breakdown of starch, while a shift to the acid side increases the activity of enzymes involved in the synthesis of starch. The breakdown of starch into sugars causes an increase in the concentration of dissolved substances, in connection with this, the osmotic potential and, as a result, the water potential become more negative. In the guard cells, water begins to flow intensively from the surrounding parenchymal cells. The stomata open. The opposite changes occur when processes shift towards starch synthesis. However, this is not the only explanation. It was shown that the guard cells of the stomata contain significantly more potassium in the light compared to the dark. It has been established that the amount of potassium in the guard cells increases by 4-20 times when the stomata open, while this indicator decreases in the accompanying cells. There is a redistribution of potassium. When the stomata open, a significant gradient of the membrane potential arises between guard and accompanying cells (I.I. Gunar, L.A. Panichkin). The addition of ATP to the epidermis floating on the KC1 solution increases the rate of stomata opening in the light. An increase in the ATP content in the guard cells of the stomata during their opening was also shown (S.A. Kubichik). It can be assumed that ATP formed in the process of photosynthetic phosphorylation in the guard cells is used to enhance the intake of potassium. This is due to the activity of H + -ATPase. Activation of the H + -pump promotes the release of H + from guard cells. This leads to transport along the K+ electrical gradient into the cytoplasm and then into the vacuole. Increased intake of K +, in turn, promotes the transport of C1 - along the electrochemical gradient. Osmotic concentration increases. In other cases, the intake of K + is balanced not by C1 -, but by malic acid salts (malates), which are formed in the cell in response to a decrease in pH as a result of the release of H +. The accumulation of osmotically active substances in the vacuole (K + , C1 - , malates) reduces the osmotic, and then the water potential of the guard cells of the stomata. Water enters the vacuole and the stomata open. In the dark, K+ is transported from the guard cells to the surrounding cells, and the stomata close. These processes are presented in the form of a diagram:

Stomatal movements are regulated by plant hormones (phytohormones). The opening of the stomata is prevented, and the closing is stimulated by the phytohormone - abscisic acid (ABA). It is interesting in this regard that ABA inhibits the synthesis of enzymes involved in the breakdown of starch. There is evidence that under the influence of abscisic acid, the ATP content decreases. At the same time, ABA reduces the intake of K +, possibly due to a decrease in the output of H + ions (inhibition of the H + pump). The role of other phytohormones, cytokinins, in the regulation of stomata opening by enhancing K+ transport to stomatal guard cells and activating H+-ATPase is discussed.

The movement of stomatal cells turned out to be temperature dependent. Studies of a number of plants have shown that stomata do not open at temperatures below 0°C. An increase in temperature above 30°C causes the stomata to close. Perhaps this is due to an increase in the concentration of CO 2 as a result of an increase in the intensity of respiration. However, there are observations that different varieties wheat stomata response to elevated temperature different. Long term exposure high temperature damages the stomata, in some cases so severely that they lose their ability to open and close.

Observations of the degree of openness of the stomata have great importance in physiological and agronomic practice. They help to establish the need to supply the plant with water. Closing of the stomata already speaks of unfavorable shifts in water metabolism and, as a result, of difficulties in feeding plants with carbon dioxide.

Of particular importance in the life of a plant are stomata related to the epidermal tissue system. The structure of the stomata is so peculiar and their significance is so great that they should be considered separately.

The physiological significance of the epidermal tissue has a dual, largely contradictory character. On the one hand, the epidermis is structurally adapted to protect the plant from drying out, which is facilitated by the tight closure of epidermal cells, the formation of a cuticle and relatively long covering hairs. But on the other hand, the epidermis must pass through itself the masses of water vapor and various gases rushing in mutually opposite directions. Gas and vapor exchange under some circumstances can be very intense. AT plant organism this contradiction is successfully resolved with the help of stomata. The stoma consists of two peculiarly altered epidermal cells, interconnected by opposite (along their length) ends and called guard cells. The intercellular space between them is called stomatal gap.

Guard cells are so called because they change their shape by active periodic changes in turgor in such a way that the stomatal opening alternately opens and closes. The following two features are of great importance for these stomatal movements. First, the guard cells, unlike the rest of the cells of the epidermis, contain chloroplasts, in which photosynthesis occurs in the light and sugar is formed. The accumulation of sugar as an osmotically active substance causes a change in the turgor pressure of the guard cells in comparison with other cells of the epidermis. Secondly, the shells of guard cells thicken unevenly, therefore, a change in turgor pressure causes an uneven change in the volume of these cells, and, consequently, a change in their shape. A change in the shape of the guard cells causes a change in the width of the stomatal opening. Let's explain this with the following example. The figure shows one of the types of stomata dicot plants. The outermost part of the stomata is made up of membranous protrusions formed by the cuticle, sometimes insignificant, and sometimes quite significant. They restrict from the outer surface small space, the lower border of which is the stomata gap itself, which is called front patio stomata. Behind the slit of the stomata, inside, there is another small space, delimited by small internal protrusions of the side walls of the guard cells, called patio stomata. Patio directly opens into a large intercellular space called air cavity.

