Sheet- This is an extremely important organ of the plant. Its main functions are photosynthesis and transpiration. The leaf consists of a leaf blade and a petiole, which in appearance resembles a stem, but in origin they are part of the leaf.
Cellular structure of the leaf
The surface of any leaf is covered with a skin; it protects the leaf from damage, drying out, and penetration of pathogenic bacteria. The cells of the leaf skin are tightly adjacent to each other, as this is a covering tissue. Most cells are colorless and transparent, allowing light to penetrate into the interior of the leaf.
The outside of the sheet is covered epidermis. It is a living tissue consisting of one or more layers of cells, which, as a rule, do not have well-differentiated chloroplasts. The cells are tightly interconnected, which contributes to the role of the epidermis in protecting leaf tissue from excess water loss and providing mechanical support.
Note 1
A characteristic feature of this tissue is the presence of various types of outgrowths on the outer surface of the cells (hairs, spines, cuticles). To carry out water and gas exchange between the plant and the environment, there are stomata between the cells of the epidermis.
Definition 1
The main tissue located between the upper and lower epidermis is called mesophyll(from the Greek “mesos” - middle and “phyllon” - leaf). It is a photosynthetic tissue consisting of living cells with a large number of chloroplasts. In many plants, the mesophyll is differentiated into palisade and spongy parenchyma. Palisade parenchyma consists of cells located perpendicular to the surface of the epidermis and resembling a series of columns (columnar parenchyma). The cells of the palisade parenchyma have a prismatic shape and are elongated. The palisade parenchyma is located directly under the epidermis, in some plants only on the upper side of the leaf, in others on both sides.
Spongy parenchyma is characterized by cells of various shapes, often with outgrowths. They are located in such a way that there are many well-defined spaces between them (hence the name parenchyma).
The degree of mesophyll differentiation depends on the type of plant and the peculiarities of their cultivation. It is known that in conditions of bright lighting, palisade parenchyma develops well. In many cereals of the temperate zone, the mesophyll does not differentiate into palisade and spongy parenchyma.
The difference in the structure of these two tissues indicates their possible functional specialization: the palisade parenchyma is probably a highly specialized tissue that performs the function of photosynthesis. This is confirmed by the fact that most chloroplasts are located in this tissue, concentrating near the cell walls, which contributes to their better lighting and supply of carbon dioxide. Spongy parenchyma performs, to a lesser extent, the function of photosynthesis, and also the function of storage tissue (reserve starch is deposited in the cells).
The conductive tissue of the leaf consists of vascular-fibrous bundles concentrated in the veins. Through them, water and nutrients enter the leaf, and the products of photosynthesis are removed. The conductive tissue of the leaf blade and petiole is a continuous whole with the conductive system of the stem. The vein may consist of one or a group of closely connected bundles.
The structure of the vascular-fibrous bundles of the main veins of the leaf is typical, but as the bundles fragment, a decrease in vessels and sieve tubes is observed. In the smallest branching veins, phloem is completely absent, and the xylem is also simplified - there are no tracheas in it, a small amount of tracheids remains. The veins end in single tracheids.
The strength of the leaf blade is determined by the development of a system of mechanical tissues: sclerenchyma sheaths of bundles, strands of mechanical tissue located opposite the vascular bundles and interlocking with the sclerenchyma sheaths, stony cells, supporting cells and others.
Structure and functions of the stomata
The stomata looks like a gap located between two cells with a peculiar structure. These are two sickle-shaped cells that connect with each other at opposite ends (guard cells), significantly different from other epidermal cells in shape and the presence of chloroplasts. Stomata are predominantly located on the lower side of the leaf blade, but in some plants they are also located on the upper side (in the cereal cabbage). In aquatic plants (semi-submerged, for example, the water lily), stomata are located only on the upper side of the plate.
The number of stomata on plant leaves varies - from $40$ to $600$ per $1\mm^2$ and even more.
On leaves with parallel veins (in conifers), the stomata are arranged in parallel rows, on the leaves of other plants - in no particular order.
The opening of stomata depends on other factors: the need for gas exchange associated with photosynthesis and leaf respiration, and control of leaf water balance.
The mechanism of stomatal movement depends on the structural features of guard cells and is associated with changes in their turgor pressure. A characteristic feature of the structure of stomatal guard cells is the uneven thickening of their membranes. As a result of this, the rear wall of the guard cell, thinner and more elastic, protrudes in the direction from the gap with increasing turgor, the front wall becomes straight or concave, and the entire cell bends in the direction from the gap. At the same time, the stomata open.
