Lesson “Cellular structure of a leaf”

Target: show the relationship between the structure of a leaf and its functions; develop the concept of the cellular structure of plants; continue to develop the skills of independent work with instruments, the ability to observe, compare, juxtapose, and draw conclusions on your own;

develop love and respect for nature. Equipment

: tables “Diversity of leaves”, “Cellular structure of leaves”; herbarium – leaf venation, simple and compound leaves; houseplants; preparations of the skin of Tradescantia and geranium leaves.

DURING THE CLASSES

Every spring and summer, on the streets, squares, in the schoolyard, and at home - all year round on the windowsills we are surrounded by elegant green plants. We're used to them. We are so used to it that we often don’t notice the difference between them.

Previously, many people thought that all leaves were the same, but the last lesson showed the variety of their amazing forms and their beauty. Let's remember the material covered.

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

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

Now look - there are assignments on the tables. Please complete them.

1. A leaf is a part... . The leaves consist of... and... .

2. The picture shows leaves with different types of venation. Label which leaf has which veining.

From the external description we move on to studying the internal structure of the leaf. In one of the lessons we learned that the leaf is necessary for the plant to provide aerial nutrition, but how does it work? A leaf consists of cells, but the cells are not identical and perform different functions. What fabric covers the sheet? Covering or protective!
The areas are not measured,
The rooms are not counted
The walls are like glass
Everything is visible through and through!
And there are windows in the walls,
They open on their own
They close themselves!

Let's look at this mystery. The green tower is a leaf, the rooms are cells. Transparent, like glass, walls are a covering fabric. That's what we'll 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 the leaf.

One student makes a preparation of the skin of the upper side of the leaf, the second - the lower side.

We prepared and set up the microscope. Let's look at the top skin first. Why is she like glass?

Because it is transparent and therefore transmits rays of light.

What does “windows in the walls” mean?

Try to find them! To do this, it is better to examine the skin of the underside of the leaf. How are some cells different from others?

Stomatal cells form a “window”: they are guard cells 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 and 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 skin. There are more stomata on the underside. Different plants have leaves with different numbers of stomata.

Now we need to formalize our observations in the form of a lab report.

To do this, complete the following tasks.

To live, plants must absorb carbon dioxide from the air for photosynthesis and draw water from the soil. They do both with the help of stomata - pores on the surface of the leaf, surrounded by guard cells, which these stomata either open or close. Water evaporates through the pores and maintains a constant flow of liquid from the roots to the leaves, but at the same time the plants regulate the level of evaporation so as not to dry out in hot weather. On the other hand, photosynthesis constantly requires carbon dioxide. It is obvious that stomata sometimes have to solve almost mutually exclusive tasks: not allowing the plant to dry out and at the same time delivering air with carbon dioxide.

The method of regulating the functioning of stomata has long occupied science. The generally accepted point of view is that plants take into account the amount of solar radiation in the blue and red ranges of the spectrum and, depending on this, keep their stomata open or closed. But recently, several researchers have proposed an alternative hypothesis: the state of stomata depends on the total amount of absorbed radiation (and not just on its blue and red parts). Sunlight not only warms the air and the plant, it is necessary for the photosynthesis reaction. Taking into account the total dose of radiation, the stomata could more accurately respond to changes in illumination - and therefore more accurately control the evaporation of moisture.

Researchers from the University of Utah (USA), who tested this theory, were forced to admit that a revolution in plant physiology is not yet in sight. The conclusion that plants emit net radiation was based on temperature measurements at the leaf surface. Keith Mott and David Peak managed to find a way to determine the internal temperature of the leaf: according to scientists, it is the difference between the external and internal temperatures that determines the rate of evaporation. As the authors write in the journal PNAS, they were unable to find a correspondence between the temperature difference inside and on the surface of the leaf and the total radiation dose. It turns out that the stomata also ignored this total radiation.

