Determination of the state of stomata in indoor plants

The leaf of a plant performs various functions. This is the main organ in which photosynthesis, gas exchange and transpiration (evaporation of water) take place. For the implementation of gas exchange in the terrestrial organs of the plant, there are special formations - stomata.

Stomata, although they are part of the epidermis (leaf skin), are a special group of cells. The stomatal apparatus consists of two guard cells, between which there is a stomatal gap, 2–4 peristomatal cells, and a gas-air chamber located under the stomatal gap.

The guard cells of the stomata have an elongated-curved, "bean-shaped" shape. Their walls facing the stomatal fissure are thickened. Stomatal cells are able to change their shape - due to this, the opening or closing of the stomatal gap occurs. These cells contain chloroplasts (green plastids). The opening and closing of the stomatal fissure occurs due to changes in turgor (osmotic pressure) in guard cells. The chloroplasts of the guard cells contain starch, which can be converted into sugar. When starch is converted to sugar, the osmotic pressure increases and the stomata open. With a decrease in sugar content, the reverse process occurs, and the stomata close.

Stomatal slits are often wide open early in the morning and closed (or semi-closed) during the daytime. The number of stomata depends on environmental conditions (temperature, light, humidity). The degree of their disclosure in different time days varies greatly in different species. In the leaves of plants in humid habitats, the density of stomata is 100–700 per 1 mm2.

Most land plants have stomata only on the underside of the leaf. They can also be found on both sides of the leaf, as, for example, in cabbage or sunflower. At the same time, the density of stomata on the upper and lower sides of the leaf is not the same: cabbage has 140 and 240 per 1 mm 2, and sunflower has 175 and 325 per 1 mm 2, respectively. In aquatic plants, such as water lilies, stomata are located only on the upper side of the leaf with a density of about 500 per 1 mm 2. Underwater plants do not have stomata at all.

Objective:

determination of the state of stomata in various indoor plants.

Tasks

1. To study the question of the structure, location and number of stomata in various plants according to additional literature.

2. Select plants for research.

3. Determine the state of stomata, the degree of their opening in various indoor plants available in the biology room.

Materials and methods

The determination of the state of stomata was carried out according to the method described in " methodological recommendations on plant physiology” (compiled by E.F. Kim and E.N. Grishina). The essence of the technique is that the degree of opening of the stomata is determined by the penetration into the pulp of the leaf of some chemical substances. Various liquids are used for this purpose: ether, alcohol, gasoline, kerosene, benzene, xylene. We used alcohol, benzene and xylene provided to us in the chemistry lab. The penetration of these fluids into the flesh of the leaf depends on the degree of opening of the stomata. If a light spot appears on the leaf 2–3 minutes after applying a drop of liquid to the underside of the leaf blade, this means that the liquid penetrates through the stomata. In this case, alcohol penetrates into the leaf only with wide open stomata, benzene already with an average opening width, and only xylene penetrates through almost closed stomata.

At the first stage of the work, we tried to establish the possibility of determining the state of stomata (degree of opening) in various plants. Agave, cyperus, tradescantia, geranium, oxalis, syngonium, Amazonian lily, begonia, sanchetia, dieffenbachia, clerodendron, passionflower, pumpkin and beans were used in this experiment. Oxalis, geranium, begonia, sanchetia, clerodendron, passionflower, pumpkin and beans were selected for further work. In other cases, the degree of stomata opening could not be determined. This may be due to the fact that agave, cyperus, lily have rather hard leaves covered with a coating that prevents the penetration of substances through the stomatal gap. Another possible reason could be that by the time of the experiment (14.00 h) their stomata were already closed.

The study was carried out during the week. Every day after school, at 14.00, we determined the degree of stomata opening using the above method.

Results and discussion

The data obtained are presented in the table. The given data are averaged, because in different days the condition of the stomata was not the same. So, out of six measurements, a wide opening of stomata was recorded twice in oxalis, once in geranium, and twice in begonia. average degree stomata opening. These differences do not depend on the time of the experiment. Perhaps they are related to climatic conditions, although the temperature regime in the study and the illumination of plants were fairly constant. Thus, the obtained averaged data can be considered a certain norm for these plants.