Changes in turgor pressure of guard cells are associated with significant energy expenditure. Organic acids participate in the regulation of the osmotic pressure of guard cells, and monovalent cations, especially potassium, also play a significant role. The entry of monovalent cations into the vacuole of guard cells increases their osmotic potential, water enters the cells, and the stomata opens. The release of osmotic active substances from the vacuoles into the cytoplasm of the guard cells or from the cell generally reduces the osmotic pressure and the stomata closes. The electrical neutrality of guard cells with open stomata is maintained mainly due to the formation of organic anions.
The entry of water into the cell
Note 2
The entry of water into a cell is a very complex process, determined by many factors. The entire system of cytoplasmic colloids actively participates in the absorption of water.
The force with which a cell sucks in water is called suction.
The flow of water into a living cell, the semi-permeability and elasticity of the cytoplasm can be demonstrated by the following experiment. A drop of a $6-8\%$ solution of potassium nitrate $(KNO_3)$ is applied to a glass slide, close to the cover glass, where the Elodea leaf is in water. On the other side of the cover glass, also close to it, we bring filter paper, which draws off the water until the nitrate solution, entering under the cover glass, completely replaces it. After some time, even with a low microscope magnification, we observe that the protoplast moves away from the cell membrane. This process is called plasmolysis. Later, the protoplast separates from the entire inner surface of the shell, becomes rounded and is located directly in the middle of the cell or near one of its walls. Thus, the space between the protoplast and the cell membranes is filled with a plasmolytic solution.
Evaporation of water by leaves
The evaporation of water by plants is called transpiration. The entire surface of the plant body evaporates water, especially the leaf. Taking into account the different forms of water evaporation, two types of transpiration are distinguished. Cuticular transpiration is characterized by evaporation of the entire leaf surface. Stomatal transpiration accordingly occurs through the stomata of the leaf.
The biological role of transpiration promotes the entry of carbon dioxide into the leaf, which provides carbon nutrition to the plant. The role is also to protect the sheet from overheating.
Question 1. What cells form the leaf blade?
The top and bottom of the leaf is covered with one of the types of integumentary tissue - skin. As a rule, skin cells fit tightly to each other, are transparent, and do not contain chloroplasts. In addition, the skin contains stomatal guard cells arranged in pairs (with chloroplasts). Between the upper and lower skins is contained the pulp of the leaf, formed by the main tissue, the cells of which have numerous chloroplasts. In the thickness of the leaf there are veins of conductive and mechanical tissues. They contain vessels, sieve tubes and fibers, respectively.
Question 2: What is the significance of the leaf skin? What tissue cells is it formed from?
The leaf is covered with a single-layer skin on the upper and lower sides. The skin is a living covering tissue. Its cells are tightly closed together and lack chloroplasts. They are transparent and allow sunlight to pass through the leaf well. The skin protects the leaf from excess moisture loss and serves for mechanical support. Hairs and spines of various shapes may be located on the surface of the skin cells. Often the skin secretes a cuticle, or waxy coating, which protects the plant from evaporation. To ensure water and gas exchange between skin cells there are stomata.
Question 3. What are stomata and where are they located?
Gas exchange and water evaporation occur through special formations - stomata. The stomata consists of two epidermal cells called guard cells. Between the guard cells there is a gap that opens or closes depending on the amount of turgor pressure in them. The gap leads into an air cavity, the edges of which are composed of parenchyma cells. There are from 40 to 300 stomata on 1 mm of leaf surface. Moreover, in terrestrial plants the stomata are located on the underside of the leaf, in aquatic plants - on the upper. Through the stomata, CO enters the intercellular spaces of the photosynthetic parenchyma and O2 and H2O exit.
Stomata are usually located on the lower side of the leaf blade, and in aquatic plants (water lily, egg capsule) - only on the upper side. A number of plants (cereals, cabbage) have stomata on both sides of the leaf.
Question 4. What structure do the leaf pulp cells have? What type of fabric are they?
The pulp of the leaf consists of cells of the main tissue. Two or three layers of pulp, located directly under the upper skin, are formed by elongated cells tightly adjacent to each other. In appearance, they resemble columns of the same size, which is why the upper part of the main leaf tissue is called columnar. There are especially many chloroplasts in the cytoplasm of soft cells. Beneath the columnar tissue lie more rounded or irregularly shaped cells. They lie loosely (not adjacent to each other), with large intercellular spaces between them. These cells form spongy tissue.
Question 5. Which leaf cells contain the most chloroplasts?