In the light, sugar is formed in the guard cells, it draws water from neighboring cells, the turgor of the guard cells increases, thin places of their membrane stretch more than thick ones. Therefore, the convex protrusions protruding into the stomata gap become flat and the stoma opens. White sugar, for example, turns into starch at night, then the turgor in the guard cells falls, this causes a weakening of the stretching of the thin sections of the membrane, they protrude towards each other and the stoma closes. At different plants The mechanism of closing and opening of the stomata gap can be different. For example, in grasses and sedges, guard cells have widened ends and narrowed in the middle. The membranes in the middle parts of the cells are thickened, while their expanded ends retain thin cellulose membranes. An increase in turgor causes swelling of the ends of the cells and, as a result, the separation of the direct median parts from each other. This leads to the opening of the stomata.

Features in the mechanism of operation of the stomatal apparatus are created both by the shape and structure of the guard cells, and by the participation in it of the epidermal cells adjacent to the stomata. If the cells directly adjacent to the stomata differ in their appearance from other cells of the epidermis, they are called accompanying cells of the stomata.

Most often, accompanying and trailing cells have a common origin.

The guard cells of the stomata are either slightly elevated above the surface of the epidermis, or, conversely, lowered into more or less deep pits. Depending on the position of the guard cells in relation to the general level of the epidermal surface, the very mechanism for adjusting the width of the stomatal fissure somewhat changes. Sometimes the guard cells of the stomata become lignified, and then the regulation of the opening of the stomatal fissure is determined by the activity of neighboring epidermal cells. Expanding and shrinking, i.e., changing their volume, they entrain the guard cells adjacent to them. However, often stomata with lignified guard cells do not close at all. In such cases, the regulation of the intensity of gas and vapor exchange is carried out differently (by the so-called incipient drying). In stomata with lignified guard cells, the cuticle often covers with a fairly thick layer not only the entire stomatal opening, but even extends to the air cavity, lining its bottom.

Most plants have stomata on both sides of the leaf or only on the underside. But there are also plants in which stomata are formed only on the upper side of the leaf (on leaves floating on the surface of the water). As a rule, there are more stomata on leaves than on green stems.

The number of stomata on the leaves of different plants is very different. For example, the number of stomata on the underside of a awnless bonfire leaf is on average 30 per 1 mm 2, in a sunflower growing under the same conditions - about 250. Some plants have up to 1300 stomata per 1 mm 2.

In specimens of the same plant species, the density and size of stomata in strong degree depend on environmental conditions. For example, on the leaves of a sunflower grown in full light, there were an average of 220 stomata per 1 mm 2 of the leaf surface, and in a specimen grown next to the first, but with slight shading, about 140. On one plant grown in full light, the density stomata increases from lower leaves to the top.

The number and size of stomata strongly depend not only on the growing conditions of the plant, but also on the internal relationships of life processes in the plant itself. These values ​​(coefficients) are the most sensitive reagents for each combination of factors that determine the growth of a plant. Therefore, the determination of the density and size of the stomata of the leaves of plants grown in various conditions, gives some idea of ​​the nature of the relationship of each plant with its environment. All methods for determining the size and number of anatomical elements in one or another organ belong to the category of quantitative-anatomical methods, which are sometimes used in environmental studies, as well as to characterize the varieties of cultivated plants, since each variety of any cultivated plant there are certain limits on the size and number of anatomical elements per unit area. The methods of quantitative anatomy can be applied with great benefit both in crop production and in ecology.

Along with stomata intended for gas and vapor exchange, there are also stomata through which water is released not in the form of vapor, but in a drop-liquid state. Sometimes such stomata are quite similar to ordinary ones, only somewhat larger than them, and their guard cells are devoid of mobility. Quite often, in a fully mature state, such a stomata lacks guard cells and only a hole remains, bringing water out. Stomata that secrete liquid water are called water, and all formations involved in the release of drop-liquid water - hydathodes.

The structure of hydathodes is varied. Some hydathodes have a parenchyma under the opening that removes water, which is involved in the transfer of water from the water supply system and in its release from the organ; in other hydathodes, the plumbing system goes directly to the outlet. Hydathodes are especially often formed on the first leaves of seedlings of various plants. So, in humid and warm weather, young leaves of cereals, peas and many meadow grasses drop by drop release water. This phenomenon can be observed in the first half of summer in the early morning of every fine day.