The conducted research indicates that in different plants at the same time and under the same conditions, the degree of opening of stomata is not the same. There are plants with wide open stomata (begonia, sanchetia, pumpkin), the average size of the stomatal gap (sour, geranium, beans). Narrow stomatal slits are found only in Clerodendron.

We regard these results as preliminary. In the future, we plan to establish whether and how biological rhythms differ in the opening and closing of stomata in different plants. To do this, timing of the state of stomatal fissures during the day will be carried out.

plant stomata

found in their skin (epidermis). Each plant is in constant exchange with the surrounding atmosphere. It constantly absorbs oxygen and releases carbon dioxide. In addition, with its green parts, it absorbs carbon dioxide and releases oxygen. Then, the plant constantly evaporates water. Since the cuticle, which covers the leaves and young stems, very weakly passes gases and water vapor through itself, there are special holes in the skin for unhindered exchange with the surrounding atmosphere, called U. On the transverse section of the leaf (Fig. 1), U. appears in slit ( S) leading to the air cavity ( i).

Fig. 1. Stoma ( S) of a hyacinth leaf in section.

On both sides of the U. there is one closing cell. The shells of the guard cells give two outgrowths towards the stomatal opening, due to which it breaks up into two chambers: the anterior and posterior courtyard. When viewed from the surface, U. appears as an oblong slit surrounded by two semilunar guard cells (Fig. 2).

During the day, U. are open, but at night they are closed. U. are also closed during the day during a drought. U.'s closing is made by guard cells. If a piece of the skin of the leaf is put into water, then the U. continues to remain open. If the water is replaced with a sugar solution that causes cell plasmolysis, then the U. will close. Since the plasmolysis of cells is accompanied by a decrease in their volume, it follows that the closure of cells is the result of a decrease in the volume of guard cells. During drought, the guard cells lose part of their water, decrease in volume and close the U. The leaf is covered with a continuous layer of cuticle, which is poorly permeable to water vapor, which prevents further drying. Night closing U. is explained by the following considerations. Guard cells constantly contain chlorophyll grains and are therefore capable of assimilating atmospheric carbon dioxide, i.e., of self-feeding. Organic substances accumulated in the light strongly attract water from the surrounding cells, so the guard cells increase in volume and open. At night, the organic substances produced in the light are consumed, and with them the ability to attract water is lost, and the U. closes. U. are both on the leaves and on the stems. On leaves, they are placed either on both surfaces, or on one of them. Herbaceous, soft leaves have U. both on the upper and on the lower surface. Hard leathery leaves have U. almost exclusively on the lower surface. In leaves floating on the surface of the water, U. are exclusively on the upper side. The amount of U. in different plants is very different. For most leaves, the number of U. per square millimeter fluctuates between 40 and 300. Largest number U. is located on the lower surface of the Brassica Rapa leaf - per 1 sq. mm 716. There is some relationship between the amount of U. and the humidity of the place. AT general plants wet areas have more U. than plants in dry areas. In addition to ordinary U., which serve for gas exchange, many plants also have water U. They serve to release water not in a gaseous state, but in a liquid state. Instead of the air cavity lying under the ordinary U., under the water U. there is a special aquifer, consisting of cells with thin membranes. Water U. are found for the most part in plants of damp areas and are found on various parts leaves, regardless of the ordinary U., which are located right there. Water U. emit drops of water for the most part when, due to the high humidity of the air, air-bearing U. can not evaporate water. All such formations are called hydathod(Hydathode). An example is the hydathodes of Gonocaryum pyriforme (Fig. 3).

A cross-section through a leaf shows that some of the skin cells have changed in a special way and turned into hydathodes. Each hydathode consists of three parts. A slanting outgrowth protrudes outward, pierced by a narrow tubule through which the water of the hydathode flows. The middle part looks like a funnel with very thickened walls. The lower part of the hydathode consists of a thin-walled bubble. Some plants give off their leaves large quantities water, without having any specially arranged hydathods. Eg. different kinds Salacia secrete such large quantities of water between 6-7 o'clock in the morning that they fully deserve the name rain shrubs: with a light touch on the branches, real rain falls from them. Water is secreted by simple pores that cover in large quantities the outer membranes of the skin cells.