According to the researchers, the most likely mechanism that controls stomata could be something like a self-organizing network, vaguely reminiscent of a neural network (no matter how crazy it may sound when applied to plants). Even the generally accepted hypothesis about the blue and red parts of the spectrum does not explain everything in the work of stomata. In this regard, is it possible to imagine that all guard cells are somehow connected with each other and can exchange certain signals? If united, they could quickly and accurately respond to both changes in the external environment and plant requests.

Plant stomata

are located 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 allows gases and water vapor to pass through itself, for unhindered exchange with the surrounding atmosphere there are special holes in the skin called U. In the cross section of the leaf (Fig. 1), the U appears in in the form of a slit ( S), leading into the air cavity ( i).

Fig. 1. Stomata ( S) cross-section of a hyacinth leaf.

On both sides of the U. there is one guard cell. The shells of the guard cells give off two projections towards the stomatal opening, thanks to which it splits into two chambers: the front and rear courtyards. When viewed from the surface, the U appears as an oblong slit, surrounded by two semilunar guard cells (Fig. 2).

During the day the U. are open, but at night they are closed. The houses are also closed during the day during a drought. The closure of the cell is carried out by guard cells. If a piece of leaf skin is placed in water, the leaves continue to remain open. If the water is replaced with a sugar solution, which causes plasmolysis of the cells, then the cells will close. Since plasmolysis of cells is accompanied by a decrease in their volume, it follows that the closure of the cell is the result of a decrease in the volume of the guard cells. During drought, the guard cells lose part of their water, decrease in volume and close the leaf. The leaf turns out to be covered with a continuous layer of cuticle, which is weakly permeable to water vapor, which is what protects it from further drying out. The night closing of the U. is explained by the following considerations. Guard cells constantly contain chlorophyll grains and are therefore capable of assimilating atmospheric carbon dioxide, i.e., self-feeding. Organic substances accumulated in the light strongly attract water from surrounding cells, so the guard cells increase in volume and open. At night, the organic substances produced in the light are consumed, and along with them the ability to attract water is lost, and the walls close. U. are found 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. on both the upper and lower surfaces. The hard, leathery leaves have U. almost exclusively on the lower surface. In leaves floating on the surface of the water, the volts are located exclusively on the upper side. The amount of U. in different plants is very different. For most leaves, the number of volts per square millimeter ranges between 40 and 300. The largest number of volts is located on the lower surface of the Brassica Rapa leaf - per 1 square millimeter. mm 716. There is some relationship between the amount of water and the humidity of the place. In general, plants in humid areas have more voltage than plants in dry areas. In addition to ordinary U., which serve for gas exchange, many plants also have U. They serve to release water not in a gaseous state, but in a liquid one. Instead of the air-bearing cavity lying under ordinary U., under water U. there is a special aquiferous tissue consisting of cells with thin membranes. Aquatic U. are found mostly in plants in damp areas and are found on various parts of the leaves, regardless of the ordinary U. that are located nearby. Aquatic U. secrete drops of water for the most part when, due to the high humidity of the air, airborne U. are unable to evaporate water. In addition to aquatic U. 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).

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

V. Palladin.

Encyclopedic Dictionary F.A. Brockhaus and I.A. Efron. - S.-Pb.: Brockhaus-Efron. 1890-1907 .

See what “plant stomata” is in other dictionaries:

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...

Stomata of a tomato leaf under an electron microscope Stomata (Latin stoma, from Greek στόμα “mouth, mouth”) in botany is a pore located on the lower or upper layer of the epidermis of a plant leaf, through which water evaporates and gases exchange with ... ... Wikipedia

The first attempts to classify flowering plants, as well as the plant world in general, were based on a few, arbitrarily taken, easily conspicuous external features. These were purely artificial classifications, in which in one... ... Biological encyclopedia