The cells adjacent to the upper epidermis form columnar tissue. They are located perpendicular to the surface of the sheet and tightly adjacent to each other. In most plants, the columnar tissue is single-layered, sometimes double-layered. Its cells contain many chloroplasts that carry out photosynthesis.
Question 6. What function do the conducting bundles of the sheet perform? What tissue cells are they formed by?
The conductive bundle of the leaf, or vein, consists of wood vessels, sieve tubes of bast and mechanical tissue. There is no cambium between the bast and the wood in the bunches. The wood in the bundle faces the top side of the leaf, and the bast faces the bottom. Water and minerals dissolved in it move through the vessels of the conductive bundles. Through sieve tubes from leaves - solutions of organic substances.
Every moment of our life we breathe. We absorb invaluable oxygen and release carbon dioxide that we do not need. And without this process our life is simply impossible. And it is plants that provide its basis. They are the ones who simply magically transform carbon dioxide into the oxygen we need so much. But how do they manage to do this? What structures provide this? The answer lies right under our noses. These are leaves! Or rather, the cells that form them. And in today's lesson we will get acquainted with this amazing factory.
Photosynthesis is the process of formation of organic substances from carbon dioxide and water in the light with the participation of photosynthetic pigments (chlorophyll in plants, bacteriochlorophyll and bacteriorhodopsin in bacteria).
Leaf structure closely related to its functioning. The sheet consists of 3 layers
The top and bottom of the leaf are covered with transparent skin (see Fig. 1), which protects it from damage and drying out.
Rice. 1. Leaf peel
The skin (epidermis) is one of the types of integumentary tissue of a plant. Usually consists of 1 layer of cells. Its cells secrete a wax-like cuticle that covers the surface of the leaf, protecting the leaf from water evaporation (transpiration).
Among the skin cells there are guard cells, the cytoplasm of which contains chloroplasts. There is a gap between the cells. These formations are called stomata (see Fig. 2).
Rice. 2. Stomata
Stomata perform the functions of gas exchange and water evaporation. Stomatal cells are able to close, preventing excessive evaporation of water. The movements of stomatal cells depend on the plant’s water supply, light, and temperature.
Plastids- these are membrane organelles found in photosynthetic eukaryotic organisms (higher plants, lower algae, some unicellular organisms). Plastids are surrounded by two membranes; their matrix has its own genomic system; the functions of plastids are related to the energy supply of the cell, which is used for the needs of photosynthesis.
Stomata are mainly located on the inside of the leaf. In floating leaves of aquatic plants, stomata are located only on the upper side of the leaf. Underwater leaves of aquatic plants do not have stomata.
The lower layer of the epidermis often has hairs that reduce overheating of the plant and reduce the rate of transpiration.
There are from 50 to 500 stomata per 1 mm2 of leaf surface.
Leaf skin structure
Take a geranium leaf. Break it and use a dissecting needle to remove a piece of skin. Prepare the preparation and examine it under a microscope (see Fig. 3).
Rice. 3. Geranium leaf peel preparation
Find colorless cells and describe them. Find stomatal cells, indicate their difference from other cells of the leaf skin. Draw the skin of the leaf and make captions.
Parenchyma- leaf pulp, consists of cells of the main tissue.
At the top there is a columnar parenchyma (see Fig. 4), formed by columnar-shaped cells. They have many chloroplasts, the main photosynthetic cells of the leaf.
Rice. 4. Columnar parenchyma cells
Below is the spongy parenchyma (see Fig. 5). Consists of irregularly shaped cells that are loosely adjacent to each other. Few chloroplasts. The intercellular spaces are filled with air. Cells of spongy parenchyma carry out intense effective gas exchange.
Rice. 5. Cells of spongy parenchyma
Gas exchange in biology is the absorption and release of gas, especially oxygen and carbon dioxide, in living organisms. Involves inhaling oxygen and exhaling carbon dioxide. In plants, algae and bacteria that carry out photosynthesis, the opposite process can occur, where carbon dioxide is inhaled and pure oxygen is exhaled.
Veins(see Fig. 6) - conductive bundles of the sheet. Consist of vessels, sieve tubes and fibers.
Fibers- highly elongated cells with thick walls. Gives strength to the sheet.
Vessels transport minerals from roots to leaves. They are made up of dead cells.
Sieve tubes consist of living cells through which solutions of organic substances move from the leaves.