V. Palladin.

Encyclopedic Dictionary F.A. Brockhaus and I.A. Efron. - St. Petersburg: Brockhaus-Efron. 1890-1907 .

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The leaf is a vegetative organ of plants, is part of the shoot. The functions of the leaf are photosynthesis, water evaporation (transpiration) and gas exchange. In addition to these basic functions, as a result of idioadaptations to various conditions of existence, leaves, changing, can serve the following purposes.

• Accumulation of nutrients (onion, cabbage), water (aloe);
• protection against being eaten by animals (thorns of cactus and barberry);
• vegetative propagation (begonia, violet);
• catching and digesting insects (dew, venus flytrap);
• movement and strengthening of a weak stem (pea tendrils, wikis);
• removal of metabolic products during leaf fall (in trees and shrubs).

General characteristics of a plant leaf

The leaves of most plants are green, most often flat, usually bilaterally symmetrical. Sizes from a few millimeters (duckweed) to 10-15m (in palms).

The leaf is formed from cells educational fabric cones of stem growth. The leaf rudiment is differentiated into:

• leaf blade;
• petiole, with which the leaf is attached to the stem;
• stipules.

Some plants do not have petioles, such leaves, unlike petioles, are called sedentary. Stipules are also not found in all plants. They are paired appendages of various sizes at the base of the leaf petiole. Their form is diverse (films, scales, small leaves, spines), their function is protective.

simple and compound leaves distinguished by the number of leaf blades. A simple sheet has one plate and disappears entirely. The complex has several plates on the petiole. They are attached to the main petiole with their small petioles and are called leaflets. When a compound leaf dies, the leaflets fall off first, and then the main petiole.

Leaf blades are diverse in shape: linear (cereals), oval (acacia), lanceolate (willow), ovate (pear), arrow-shaped (arrowhead), etc.

Leaf blades are pierced in different directions by veins, which are vascular-fibrous bundles and give the sheet strength. The leaves of dicotyledonous plants most often have reticulate or pinnate venation, while the leaves of monocotyledonous plants have a parallel or arcuate venation.

The edges of the leaf blade can be solid, such a sheet is called whole-edge (lilac) or notched. Depending on the shape of the notch, along the edge of the leaf blade, there are serrate, serrate, crenate, etc. In serrated leaves, the serrations have more or less equal sides (beech, hazel), in serrate - one side of the tooth is longer than the other (pear), crenate - have sharp notches and blunt bulges (sage, budra). All these leaves are called whole, since their recesses are shallow, do not reach the width of the plate.

In the presence of deeper recesses, the leaves are lobed, when the depth of the recess is equal to half the width of the plate (oak), separate - more than half (poppy). In dissected leaves, the recesses reach the midrib or to the base of the leaf (burdock).

Under optimal growth conditions, the lower and upper leaves of the shoots are not the same. There are lower, middle and upper leaves. Such differentiation is determined even in the kidney.

The lower, or first, leaves of the shoot are the scales of the kidneys, the outer dry scales of the bulbs, the cotyledon leaves. The lower leaves usually fall off during the development of the shoot. The leaves of the basal rosettes also belong to the grassroots. Median, or stem, leaves are typical for plants of all kinds. Upper leaves usually have smaller sizes, are located near flowers or inflorescences, are painted in various colors, or are colorless (covering leaves of flowers, inflorescences, bracts).

Sheet arrangement types

There are three main types of leaf arrangement:

• Regular or spiral;
• opposite;
• whorled.

At the next arrangement, single leaves are attached to the stem nodes in a spiral (apple, ficus). With the opposite - two leaves in the node are located one against the other (lilac, maple). Whorled leaf arrangement - three or more leaves in a node cover the stem with a ring (elodea, oleander).

