Separate sheet - a sheet with a plate, dissected up to ½ the width of the half-sheet.
The leaf is an extremely important plant organ. The leaf is part of the escape. Its main functions are photosynthesis and transpiration. The leaf is characterized by high morphological plasticity, a variety of shapes and great adaptive capabilities. The base of the leaf can expand in the form of oblique leaf-shaped formations - stipules on each side of the leaf. In some cases, they are so large that they play a role in photosynthesis. Stipules are free or adhering to the petiole, they can shift to the inside of the leaf and then they are called axillary. The bases of the leaves can be turned into a sheath that surrounds the stem and prevents it from bending.
External leaf structure
Leaf blades vary in size: from a few millimeters to 10-15 meters and even 20 (in palm trees). The life span of the leaves does not exceed several months, in some - from 1.5 to 15 years. The size and shape of the leaves are hereditary traits.
Leaf parts
The leaf is a lateral vegetative organ growing from the stem, having bilateral symmetry and a growth zone at the base. The leaf usually consists of a leaf blade, petiole (with the exception of sessile leaves); stipules are characteristic of a number of families. The leaves are simple, having one leaf blade, and complex - with several leaf blades (leaves).
leaf blade- an extended, usually flat part of the leaf, performing the functions of photosynthesis, gas exchange, transpiration and, in some species, vegetative reproduction.
Leaf base (leaf cushion)- the part of the leaf that connects it to the stem. Here is the educational tissue that gives rise to the leaf blade and petiole.
Stipules- paired leaf-shaped formations at the base of the leaf. They can fall off when the sheet is unfolded or remain. They protect the axillary lateral buds and the intercalary educational tissue of the leaf.
Petiole- the narrowed part of the leaf, connecting the leaf blade with the stem with its base. It performs the most important functions: it orients the leaf in relation to the light, it is the location of the intercalated educational tissue, due to which the leaf grows. In addition, it has a mechanical significance for attenuating blows to the leaf blade from rain, hail, wind, etc.
simple and compound leaves
A leaf may have one (simple), several or many leaf blades. If the latter are equipped with joints, then such a sheet is called complex. Due to the articulations on the common leaf petiole, the leaflets of compound leaves fall off one by one. However, in some plants, compound leaves may fall off entirely.
In shape, whole leaves are distinguished as lobed, separate and dissected.
vaned I call a sheet in which the cuts along the edges of the plate reach one quarter of its width, and with a larger recess, if the cuts reach more than a quarter of the plate's width, the sheet is called separate. The blades of a split sheet are called lobes.
Dissected a leaf is called, in which cuts along the edges of the plate reach almost to the midrib, forming segments of the plate. Separated and dissected leaves can be palmate and pinnate, doubly palmate and doubly pinnate, etc. accordingly, a palmately divided leaf, a pinnate leaf are distinguished; unpaired-pinnate leaf of a potato. It consists of a final lobe, several pairs of lateral lobules, between which are even smaller lobules.
If the plate is elongated, and its lobes or segments are triangular, the leaf is called plow-shaped(dandelion); if the lateral lobes are not equal in size, they decrease towards the base, and the final lobe is large and rounded, a lyre-shaped leaf (radish) is obtained.
As for compound leaves, among them there are ternary, palmately compound and pinnately compound leaves. If a complex leaf consists of three leaves, it is called ternary, or trifoliate (maple). If the petioles of the leaflets are attached to the main petiole as if at one point, and the leaflets themselves diverge radially, the leaf is called palmate (lupine). If on the main petiole the lateral leaflets are located on both sides along the length of the petiole, the leaf is called pinnate.
If such a leaf ends at the top with an unpaired single leaflet, it turns out to be an unpaired leaf. If there is no terminal, the leaf is called paired.
If each leaflet of a pinnate leaf, in turn, is complex, then a doubly pinnate leaf is obtained.
Forms of whole leaf blades
A compound leaf is one that has several leaf blades on the petiole. They are attached to the main petiole with their own petioles, often on their own, one by one, fall off, and are called leaflets.