The most well-defined hydathodes are located along the edges of the leaves. Often, one or more hydathodes are borne by each of the teeth that turn off the edges of the leaves.

Lesson " Cell structure sheet"

Target: show the relationship between the leaf structure and its functions; develop the concept of the cellular structure of plants; continue building skills independent work with instruments, the ability to observe, compare, contrast, draw their own conclusions; develop love and respect for nature.

Equipment: tables "Variety of leaves", "Cellular structure of the leaf"; herbarium - leaf venation, leaves are simple and complex; houseplants; preparations of the peel of tradescantia leaves, geraniums.

DURING THE CLASSES

Every spring, summer on the streets, squares, in school yard, and at home all year round elegant green plants surround us on the windowsills. We are used to them. We are so used to it that we often do not notice the difference between them.

Previously, it seemed to many that all leaves are the same, but the last lesson showed the variety of their amazing forms, their beauty. Let's remember what we've learned.

Plants, depending on the number of cotyledons, are divided into two groups. Which? That's right, monocots and dicots! Now look: it turns out that each leaf knows what class its plant belongs to, and the lace of leaf arrangement helps the leaves make better use of light.

So, take the first envelope. It contains the leaves of various plants. Divide them into two groups according to the type of venation. Well done! And now the leaves from the second envelope are also divided into two groups, but at your discretion. Who can say what principle you were guided by when putting things in order? That's right, you divided the leaves complex and simple.

And now look - on the tables of the task. Please complete them.

1. A sheet is a part .... Leaves are made up of... and... .

2. The figure shows leaves with different types venation. Sign which leaf has which venation.

From external description Let's move on to the study of the internal structure of the leaf. In one of the lessons, we learned that a plant needs a leaf for air nutrition, but how does it work? The leaf consists of cells, while the cells are not the same and perform different functions. What fabric covers the sheet? Integumentary or protective!

In the green chamber
Areas are not measured
Rooms not counted
Walls are like glass
You can see right through everything!
And in the walls - windows,
open themselves
They close themselves!

Let's solve this riddle. The green tower is a leaf, the rooms are cells. Transparent, like glass, the walls are an integumentary fabric. That's what we're going to look at today. To do this, you need to prepare the drug. We learned how to do this correctly when we studied the skin of a leaf.

One student makes a preparation of the skin of the upper side of the leaf, the second - the bottom. Ready and set up the microscope. Let's take a look at the top skin first. Why is she like glass? Because it is transparent and therefore transmits light rays.

And what does "windows in the walls" mean? Try to find them! To do this, it is better to consider the skin of the underside of the leaf. How are some cells different from others?

Stomatal cells form a “window”: they are trailing and, unlike other cells of the integumentary tissue, have a green color, because contain chloroplasts. The gap between them is called stomatal.

Why do you think stomata are needed? To ensure evaporation, penetration of air into the sheet. And they open and close to regulate the penetration of air and water. Consider the differences in the structure of the upper and lower skins. There are more stomata on the underside. Different plants have leaves different amount stomata.

Now we need to document our observations as a lab report. To do this, complete the following tasks.

Laboratory work "Structure of the skin of a leaf"

1. Find colorless cells of the integumentary tissue on the micropreparation, examine them. Describe what shape they have? What is their structure? What role do they play in the life of the leaf?

2. Find the stomata. Draw the shape of the guard cells. Note how the guard cells differ from the cells of the integumentary tissue. Locate the stomatal gap between the guard cells.

3. Sketch the skin in a notebook, in the figure sign: the main cells of the skin, guard cells, stomata, stomatal fissure.

Although scientists have long known about the evaporation of water from the surface of a leaf, the first to observe stomata was the Italian naturalist Marcello Malpighi, who published this discovery in 1675 in his work Anatome plantarum. However, he did not understand their true function. At the same time, his contemporary Nehemiah Grew developed the hypothesis of the participation of stomata in the ventilation of the internal environment of the plant and compared them with insect tracheae. Progress in the study came in the 19th century, and at the same time, in 1827, the word "stoma" was first used by the Swiss botanist Decandole. The study of stomata at that time was carried out by Hugo von Mol, who discovered the basic principle of opening stomata, and Simon Schwendener, who classified stomata by the type of their construction.

Some aspects of the functioning of stomata continue to be intensively studied at the present time; The material is mainly Commelina vulgaris ( Commelina communis), garden bob ( Vicia faba), Sweet corn ( Zea mays) .