Any leaf arrangement allows plants to capture the maximum amount of light, since the leaves form a leaf mosaic and do not obscure each other.

Cellular structure of the leaf

The leaf, like all other plant organs, has a cellular structure. The upper and lower surfaces of the leaf blade are covered with skin. Living colorless cells of the skin contain the cytoplasm and nucleus, are located in one continuous layer. Their outer shells are thickened.

Stomata are the respiratory organs of a plant.

In the skin are stomata - gaps formed by two trailing, or stomatal, cells. Guard cells are crescent-shaped and contain cytoplasm, nucleus, chloroplasts, and a central vacuole. The membranes of these cells are thickened unevenly: the inner, facing the gap, is thicker than the opposite.

A change in the turgor of the guard cells changes their shape, due to which the stomatal opening is open, narrowed or completely closed, depending on the conditions. environment. So, during the day, the stomata are open, and at night and in hot, dry weather they are closed. The role of stomata is to regulate the evaporation of water by the plant and gas exchange with the environment.

Stomata are usually located on the lower surface of the leaf, but there are also on the upper, sometimes they are distributed more or less evenly on both sides (corn); in aquatic floating plants, stomata are located only on the upper side of the leaf. The number of stomata per unit leaf area depends on the plant species and growth conditions. On average, there are 100-300 of them per 1 mm 2 of the surface, but there can be much more.

Leaf pulp (mesophile)

Between the upper and lower skin of the leaf blade is the pulp of the leaf (mesophile). Under the top layer is one or more layers of large rectangular cells that have numerous chloroplasts. This is a columnar, or palisade, parenchyma - the main assimilation tissue in which photosynthesis processes are carried out.

Under the palisade parenchyma there are several layers of irregularly shaped cells with large intercellular spaces. These layers of cells form a spongy, or loose, parenchyma. Spongy parenchyma cells contain fewer chloroplasts. They perform the functions of transpiration, gas exchange and storage of nutrients.

The flesh of the leaf is permeated with a dense network of veins, vascular-fibrous bundles that supply the leaf with water and substances dissolved in it, as well as the removal of assimilants from the leaf. In addition, the veins perform a mechanical role. As the veins move away from the base of the leaf and approach them to the top, they become thinner due to branching and gradual loss of mechanical elements, then sieve tubes, and finally tracheids. The smallest branches at the very edge of the leaf usually consist only of tracheids.

Diagram of the structure of a plant leaf

The microscopic structure of the leaf blade varies significantly even within the same systematic group of plants, depending on different growth conditions, primarily on lighting conditions and water supply. Plants in shaded places often lack palisade perenchyma. The cells of the assimilation tissue have larger palisades, the concentration of chlorophyll in them is higher than in photophilous plants.

Photosynthesis

In the chloroplasts of the pulp cells (especially the columnar parenchyma), the process of photosynthesis takes place in the light. Its essence lies in the fact that green plants absorb solar energy and create complex organic substances from carbon dioxide and water. This releases free oxygen into the atmosphere.

Organic substances created by green plants are food not only for the plants themselves, but also for animals and humans. Thus, life on earth depends on green plants.

All oxygen contained in the atmosphere is of photosynthetic origin, it accumulates due to the vital activity of green plants and its quantitative content is maintained constant due to photosynthesis (about 21%).

Using carbon dioxide from the atmosphere for the process of photosynthesis, green plants thereby purify the air.

Evaporation of water from leaves (transpiration)

In addition to photosynthesis and gas exchange, the process of transpiration occurs in the leaves - the evaporation of water by the leaves. The stomata play the main role in evaporation, and the entire surface of the leaf also partially takes part in this process. In this regard, stomatal transpiration and cuticular transpiration are distinguished - through the surface of the cuticle covering the leaf epidermis. Cuticular transpiration is much less than stomatal: in old leaves, 5-10% of total transpiration, but in young leaves with a thin cuticle, it can reach 40-70%.