The forms of leaf blades of various plants differ in outline, degree of dissection, shape of the base and top. The outlines can be oval, round, elliptical, triangular and others. The leaf blade is elongated. Its free end can be sharp, blunt, pointed, pointed. Its base is narrowed and drawn to the stem, it can be rounded, heart-shaped.
Attaching the leaves to the stem
The leaves are attached to the shoot with long, short petioles or are sessile.
In some plants, the base of the sessile leaf fuses with the shoot for a long distance (descending leaf) or the shoot pierces the leaf blade through and through (pierced leaf).
Blade edge shape
Leaf blades are distinguished by the degree of dissection: shallow cuts - serrated or palmate edges of the leaf, deep cuts - lobed, separate and dissected edges.
If the edges of the leaf blade do not have any notches, the leaf is called whole-edge. If the notches along the edge of the sheet are shallow, the sheet is called whole.
vane leaf - a leaf, the plate of which is divided into lobes up to 1/3 of the width of the half-leaf.
Separated sheet - a sheet with a plate, dissected up to ½ the width of the half-sheet.
Dissected leaf - a leaf, the plate of which is dissected to the main vein or to the base of the leaf.
The edge of the leaf blade is serrate (acute corners).
The edge of the leaf blade is crenate (rounded protrusions).
The edge of the leaf blade is notched (rounded notches).
Venation
It is easy to notice numerous veins on each leaf, especially distinct and embossed on the underside of the leaf.
Veins- these are vascular bundles connecting the leaf to the stem. Their functions are conductive (supplying leaves with water and mineral salts and removing assimilation products from them) and mechanical (veins are a support for the leaf parenchyma and protect the leaves from tearing). Among the variety of venation, a leaf blade is distinguished with one main vein, from which lateral branches diverge in a pinnate or palmate-pinnate type; with several main veins, differing in thickness and direction of distribution along the plate (arc-nervous, parallel types). There are many intermediate or other forms between the described types of venation.
The original part of all the veins of the leaf blade is located in the petiole of the leaf, from where the main, main vein emerges in many plants, branching later in the thickness of the blade. As you move away from the main, the lateral veins become thinner. The thinnest are mostly located on the periphery, and also far from the periphery - in the middle of areas surrounded by small veins.
There are several types of venation. In monocot plants, venation is arcuate, in which a series of veins enter the plate from the stem or sheath, arcuately directed towards the top of the plate. Most cereals have parallel nerve venation. Arc nerve venation also exists in some dicotyledonous plants, such as plantain. However, they also have a connection between the veins.
In dicotyledonous plants, the veins form a highly branched network and, accordingly, retico-nervous venation is distinguished, which indicates a better supply of vascular bundles.
The shape of the base, apex, petiole of the leaf
According to the shape of the top of the plate, the leaves are blunt, sharp, pointed and pointed.
According to the shape of the base of the plate, the leaves are wedge-shaped, heart-shaped, spear-shaped, arrow-shaped, etc.
The internal structure of the leaf
The structure of the skin of the leaf
Upper skin (epidermis) - integumentary tissue on the reverse side of the leaf, often covered with hairs, cuticle, wax. Outside, the leaf has a skin (integumentary 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. The cells of the skin are alive, they are different 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 that give them a 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 approach each other and the gap disappears. These cells were called trailing cells, and the gap that appeared between them was called stomatal. The stomata open when the guard cells are saturated with water. With the outflow of water from the guard cells, the stomata close.
The structure of the stomata
Through the stomatal gaps, air enters the inner cells of the leaf; through them, gaseous substances, including water vapor, exit the leaf to the outside. With insufficient supply of water to the plant (which can happen in dry and hot weather), the stomata close. In this way, plants protect themselves from drying out, since water vapor does not go outside with closed stomatal slits and is stored in the intercellular spaces of the leaf. Thus, plants conserve water during the dry period.