Structure

The size of the stomata (length) ranges from 0.01-0.06 mm (the stomata of polyploid plants are also larger in leaves growing in the shade. The largest stomata were found in an extinct plant Zosterophyllum, 0.12 mm (120 µm) . The pore is made up of a pair of specialized cells called guard cells ( cellulae claudentes), which regulate the degree of pore openness, between them there is a stomatal gap ( porus stomatalis). The walls of the guard cells are thickened unevenly: those directed towards the gap (abdominal) are thicker than the walls directed away from the gap (dorsal). The gap can expand and narrow, regulating transpiration and gas exchange. When there is little water, the guard cells are tightly adjacent to each other and the stomatal opening is closed. When there is a lot of water in the guard cells, it presses on the walls and the thinner walls stretch more, and the thicker ones are drawn inward, a gap appears between the guard cells. Under the gap there is a substomatal (air) cavity, surrounded by cells of the pulp of the leaf, through which gas exchange takes place directly. Air containing carbon dioxide (carbon dioxide) and oxygen penetrates into the leaf tissue through these pores, and is further used in the process of photosynthesis and respiration. Excess oxygen produced during photosynthesis by the inner cells of the leaf is released back into environment through the same pores. Also, in the process of evaporation, water vapor is released through the pores. Epidermal cells adjacent to the trailing cells are called accompanying (side, neighboring, parotid). They are involved in the movement of guard cells. Trailing and accompanying cells form a stomatal complex (stomatal apparatus). The presence or absence of stomata (the visible parts of the stomata are called stomatal lines) is often used in the classification of plants.

Stomata types

The number of accompanying cells and their location relative to the stomatal opening make it possible to distinguish a number of types of stomata:

• anomocytic - accompanying cells do not differ from the rest of the cells of the epidermis, the type is very common for all groups of higher plants, with the exception of conifers;
• diacytic - characterized by only two accompanying cells, the common wall of which is at right angles to the trailing cells;
• paracytic - accompanying cells are located parallel to the closing and stomatal gaps;
• anisocytic - guard cells are surrounded by three accompanying cells, one of which is noticeably larger or smaller than the others, this type is found only in flowering plants;
• tetracytic - four accompanying cells, characteristic of monocots;
• encyclocytic - accompanying cells form a narrow wheel around guard cells;
• actinocytic - several accompanying cells, radially diverging from the trailing cells;
• pericytic - the guard cells are surrounded by one secondary accompanying cell, the stomata is not connected to the accompanying cell by an anticlinal cell wall;
• desmocytic - guard cells are surrounded by one accompanying cell, the stomata are connected to it by an anticlinal cell wall;
• polocytic - guard cells are not completely surrounded by one accompanying one: one or two epidermal cells adjoin one of the stomatal poles; the stoma is attached to the distal side of a single accompanying cell, which is U-shaped or horseshoe-shaped;
• stephanocyte - a stomata surrounded by four or more (usually five to seven) poorly differentiated accompanying cells, forming a more or less distinct rosette;
• laterocyte - this type of stomatal apparatus is considered by most botanists as a simple modification of the anomocytic type.

Location of stomata

Dicotyledonous plants tend to have more stomata at the bottom of the leaf than at the top. This is due to the fact that the upper part of a horizontally arranged leaf, as a rule, is better lit, and a smaller number of stomata in it prevents excessive evaporation of water. Leaves with stomata located on the underside are called hypostomatic.

In monocot plants, the presence of stomata in the upper and lower parts of the leaf is different. Very often the leaves of monocotyledonous plants are arranged vertically, in which case the number of stomata on both parts of the leaf may be the same. Such leaves are called amphistomatic.

Floating leaves on the underside of the leaf lack stomata, as they can absorb water through the cuticle. Leaves with stomata located on the upper side are called epistomatic. Underwater leaves have no stomata at all.

The stomata of coniferous plants are usually hidden deep under the endodermis, which makes it possible to greatly reduce water consumption in winter for evaporation, and in summer during drought.

Mosses (with the exception of Anthocerota) have no true stomata.

Stomata also differ in their level of location relative to the surface of the epidermis. Some of them are located flush with other epidermal cells, others are raised above or immersed below the surface. In monocots, whose leaves grow predominantly in length, the stomata form regular parallel rows, while in dicots they are arranged randomly.

Carbon dioxide

Since carbon dioxide is one of the key reactants in the process of photosynthesis, most plants have stomata open during the daytime. The problem is that when the air enters, it mixes with the water vapor evaporating from the leaf, and so the plant cannot get carbon dioxide without losing some water at the same time. Many plants have protection against water evaporation in the form of wax deposits that clog their stomata.