Since transpiration is carried out mainly through the stomata, where carbon dioxide also enters for the process of photosynthesis, there is a relationship between the evaporation of water and the accumulation of dry matter in the plant. The amount of water that a plant evaporates to build 1g of dry matter is called transpiration coefficient. Its value ranges from 30 to 1000 and depends on the growth conditions, type and variety of plants.

The plant uses an average of 0.2% of the passed water to build its body, the rest is spent on thermoregulation and transport of minerals.

Transpiration creates a suction force in the cell of the leaf and root, thereby maintaining the constant movement of water throughout the plant. In this regard, the leaves are called the upper water pump, in contrast to the root system - the lower water pump, which pumps water into the plant.

Evaporation protects the leaves from overheating, which has great importance for all life processes of a plant, especially photosynthesis.

Plants of arid places, as well as in dry weather, evaporate more water than under high humidity conditions. Evaporation of water, except for stomata, is regulated by protective formations on the skin of the leaf. These formations are: cuticle, wax coating, pubescence from various hairs, etc. In succulent plants, the leaf turns into spines (cacti), and the stem performs its functions. Plants of wet habitats have large leaf blades, there are no protective formations on the skin.

Transpiration is the mechanism by which water is evaporated from the leaves of a plant.

With difficult evaporation in plants, guttation- the release of water through the stomata in a drop-liquid state. This phenomenon occurs in nature usually in the morning, when the air approaches saturation with water vapor, or before rain. Under laboratory conditions, guttation can be observed by covering young wheat seedlings with glass caps. After a short time, droplets of liquid appear on the tips of their leaves.

Isolation system - leaf fall (leaf fall)

The biological adaptation of plants to protection from evaporation is leaf fall - a massive fall of leaves in the cold or hot season. In temperate zones, trees shed their leaves for the winter when the roots cannot supply water from the frozen soil and frost dries out the plant. In the tropics, leaf fall is observed during the dry season.

Preparation for shedding leaves begins with a weakening of the intensity of life processes in late summer - early autumn. First of all, chlorophyll is destroyed, other pigments (carotene and xanthophyll) last longer and give the leaves an autumn color. Then, at the base of the leaf petiole, parenchymal cells begin to divide and form a separating layer. After that, the leaf comes off, and a trace remains on the stem - a leaf scar. By the time of leaf fall, the leaves are aging, unnecessary metabolic products accumulate in them, which are removed from the plant along with the fallen leaves.

All plants (usually trees and shrubs, less commonly herbs) are divided into deciduous and evergreen. In deciduous leaves develop during one growing season. Every year, with the onset of adverse conditions, they fall. Leaves of evergreen plants live from 1 to 15 years. The death of part of the old and the appearance of new leaves occurs constantly, the tree seems evergreen (coniferous, citrus).

And it releases carbon dioxide. In addition, with its green parts, it absorbs carbon dioxide and releases oxygen. Then, the plant constantly evaporates water. Since the cuticle, which covers the leaves and young stems, very weakly passes gases and water vapor through itself, there are special holes in the skin for unhindered exchange with the surrounding atmosphere, called U. On the transverse section of the leaf (Fig. 1), U. appears in slit ( S) leading to the air cavity ( i).

Fig. 1. Stoma ( S) cutaway of a hyacinth leaf.

On both sides of the U. there is one closing cell. The shells of the guard cells give two outgrowths towards the stomatal opening, due to which it breaks up into two chambers: the anterior and posterior courtyard. When viewed from the surface, U. appears as an oblong slit surrounded by two semilunar guard cells (Fig. 2).

Fig. 2. Stomata of a Sedum purpurascens leaf from the surface.