Main sheet fabric
columnar fabric- the main tissue, the cells of which are cylindrical, 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 shell, cytoplasm, nucleus, chloroplasts, vacuole. The presence of chloroplasts gives the green color to the tissue and the entire leaf. Cells that are adjacent to the upper skin of the leaf, elongated and arranged vertically, are called columnar tissue.
sponge tissue- the main tissue, the cells of which have a rounded shape, are located loosely and large intercellular spaces are formed between them, also filled with air. In the intercellular spaces of the main tissue, water vapor accumulates, coming here from the cells. Serves for photosynthesis, gas exchange and transpiration (evaporation).
The number of layers of cells of columnar and spongy tissues depends on the illumination. In leaves grown in the light, columnar tissue is more developed than in leaves grown in dark conditions.
Conductive fabric- the main tissue of the leaf, penetrated by veins. The veins are conductive bundles, since they are formed by conductive tissues - bast and wood. The bast transfers sugar solutions from the leaves to all organs of the plant. The movement of sugar goes 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 short sides in the shells, there are small holes. Through the holes in the shells, the sugar solution passes from one cell to another. Sieve tubes are adapted to the transfer of organic matter over long distances. Smaller living cells adhere tightly along the entire length to the side wall of the sieve tube. They accompany tube cells and are called companion cells.
The structure of leaf veins
In addition to the bast, wood is also included in the conductive bundle. Through the vessels of the leaf, as well as in the root, water moves with minerals dissolved in it. Plants absorb water and minerals from the soil through their roots. Then, from the roots through the vessels of the wood, these substances enter the above-ground organs, including the cells of the leaf.
The composition of numerous veins includes fibers. These are long cells with pointed ends and thickened lignified shells. Large leaf veins are often surrounded by mechanical tissue, which consists entirely of thick-walled cells - fibers.
Thus, along the veins, a solution of sugar (organic matter) is transferred from the leaf to other plant organs, and from the root - water and minerals to the leaves. Solutions move from the leaf through sieve tubes, and to the leaf through the vessels of the wood.
The underskin is the integumentary tissue on the underside of the leaf, usually bearing stomata.
leaf life
Green leaves are organs of air nutrition. The green leaf performs an important function in the life of plants - organic substances are formed here. The structure of the leaf is well suited to this function: it has a flat leaf blade, and the pulp of the leaf contains a huge amount of chloroplasts with green chlorophyll.
Substances necessary for the formation of starch in chloroplasts
Target: find out what substances are necessary for the formation of starch?
What we do: put two small indoor plants in a dark place. After two or three days, we will put the first plant on a piece of glass, and next we will place a glass with a solution of caustic alkali (it will absorb all carbon dioxide from the air), and we will cover all this with a glass cap. In order to prevent air from entering the plant from the environment, we grease the edges of the cap with petroleum jelly.
We will also put the second plant under the cap, but only next to the plant we will place a glass with soda (or a piece of marble) moistened with a solution of hydrochloric acid. As a result of the interaction of soda (or marble) with acid, carbon dioxide is released. A lot of carbon dioxide is formed in the air under the cap of the second plant.
Both plants will be placed in the same conditions (in the light).
The next day, take a leaf from each plant and first treat with hot alcohol, rinse and act with a solution of iodine.
What we observe: in the first case, the leaf color did not change. The leaf of the plant that was under the cap, where there was carbon dioxide, became dark blue.
Conclusion: this proves that carbon dioxide is necessary for the plant to form organic matter (starch). This gas is part of the atmospheric air. Air enters the leaf through the stomata and fills the spaces between the cells. From the intercellular spaces, carbon dioxide penetrates into all cells.
Formation of organic matter in leaves
Target: find out in which cells of the green leaf organic substances (starch, sugar) are formed.
What we do: houseplant geranium bordered will be placed for three days in a dark closet (so that there is an outflow of nutrients from the leaves). After three days, take the plant out of the closet. We attach a black paper envelope with the word “light” cut out to one of the leaves and put the plant in the light or under an electric light bulb. After 8-10 hours, cut the leaf. Let's take off the paper. We lower the leaf into boiling water, and then for a few minutes into hot alcohol (chlorophyll dissolves well in it). When the alcohol turns green and the leaf becomes discolored, rinse it with water and place it in a weak solution of iodine.