Rice. 6. Leaf veins, microscopic specimen
Cellular structure of the leaf
Examine the finished micropreparations of a cross section of a leaf. Examine the cells of the leaf skin, find the stomata. Examine the leaf parenchyma cells. Describe their location and shape. Consider the intercellular spaces, remember their purpose. Find the vascular bundles of the leaf, indicate the cells that form them and their functions. Compare the appearance of the microslide with the picture (see Fig. 7).
Rice. 7. Diagram of the cellular structure of a leaf
Draw a cross section of a leaf, label all its parts and cell types.
Experiment
Place 2 onions in jars of water until it touches the base. Place the jars in a cool place. Place one jar in a lighted place and the other in a dark place. Watch the development of leaves on the bulbs. How is it different? Why?
Bibliography
- Biology. Bacteria, fungi, plants. 6th grade: textbook. for general education institutions / V.V. Beekeeper. - 14th ed., stereotype. - M.: Bustard, 2011. - 304 p.: ill.
- Tikhonova E.T., Romanova N.I. Biology, 6. - M.: Russian Word.
- Isaeva T.A., Romanova N.I. Biology, 6. - M.: Russian Word.
- biolicey2vrn.ucoz.ru ().
- Engschool18.ru ().
- Kaz-ekzams.ru ().
Homework
- Biology. Bacteria, fungi, plants. 6th grade: textbook. for general education institutions / V.V. Beekeeper. - 14th ed., stereotype. - M.: Bustard, 2011. - 304 p.: ill. - With. 119, tasks and questions 2, 5 ().
- How do stomatal cells function? What is their function?
- How are leaf veins arranged?
- * Imagine that the skin cells of the leaves suddenly disappeared. What will happen to them?
The structure of the leaf skin The upper skin (epidermis) is the covering tissue on the reverse side of the leaf, often covered with hairs, cuticle, and wax. On the outside, the leaf has a skin (covering tissue), which protects it from the adverse effects of the external environment: from drying out, from mechanical damage, from penetration of pathogenic microorganisms into the internal tissues. Skin cells are living, they vary in size and shape. Some of them are larger, colorless, transparent and fit tightly to each other, which increases the protective qualities of the integumentary tissue. The transparency of the cells allows sunlight to penetrate into the leaf.
Other cells are smaller and contain chloroplasts, which give them their green color. These cells are arranged in pairs and have the ability to change their shape. In this case, the cells either move away from each other and a gap appears between them, or they move closer to each other and the gap disappears. These cells were called guard cells, and the gap that appeared between them was called stomatal. The stomata opens when the guard cells are saturated with water. When water drains from the guard cells, the stomata closes.
The structure of the stomata Through the stomatal slits, air enters the internal cells of the leaf; through them, gaseous substances, including water vapor, escape from the leaf to the outside. If the plant is insufficiently supplied with water (which can happen in dry and hot weather), the stomata close. By this, plants protect themselves from desiccation, since water vapor does not escape outside when the stomatal slits are closed and is stored in the intercellular spaces of the leaf. In this way, plants retain water during dry periods.
Main leaf tissue Columnar tissue is the main tissue, the cells of which are cylindrical in shape, tightly adjacent to each other and located on the upper side of the leaf (facing the light). Serves for photosynthesis. Each cell of this tissue has a thin membrane, cytoplasm, nucleus, chloroplasts, and vacuole. The presence of chloroplasts gives the green color to the tissue and the entire leaf. The cells that are adjacent to the upper skin of the leaf, elongated and arranged vertically, are called columnar tissue.
Spongy tissue is the main tissue, the cells of which are round in shape, arranged loosely, and large intercellular spaces, also filled with air, are formed between them. Water vapor coming from the cells accumulates in the intercellular spaces of the main tissue. Serves for photosynthesis, gas exchange and transpiration (evaporation).
The number of cell layers of columnar and spongy tissues depends on lighting. In leaves grown in light, columnar tissue is more developed than in leaves grown in dark conditions.
Conductive tissue is the main tissue of the leaf, penetrated by veins. Veins are conductive bundles, since they are formed by conductive tissues - bast and wood. The bast carries out the transfer of sugar solutions from the leaves to all organs of the plant. The movement of sugar occurs through the sieve tubes of the bast, which are formed by living cells. These cells are elongated, and in the place where they touch each other with their short sides in the membranes, there are small holes. Through holes in the membranes, the sugar solution passes from one cell to another. Sieve tubes are adapted to transport organic matter over long distances. Living cells of smaller sizes adhere tightly along the entire length to the side wall of the sieve tube. They accompany the cells of the tube and are called companion cells.