Encyclopedic Dictionary F.A. Brockhaus and I.A. Ephron

Groups of cells located in the plant body in a known order, having a specific structure and serving for various vital functions of the plant organism. The cells of almost all multicellular plants are not homogeneous, but are collected in T. In lower ... Encyclopedic Dictionary F.A. Brockhaus and I.A. Ephron- are processes and phenomena of this kind that occur in a living plant organism, which never occur during normal life. According to Frank’s definition, plant disease is a deviation from the normal state of the species... Encyclopedic Dictionary F.A. Brockhaus and I.A. Ephron

Contents: Subject of F.F. nutrition. F. growth. F. plant forms. F. reproduction. Literature. Plant physiology studies the processes occurring in plants. This part of the broad science of plant botany differs from its other parts of taxonomy,... ... Encyclopedic Dictionary F.A. Brockhaus and I.A. Ephron

Leaf (folium), an organ of higher plants that performs the functions of photosynthesis and transpiration, as well as providing gas exchange with the air and participating in other important processes of plant life. Morphology, anatomy of the leaf and its... ... Great Soviet Encyclopedia

Scientists still cannot explain the mechanism that controls plant stomata. Today, we can only say with certainty that the dose of solar radiation is not a clear and decisive factor influencing the closing and opening of stomata, writes PhysOrg.

To live, plants must absorb carbon dioxide from the air for photosynthesis and draw water from the soil. They do both with the help of stomata - pores on the surface of the leaf, surrounded by guard cells, which these stomata either open or close. Water evaporates through the pores and maintains a constant flow of liquid from the roots to the leaves, but at the same time the plants regulate the level of evaporation so as not to dry out in hot weather. On the other hand, photosynthesis constantly requires carbon dioxide. It is obvious that stomata sometimes have to solve almost mutually exclusive tasks: not allowing the plant to dry out and at the same time delivering air with carbon dioxide.

The method of regulating the functioning of stomata has long occupied science. The generally accepted point of view is that plants take into account the amount of solar radiation in the blue and red ranges of the spectrum and, depending on this, keep their stomata open or closed. But recently, several researchers have proposed an alternative hypothesis: the state of stomata depends on the total amount of absorbed radiation (and not just on its blue and red parts). Sunlight not only heats the air and the plant, it is necessary for the photosynthesis reaction. Taking into account the total dose of radiation, the stomata could more accurately respond to changes in illumination - and therefore more accurately control the evaporation of moisture.

Researchers from the University of Utah (USA), who tested this theory, were forced to admit that a revolution in plant physiology is not yet in sight. The conclusion that plants emit net radiation was based on temperature measurements at the leaf surface. Keith Mott and David Peak managed to find a way to determine the internal temperature of the leaf: according to scientists, it is the difference between the external and internal temperatures that determines the rate of evaporation. As the authors write in the journal PNAS, they were unable to find a correspondence between the temperature difference inside and on the surface of the leaf and the total radiation dose. It turns out that the stomata also ignored this total radiation.

According to the researchers, the most likely mechanism that controls stomata could be something like a self-organizing network, vaguely reminiscent of a neural network (no matter how crazy it may sound when applied to plants). Even the generally accepted hypothesis about the blue and red parts of the spectrum does not explain everything in the work of stomata. In this regard, is it possible to imagine that all guard cells are somehow connected with each other and can exchange certain signals? If united, they could quickly and accurately respond to both changes in the external environment and plant requests.

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

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

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

3. Photoactive reaction. Photoactive movements manifest themselves 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, necessary for photosynthesis, diffuses to the chloroplasts.