What we observe: blue letters will appear on a discolored sheet (starch turns blue from iodine). The letters appear on the part of the sheet on which the light fell. This means that starch has formed in the illuminated part of the leaf. It is necessary to pay attention to the fact that the white strip along the edge of the sheet is not colored. This explains the fact that there is no chlorophyll in the plastids of the cells of the white stripe of the bordered geranium leaf. Therefore, starch is not detected.
Conclusion: thus, organic substances (starch, sugar) are formed only in cells with chloroplasts, and light is necessary for their formation.
Special studies of scientists have shown that sugar is formed in chloroplasts in the light. Then, as a result of transformations from sugar in chloroplasts, starch is formed. Starch is an organic substance that does not dissolve in water.
There are light and dark phases of photosynthesis.
During the light phase of photosynthesis, light is absorbed by pigments, excited (active) molecules with excess energy are formed, photochemical reactions take place, in which excited pigment molecules take part. Light reactions occur on the membranes of the chloroplast, where chlorophyll is located. Chlorophyll is a highly active substance that absorbs light, primary storage of energy and its further transformation into chemical energy. Yellow pigments, carotenoids, also take part in photosynthesis.
The process of photosynthesis can be represented as a summary equation:
6CO 2 + 6H 2 O \u003d C 6 H 12 O 6 + 6O 2
Thus, the essence of light reactions is that light energy is converted into chemical energy.
Dark reactions of photosynthesis take place in the matrix (stroma) of the chloroplast with the participation of enzymes and products of light reactions and lead to the synthesis of organic substances from carbon dioxide and water. Dark reactions do not require the direct participation of light.
The result of dark reactions is the formation of organic compounds.
Photosynthesis takes place in chloroplasts in two steps. In the granae (thylakoids), light-induced reactions occur, and in the stroma, reactions not associated with light, dark, or carbon fixation reactions.
Light reactions
1. Light, falling on the chlorophyll molecules that are in the membranes of the thylakoids of the grana, leads them to an excited state. As a result of this, electrons ē leave their orbits and are transported with the help of carriers outside the thylakoid membrane, where they accumulate, creating a negatively charged electric field.
2. The place of released electrons in chlorophyll molecules is occupied by water electrons ē, since water undergoes photodecomposition (photolysis) under the action of light:
H 2 O↔OH‾+H +; OH‾−ē→OH.
OH‾ hydroxyls, becoming OH radicals, combine: 4OH→2H 2 O + O 2, forming water and free oxygen, which is released into the atmosphere.
3. H + protons do not penetrate the thylakoid membrane and accumulate inside using a positively charged electric field, which leads to an increase in the potential difference on both sides of the membrane.
4. When a critical potential difference (200 mV) is reached, H + protons rush out through the proton channel in the ATP synthetase enzyme built into the thylakoid membrane. At the exit from the proton channel, a high level of energy is created, which goes to the synthesis of ATP (ADP + P → ATP). The resulting ATP molecules pass into the stroma, where they participate in carbon fixation reactions.
5. H + protons that have come to the surface of the thylakoid membrane combine with ē electrons, forming atomic hydrogen H, which goes to the reduction of NADP + carriers: 2ē + 2H + \u003d NADP + → NADP ∙ H 2 (carrier with attached hydrogen; reduced carrier ).
Thus, the chlorophyll electron activated by light energy is used to attach hydrogen to the carrier. NADP∙H2 passes into the stroma of the chloroplast, where it participates in carbon fixation reactions.
Carbon fixation reactions (dark reactions)
It is carried out in the stroma of the chloroplast, where ATP, NADP ∙ H 2 come from thylakoids gran and CO 2 from the air. In addition, five-carbon compounds are constantly found there - C 5 pentoses, which are formed in the Calvin cycle (CO 2 fixation cycle). Simplified, this cycle can be represented as follows:
1. CO 2 is added to C 5 pentose, as a result of which an unstable hexagonal C 6 compound appears, which splits into two three-carbon groups 2C 3 - trioses.