During the day, U. are open, but at night they are closed. U. are also closed during the day during a drought. U.'s closing is made by guard cells. If a piece of the skin of the leaf is put into water, then the U. continues to remain open. If the water is replaced with a sugar solution that causes cell plasmolysis, then the U. will close. Since the plasmolysis of cells is accompanied by a decrease in their volume, it follows that the closure of cells is the result of a decrease in the volume of guard cells. During drought, the guard cells lose some of their water, decrease in volume and close the U. The leaf is covered with a continuous cuticle layer, which poorly passes water vapor, and this is protected from further drying. Night closing U. is explained by the following considerations. Guard cells constantly contain chlorophyll grains and are therefore capable of assimilating atmospheric carbon dioxide, i.e., of self-feeding. Organic substances accumulated in the light strongly attract water from the surrounding cells, so the guard cells increase in volume and open. At night, the organic substances produced in the light are consumed, and with them the ability to attract water is lost, and the U. closes. U. are both on the leaves and on the stems. On leaves, they are placed either on both surfaces, or on one of them. Herbaceous, soft leaves have U. both on the upper and on the lower surface. Hard leathery leaves have U. almost exclusively on the lower surface. In leaves floating on the surface of the water, U. are exclusively on the upper side. The amount of U. in different plants is very different. For most leaves, the number of U., located on one square millimeter, ranges between 40 and 300. The largest number of U. is on the lower surface of the Brassica Rara leaf - per 1 square. mm 716. There is some relationship between the amount of U. and the humidity of the place. In general, plants in humid areas have more UV than plants in dry areas. In addition to ordinary U., which serve for gas exchange, many plants also have water U. They serve to release water not in a gaseous state, but in a liquid state. Instead of the air cavity lying under the ordinary U., under the water U. there is a special aquifer, consisting of cells with thin membranes. Water W. are found for the most part in plants of damp areas and are found on various parts of the leaves, regardless of the ordinary W. located there. U., there are a number of different devices for the release of water in liquid form by leaves. All such formations are called hydathod(Hydathode). An example is the hydathodes of Gonocaryum pyriforme (Fig. 3).

Fig. 3. Gonocaryum pyriforme leaf hydathode.

A cross-section through a leaf shows that some of the skin cells have changed in a special way and turned into hydathodes. Each hydathode consists of three parts. A slanting outgrowth protrudes outward, pierced by a narrow tubule through which the water of the hydathode flows. The middle part looks like a funnel with very thickened walls. The lower part of the hydathode consists of a thin-walled bubble. Some plants exude large amounts of water with their leaves, without any specially arranged hydathodes. Eg. the different species of Salacia exude such large quantities of water between 6-7 o'clock in the morning that they fully deserve the name of rain bushes: when lightly touched, real rain falls from them. Water is secreted by simple pores that cover in large quantities the outer membranes of the skin cells.

Stomata, their structure and mechanism of action

The cells of the epidermis are almost impermeable to water and gases due to the peculiar structure of their outer wall. How are gas exchange between the plant and the external environment and the evaporation of water - processes necessary for the normal life of the plant? Among the cells of the epidermis there are characteristic formations called stomata.

The stomata is a slit-like opening, bordered on both sides by two trailing cells, which are mostly semilunar in shape.

Stomata are pores in the epidermis through which gas exchange occurs. They are found mainly in the leaves, but also on the stem. Each stomata is surrounded on both sides by guard cells, which, unlike other epidermal cells, contain chloroplasts. Guard cells control the size of the stomatal opening by changing their turgidity.

These cells are alive and contain chlorophyll grains and grains of starch, which are absent in other cells of the epidermis. There are especially many stomata on the leaf. The cross section shows that directly below the stomata inside the leaf tissue is a cavity called the respiratory cavity. Within the gap, the guard cells are closer together in the middle part of the cells, and above and below they recede further from each other, forming spaces called the front and back courtyards.

The guard cells are able to increase and decrease their size, due to which the stomatal opening is either widely opened, then narrowed, or even completely closed.

Thus, the guard cells are the apparatus that regulates the process of opening and closing of the stomata.

How is this process carried out?

The walls of the guard cells facing the gap are thickened much more strongly than the walls facing the adjacent cells of the epidermis. When the plant is lit and has excess moisture, starch accumulates in the chlorophyll grains of guard cells, part of which is converted into sugar. Sugar, dissolved in cell sap, attracts water from neighboring cells of the epidermis, as a result of which turgor increases in guard cells. Strong pressure leads to protrusion of the cell walls adjacent to the epidermal ones, and the opposite, strongly thickened walls straighten. As a result, the stomatal opening opens, and gas exchange, as well as water evaporation, increase. In the dark or with a lack of moisture, the turgor pressure decreases, the guard cells take their former position, and the thickened walls close. The opening of the stomata closes.