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 type), the stomata open. When the CO 2 concentration increases, the stomata close. The guard cells of stomata always contain chloroplasts and photosynthesis occurs. In the light, CO 2 is assimilated during photosynthesis, its content decreases. According to the hypothesis of the Canadian physiologist W. Scarce, CO 2 affects the degree of openness of stomata through a change in pH in the guard cells. A decrease in CO 2 content leads to an increase in pH value (a shift to the alkaline side). On the contrary, darkness causes an increase in CO 2 content (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 value (a shift to the acidic side). Changing the pH value leads to a change in the activity of enzyme systems. In particular, a shift in pH to the alkaline side increases the activity of enzymes involved in the breakdown of starch, while a shift to the acidic side increases the activity of enzymes involved in starch synthesis. The breakdown of starch into sugars causes an increase in the concentration of dissolved substances, and therefore the osmotic potential and, as a consequence, the water potential become more negative. The guard cells begin to intensively receive water from the surrounding parenchyma cells. The stomata open. Opposite changes occur when processes shift towards starch synthesis. However, this is not the only explanation. It has been shown that stomatal guard cells contain significantly more potassium in the light compared to the dark. It has been established that the amount of potassium in guard cells when stomata open increases 4-20 times, while this indicator in accompanying cells decreases. There seems to be a redistribution of potassium. When the stomata open, a significant gradient of membrane potential arises between the guard and accompanying cells (I.I. Gunar, L.A. Panichkin). Addition of ATP to epidermis floating on KS1 solution increases the rate of stomatal opening in light. An increase in the ATP content in the guard cells of stomata during their opening has also been shown (S.A. Kubichik). It can be assumed that ATP, formed during photosynthetic phosphorylation in guard cells, is used to enhance the supply 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 results in transport along the electrical gradient of K+ into the cytoplasm and then into the vacuole. The increased supply 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 salts of malic acid (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 amount (this value depends on the plant species), the stomata open. When the CO 2 concentration increases, the stomata close. The guard cells of stomata always contain chloroplasts and photosynthesis occurs. In the light, CO 2 is assimilated during photosynthesis, its content decreases. According to the hypothesis of the Canadian physiologist W. Scarce, CO 2 affects the degree of openness of stomata through a change in pH in the guard cells. A decrease in CO 2 content leads to an increase in pH value (a shift to the alkaline side). On the contrary, darkness causes an increase in CO 2 content (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 value (a shift to the acidic side). Changing the pH value leads to a change in the activity of enzyme systems. In particular, a shift in pH to the alkaline side increases the activity of enzymes involved in the breakdown of starch, while a shift to the acidic side increases the activity of enzymes involved in starch synthesis. The breakdown of starch into sugars causes an increase in the concentration of dissolved substances, and therefore the osmotic potential and, as a consequence, the water potential become more negative. The guard cells begin to intensively receive water from the surrounding parenchyma cells. The stomata open. Opposite changes occur when processes shift towards starch synthesis. However, this is not the only explanation. It has been shown that stomatal guard cells contain significantly more potassium in the light compared to the dark. It has been established that the amount of potassium in guard cells when stomata open increases 4-20 times, while this indicator in accompanying cells decreases. There seems to be a redistribution of potassium. When the stomata open, a significant gradient of membrane potential arises between the guard and accompanying cells (I.I. Gunar, L.A. Panichkin). Addition of ATP to epidermis floating on KS1 solution increases the rate of stomatal opening in light. An increase in the ATP content in the guard cells of stomata during their opening has also been shown (S.A. Kubichik). It can be assumed that ATP, formed during photosynthetic phosphorylation in guard cells, is used to enhance the supply 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 results in transport along the electrical gradient of K+ into the cytoplasm and then into the vacuole. The increased supply 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 salts of malic acid (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 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 stomatal opening by enhancing K+ transport into stomatal guard cells and activating H+-ATPase is discussed.

The movement of stomatal cells turned out to be temperature dependent. A study of a number of plants showed that at temperatures below 0°C the stomata do not open. An increase in temperature above 30°C causes the closure of stomata. This may be due to an increase in CO 2 concentration as a result of an increase in respiration intensity. At the same time, there are observations that in different varieties of wheat the reaction of stomata to elevated temperature is different. Prolonged exposure to high temperatures damages the stomata, in some cases so much that they lose the ability to open and close.

Observations of the degree of openness of stomata are of great importance in physiological and agronomic practice. They help determine the need for water supply to the plant. The closure of stomata already indicates unfavorable changes in water metabolism and, as a consequence, difficulties in feeding plants with carbon dioxide.