2. Each of the triose 2C 3 takes one phosphate group from two ATP, which enriches the molecules with energy.
3. Each of the triose 2C 3 adds one hydrogen atom from two NADP ∙ H2.
4. After that, some trioses combine to form carbohydrates 2C 3 → C 6 → C 6 H 12 O 6 (glucose).
5. Other trioses combine to form pentoses 5C 3 → 3C 5 and are again included in the CO 2 fixation cycle.
Total reaction of photosynthesis:
6CO 2 + 6H 2 O chlorophyll light energy → C 6 H 12 O 6 + 6O 2
In addition to carbon dioxide, water takes part in the formation of starch. Her plant receives from the soil. The roots absorb water, which rises through the vessels of the vascular bundles into the stem and further into the leaves. And already in the cells of a green leaf, in chloroplasts, organic matter is formed from carbon dioxide and water in the presence of light.
What happens to organic substances formed in chloroplasts?
The starch formed in chloroplasts under the influence of special substances turns into soluble sugar, which enters the tissues of all plant organs. In the cells of some tissues, sugar can turn back into starch. Spare starch accumulates in colorless plastids.
From sugars formed during photosynthesis, as well as mineral salts absorbed by the roots from the soil, the plant creates the substances that it needs: proteins, fats and many other proteins, fats and many others.
Part of the organic substances synthesized in the leaves is spent on the growth and nutrition of the plant. The other part is kept in reserve. In annual plants, reserve substances are deposited in seeds and fruits. In biennials in the first year of life, they accumulate in the vegetative organs. In perennial grasses, substances are stored in underground organs, and in trees and shrubs - in the core, the main tissue of the bark and wood. In addition, at a certain year of life, organic substances also begin to be stored in fruits and seeds.
Types of plant nutrition (mineral, air)
In the living cells of a plant, there is a constant exchange of substances and energy. Some substances are absorbed and used by the plant, others are released into the environment. Complex substances are formed from simple substances. Complex organic substances are broken down into simple ones. Plants accumulate energy, and in the process of photosynthesis release it during respiration, using this energy to carry out various life processes.
Gas exchange
Leaves, thanks to the work of stomata, also carry out such an important function as gas exchange between the plant and the atmosphere. Through the stomata of the leaf with atmospheric air, carbon dioxide and oxygen enter. Oxygen is used for respiration, carbon dioxide is necessary for the plant to form organic substances. Through the stomata, oxygen is released into the air, which was formed during photosynthesis. Carbon dioxide, which appeared in the plant in the process of respiration, is also removed. Photosynthesis is carried out only in the light, and respiration in the light and in the dark, i.e. constantly. Respiration in all living cells of plant organs occurs continuously. Like animals, plants die when they stop breathing.
In nature, there is an exchange of substances between a living organism and the environment. The absorption of certain substances by the plant from the external environment is accompanied by the release of others. Elodea, being an aquatic plant, uses carbon dioxide dissolved in water for nutrition.
Target: Let's find out what substance releases Elodea into the external environment during photosynthesis?
What we do: we cut the stems of the branches under water (boiled water) at the base and cover with a glass funnel. A test tube filled to the brim with water is placed on the funnel tube. Do this in two ways. Put one container in a dark place, and put the other in bright sunlight or artificial light.
Add carbon dioxide to the third and fourth containers (add a small amount of baking soda or you can breathe into a tube) and also put one in the dark and the other in sunlight.
What we observe: after some time, in the fourth variant (a vessel standing in bright sunlight), bubbles begin to stand out. This gas displaces water from the test tube, its level in the test tube is displaced.
What we do: when the water is completely displaced by the gas, carefully remove the test tube from the funnel. Close the hole tightly with the thumb of the left hand, and quickly insert a smoldering splinter into the test tube with the right.
What we observe: the splinter ignites with a bright flame. Looking at the plants that were placed in the dark, we will see that no gas bubbles are released from the elodea, and the test tube remains filled with water. The same with test tubes in the first and second versions.