Vital activity of a leaf Green leaves are organs of air nutrition. Green leaves perform an important function in plant life - organic substances are formed here. The structure of the leaf corresponds well to this function: it has a flat leaf blade, and the pulp of the leaf contains a huge number of chloroplasts with green chlorophyll.
Modifications of leaves In the process of adaptation to environmental conditions, the leaves of some plants have changed because they began to play a role that is not characteristic of typical leaves. In barberry, some of the leaves have changed into spines.
Leaf aging and leaf fall Leaf fall is preceded by leaf aging. This means that in all cells the intensity of life processes - photosynthesis, respiration - decreases. The content of substances already present in the cells that are important for the plant decreases and the supply of new ones, including water, is reduced. The breakdown of substances prevails over their formation. Unnecessary and even harmful products accumulate in cells; they are called the end products of metabolism.
In most trees and shrubs, during the aging period, the leaves change color and become yellow or purple. This happens because chlorophyll is destroyed. But besides it, plastids (chloroplasts) contain yellow and orange substances. In the summer they were, as it were, disguised by chlorophyll and the plastids were green. In addition, other yellow or red-crimson coloring substances accumulate in the vacuoles. Together with plastid pigments, they determine the color of autumn leaves. Some plants have leaves that remain green until they die.
Even before the leaf falls from the shoot, a layer of cork forms at its base at the border with the stem. A separating layer is formed from it outside. Over time, the cells of this layer separate from each other, as the intercellular substance that connects them, and sometimes the cell membranes, becomes slimy and destroyed. The leaf is separated from the stem. However, it still remains on the shoot for some time thanks to the conducting bundles between the leaf and the stem. But there comes a moment when this connection is disrupted. The scar at the site of the detached leaf is covered with a protective cloth, cork.
The importance of leaf fall Autumn leaf fall in the forest has important biological significance. Fallen leaves are a good organic and mineral fertilizer. Every year in their deciduous forests, fallen leaves serve as material for mineralization produced by soil bacteria and fungi. In addition, fallen leaves stratify seeds that fell before leaf fall, protect roots from freezing, prevent the development of moss cover, etc. some types of trees shed not only foliage, but also one-year-old shoots.
Epithelial tissue is formed by cells devoid of blood vessels. It forms the surface of a biological body. Many layers of epithelium form the epidermis, the skin. Let us consider in practice the structure of the skin of a geranium leaf, looking at it under microscope. A specially prepared preparation will help you study its features. It is included in some ready-made school microscopy kits, but you can also make it yourself. As you already know, the microsample must be enclosed between grease-free, clean glasses glued with liquid or fir resin.
Geranium is a dicotyledonous herbaceous plant. Resistant to drought and frost, loves light. It is important for humans as a visual aid for basic microbiology studies, and is also used for the production of essential oil, which is actively used in perfumery. The shape of the leaves can be whole lobed or dissected. The best time to study is late spring and all summer.
Preparation of geranium leaf epidermis prepared using a dissecting needle. This is a convenient device in which the needle tip is bent at a right angle and the fingers are grasped by the wooden handle. This tool is very effective in creating histological samples at home or professionally. With its help, you need to carefully peel off a piece of the skin of a softly hairy leaf and then straighten it in water, previously applied with a pipette to a glass slide, then carefully cover it with a coverslip. To give maximum contrast, you can add a drop of Lugol's solution (molecular iodine substance) or brilliant green.
Under a microscope, you can clearly see the elements of the geranium epidermis:
- Primary (living, colorless, well-transmitting sunlight) and secondary (bean-shaped) cells;
- Pores - stomata;
- Stomatal fissure (intercellular space);
- Substomatal air cavity;
- Trichomes (multicellular glandular hairs).
Observations are carried out in a bright field. Bottom illumination will ensure the penetration of light waves through the transparent areas of the microspecimen, thereby providing good contrast of its cellular structure. First, viewing must be carried out at low magnification - this will allow you to “find” the bio-sample in the field of view, place it in the center of the table by rotating the handles of the preparation guide. A minimum magnification of 40x will be obtained by combining a 4x objective (it must be selected on the turret) and a 10x eyepiece. Achieve clear images by tightening the focusing mechanism screw (microscrew). If you insert a digital camera-eyepiece into the eyepiece tube and connect it to a computer, then the picture of the epidermis can be observed on the monitor screen. The computer visualization technique is applicable if it is necessary to take photographs of the microworld or carry out linear (and angular) measurements of the most interesting parts, zoomed in hundreds of times. If there is no video camera, then the results of the study can be recorded in a drawing; this, in particular, is practiced in schools during laboratory work.