Stomata are located on all young non-lignified ground organs of the plant. There are especially many of them on the leaves, and here they are located mainly on the lower surface. If the leaf is located vertically, then stomata develop on both sides of it. Some leaves floating on the surface of the water aquatic plants(for example, water lilies, capsules) stomata are located only on the upper side of the leaf.

The number of stomata per 1 square. mm of the leaf surface is on average 300, but sometimes it reaches 600 or more. In cattail (Typha) there are over 1300 stomata per 1 sq. mm. Leaves immersed in water do not have stomata. The stomata are most often evenly distributed over the entire surface of the skin, but in some plants they are collected in groups. In monocot plants, as well as on the needles of many conifers, they are located in longitudinal rows. In plants of arid regions, stomata are often immersed in leaf tissue. Stomatal development usually proceeds as follows. In individual cells of the epidermis, arcuate walls are formed, dividing the cell into several smaller ones so that the central one becomes the ancestor of the stomata. This cell is divided by a longitudinal (along the axis of the cell) septum. Then this septum splits, and a gap is formed. The cells limiting it become guard cells of the stomata. Some liver mosses have peculiar stomata, devoid of guard cells.

On fig. shows the appearance of stomata and guard cells in a micrograph obtained using a scanning electron microscope.

It can be seen here that the cell walls of the guard cells are not uniform in thickness: the wall closest to the stomatal opening is clearly thicker than the opposite wall. In addition, the cellulose microfibrils that make up the cell wall are arranged in such a way that the wall facing the hole is less elastic, and some fibers form a kind of hoops around the sausage-like guard cells. As the cell sucks in water and becomes turgid, these hoops prevent it from expanding further, only allowing it to expand in length. Since the guard cells are connected at their ends, and the thinner walls away from the stomatal fissure stretch more easily, the cells take on a semicircular shape. Therefore, a hole appears between the guard cells. (We will get the same effect if we inflate a sausage-shaped balloon with adhesive tape taped to it along one of its sides.)

Conversely, when water leaves the guard cells, the pore closes. How the change in cell turgidity occurs is not yet clear.

In one of the traditional hypotheses, the "sugar starch" hypothesis, it is assumed that during the day the concentration of sugar increases in the guard cells, and as a result, the osmotic pressure in the cells and the flow of water into them increase. However, no one has yet been able to show that enough sugar accumulates in the guard cells to cause the observed changes in osmotic pressure. It has recently been established that during the day in the light, potassium ions and their accompanying anions accumulate in the guard cells; this accumulation of ions is sufficient to cause the observed changes. In the dark, potassium ions (K+) exit the guard cells into adjacent epidermal cells. It is still unclear which anion balances the positive charge of the potassium ion. Some (but not all) of the plants studied showed the accumulation of large amounts of anions of organic acids such as malate. At the same time, starch grains, which appear in the dark in the chloroplasts of guard cells, decrease in size. This suggests that starch is converted to malate in the presence of light.

Some plants, such as Allium cepa (onion), do not have starch in their guard cells. Therefore, with open stomata, malate does not accumulate, and cations, apparently, are absorbed together with inorganic anions such as chloride (Cl-).

Some issues remain unresolved. For example, why do stomata need light to open? What role do chloroplasts play besides storage of starch? Does malate turn back to starch in the dark? In 1979, it was shown that the chloroplasts of Vicia faba guard cells (horse beans) lack the enzymes of the Calvin cycle and the thylakoid system is poorly developed, although chlorophyll is present. As a result, the usual C3 - the path of photosynthesis does not work and starch is not formed. This could help explain why starch is not formed during the day, as in normal photosynthetic cells, but at night. Another interesting fact- absence of plasmodesmata in guard cells, i.e. comparative isolation of these cells from other cells of the epidermis.