Conclusion: hence it follows that the gas that the elodea gave off is oxygen. Thus, the plant releases oxygen only when there are all conditions for photosynthesis - water, carbon dioxide, light.
Evaporation of water from leaves (transpiration)
The process of evaporation of water by leaves in plants is regulated by the opening and closing of stomata. By closing the stomata, the plant protects itself from water loss. The opening and closing of stomata is influenced by external and internal factors, primarily temperature and sunlight intensity.
Plant leaves contain a lot of water. It enters through the conducting system from the roots. Inside the leaf, water moves along the cell walls and along the intercellular spaces to the stomata, through which it leaves in the form of steam (evaporates). This process is easy to check if you perform a simple adaptation, as shown in the figure.
The evaporation of water from a plant is called transpiration. Water evaporates from the surface of the leaf of the plant, especially intensively from the surface of the leaf. There are cuticular transpiration (evaporation by the entire surface of the plant) and stomatal transpiration (evaporation through the stomata). The biological significance of transpiration is that it is a means of moving water and various substances around the plant (suction action), promotes the entry of carbon dioxide into the leaf, carbon nutrition of plants, and protects the leaves from overheating.
The rate of evaporation of water by leaves depends on:
- biological characteristics of plants;
- growth conditions (plants in arid areas evaporate little water, wet ones - much more; shady plants evaporate less water than light plants; plants evaporate a lot of water in heat, much less - in cloudy weather);
- lighting (scattered light reduces transpiration by 30-40%);
- water content in leaf cells;
- osmotic pressure of cell sap;
- soil, air and plant body temperatures;
- air humidity and wind speed.
The greatest amount of water evaporates in some species of tree species through leaf scars (the scar left by fallen leaves on the stem), which are the most vulnerable places on the tree.
The relationship between the processes of respiration and photosynthesis
The entire process of respiration takes place in the cells of the plant organism. It consists of two stages, during which organic matter is broken down into carbon dioxide and water. At the first stage, with the participation of special proteins (enzymes), glucose molecules break down into simpler organic compounds and some energy is released. This stage of the respiratory process occurs in the cytoplasm of cells.
At the second stage, simple organic substances formed at the first stage decompose into carbon dioxide and water under the action of oxygen. This releases a lot of energy. The second stage of the respiratory process proceeds only with the participation of oxygen and in special cells of the cell.
Absorbed substances in the process of transformations in cells and tissues become substances from which the plant builds its body. All transformations of substances that occur in the body are always accompanied by energy consumption. A green plant, as an autotrophic organism, absorbs the light energy of the Sun and accumulates it in organic compounds. In the process of respiration, during the breakdown of organic substances, this energy is released and used by the plant for vital processes that occur in cells.
Both processes - photosynthesis and respiration - go through numerous successive chemical reactions in which one substance is converted into another.
So, in the process of photosynthesis from carbon dioxide and water received by the plant from the environment, sugars are formed, which are then converted into starch, fiber or proteins, fats and vitamins - substances that the plant needs for nutrition and energy storage. In the process of respiration, on the contrary, the organic substances created in the process of photosynthesis are split into inorganic compounds - carbon dioxide and water. In this case, the plant receives the released energy. These transformations of substances in the body are called metabolism. Metabolism is one of the most important signs of life: with the cessation of metabolism, the life of a plant ceases.
Influence of environmental factors on leaf structure
The leaves of plants in wet places are usually large with a large number of stomata. A lot of moisture evaporates from the surface of these leaves.
The leaves of dryland plants are small and have adaptations to reduce evaporation. This is a dense pubescence, wax coating, a relatively small number of stomata, etc. Some plants have soft and juicy leaves. They store water.
The leaves of shade-tolerant plants have only two or three layers of rounded, loosely adjacent cells. Large chloroplasts are located in them so that they do not obscure each other. Shade leaves tend to be thinner and darker green in color as they contain more chlorophyll.
In plants of open places, the pulp of the leaf has several layers of columnar cells tightly adjacent to each other. They contain less chlorophyll, so the light leaves are lighter in color. Those and other leaves can sometimes be found in the crown of the same tree.
Dehydration protection
The outer wall of each cell of the skin of the leaf is not only thickened, but also protected by a cuticle, which does not pass water well. The protective properties of the skin are greatly enhanced by the formation of hairs that reflect the sun's rays. Due to this, the heating of the sheet is reduced. All this limits the possibility of evaporation of water from the surface of the sheet. With a lack of water, the stomatal gap closes and the steam does not go outside, accumulating in the intercellular spaces, which leads to the cessation of evaporation from the leaf surface. Plants of hot and dry habitats have a small plate. The smaller the leaf surface, the less the risk of excessive water loss.
Leaf modifications
In the process of adaptation to environmental conditions, the leaves of some plants have changed because they began to play a role not characteristic of typical leaves. In barberry, some of the leaves have changed into thorns.
Leaf aging and leaf fall
Leaf fall is preceded by leaf aging. This means that in all cells the intensity of vital processes decreases - photosynthesis, respiration. The content of substances that are already important for the plant in the cells is reduced and the intake of new ones, including water, is reduced. The breakdown of substances predominates over their formation. Cells accumulate unnecessary and even harmful products, they are called the end products of metabolism. These substances are removed from the plant when the leaves are shed. The most valuable compounds flow through the conducting tissues from the leaves to other organs of the plant, where they are deposited in the cells of storage tissues or are immediately used by the body for nutrition.
In most trees and shrubs, during the aging period, the leaves change color and become yellow or crimson. This is because chlorophyll is destroyed. But besides it, plastids (chloroplasts) contain yellow and orange substances. In the summer they were, as it were, masked by chlorophyll and the plastids had a green color. In addition, other dyes of yellow or red-crimson color accumulate in the vacuoles. Together with plastid pigments, they determine the color of autumn leaves. In some plants, the leaves remain green until they die.
Even before the leaf falls from the shoot, a layer of cork forms at its base on the border with the stem. A separating layer is formed outside of it. Over time, the cells of this layer separate from each other, as the intercellular substance that connected them, and sometimes the membranes of the cells, becomes mucilaginous and collapses. The leaf is separated from the stem. However, for some time it still remains on the shoot due to conductive bundles between the leaf and the stem. But there comes a moment of violation of this connection. The scar in place of the detached sheet is covered with a protective cloth, cork.
As soon as the leaves reach the maximum size, aging processes begin, leading, in the end, to the death of the leaf - its yellowing or redness associated with the destruction of chlorophyll, the accumulation of carotenoids and anthocyanins. As the leaf ages, the intensity of photosynthesis and respiration also decreases, chloroplasts degrade, some salts (calcium oxalate crystals) accumulate, and plastic substances (carbohydrates, amino acids) flow out of the leaf.
In the process of leaf aging near its base in dicotyledonous woody plants, the so-called separating layer is formed, which consists of an easily exfoliating parenchyma. On this layer, the leaf is separated from the stem, and on the surface of the future leaf scar a protective layer of cork tissue is formed in advance.
On the leaf scar, cross-sections of the leaf trace are visible in the form of dots. The sculpture of the leaf scar is different and is a characteristic feature for the taxonomy of lepidophytes.
In monocots and herbaceous dicots, the separating layer, as a rule, is not formed, the leaf dies off and collapses gradually, remaining on the stem.
In deciduous plants, the fall of leaves for the winter has an adaptive value: by shedding leaves, plants sharply reduce the evaporating surface, and protect themselves from possible breakages under the weight of snow. In evergreens, massive leaf fall is usually timed to coincide with the beginning of the growth of new shoots from the buds and therefore occurs not in autumn, but in spring.
Autumn leaf fall in the forest is of great biological importance. 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 have fallen before leaf fall, protect roots from freezing, prevent the development of moss cover, etc. some types of trees drop not only foliage, but also one-year-old shoots.
