How does sap travel up the tree? The movement of water through the plant What is the name of the process of raising water into the sky

The xylem of flowering plants consists of two types of water-carrying structures, tracheids and vessels. In sec. 8.2.1 we have already talked about how the corresponding cells look in a light microscope, as well as in microphotographs obtained using a scanning electron microscope (Fig. 8.11). The structure of the secondary xylem (wood) will be considered in Sec. 21.6.6.

Xylem, together with phloem, forms the conductive tissue of higher plants. This fabric consists of so-called conductive beams, which consist of special tubular structures. On fig. 14.15 shows how the vascular bundles are arranged and how they are located in the primary stem in dicotyledonous and monocotyledonous plants.

14.19. Summarize in tabular form the differences in primary stem structure between dicots and monocots.

14.20. What is the three-dimensional shape of the following tissue components: a) epidermis; b) xylems; c) pericycle of dicotyledonous and d) pith?

That water can rise up the xylem is very easy to demonstrate by immersing the lower end of a cut stem in a dilute solution of a dye such as eosin. The dye ascends the xylem and spreads throughout the network of leaf veins. If thin sections are made and viewed under a light microscope, the dye will be found in the xylem.

The fact that xylem conducts water is best shown by "ringing" experiments. Such experiments were carried out long before radioactive isotopes were used, which made it very easy to trace the path of a substance in a living organism. In one of the variants of the experiment, a ring of bark with phloem is cut out. If the experience is not very long, such "ringing" does not affect the rise of water along the stem. However, if the bark flap is peeled off and the xylem cut out without damaging the bark flap, the plant will quickly wither.

Any theory explaining the movement of water through the xylem cannot fail to take into account the following observations:

1. Xylem vessels are dead tubes with a narrow lumen, the diameter of which varies from 0.01 mm in "summer" wood to about 0.2 mm in "spring" wood.

2. Large quantities of water are transported relatively quickly: in tall trees, water rise rates of up to 8 m/h have been recorded, while in other plants it is often around 1 m/h.

3. To raise water through such tubes to the top of a tall tree, a pressure of about 4000 kPa is needed. The tallest trees - California giant sequoias (conifers that have no vessels and only tracheids) and Australian eucalyptus trees - are over 100 m. Water rises through thin capillary tubes due to high surface tension under the action of capillary forces; however, only due to these forces, even through the thinnest vessels of xylem, water will not rise above 3 m.

All these observations are satisfactorily explained by the theory clutch(cohesion), or theory tension. According to this theory, the rise of water from the roots is due to the evaporation of water from the cells of the leaf. As we have already said in sect. 14.3, evaporation leads to a decrease in the water potential of the cells adjacent to the xylem. Therefore, water enters these cells from xylem sap, which has a higher water potential; at the same time, it passes through the moist cellulose cell walls of the xylem vessels at the ends of the veins, as shown in Fig. 14.7.

The vessels of the xylem are filled with water, and as the water leaves the vessels, tension is created in the water column. It is transmitted down the stem all the way from leaf to root thanks to clutch(cohesion) of water molecules. These molecules tend to "stick" to each other because they are polar and are attracted to each other by electrical forces and then held together by hydrogen bonds (Sec. 5.1.2). In addition, they tend to stick to the walls of blood vessels under the action of forces adhesion. The high cohesion of water molecules means that a relatively large tensile force is required to break the water column; in other words, the water column has a high tensile strength. The tension in the xylem vessels reaches such a force that it can pull the entire column of water upwards, creating a mass flow; in this case, water enters the base of such a column in the roots from neighboring root cells. It is necessary that the walls of the xylem vessels also have high strength and are not pressed inward.

This strength is provided by lignin and cellulose. Evidence that the contents of the xylem vessels are under the influence of a large tensile force was obtained by measuring the daily changes in trunk diameter in trees using an instrument called a dendrometer. The minimum values were recorded in the daytime, when the transpiration rate is maximum. The tiny contraction of individual xylem vessels added up and gave a quite measurable decrease in the diameter of the entire trunk.

Tensile strength estimates for the xylem sap column range from about 3,000 to 30,000 kPa, with lower values coming later. The water potential of the order of -4000 kPa was recorded in the leaves, and the strength of the xylem sap column is probably sufficient to withstand the resulting tension. It is possible, of course, that the column of water may sometimes burst, especially in vessels of large diameter.

Critics of this theory point out that any discontinuity of the sap column must immediately stop the entire flow, since the vessel must be filled with air and water vapor (the phenomenon cavitation). Cavitation can be caused by strong shaking, barrel bending, or lack of water. It is well known that during the summer the water content in the tree trunk gradually decreases and the wood fills with air. This is used in the timber industry because such wood has better buoyancy. However, the rupture of the water column in some of the vessels does not greatly affect the rate of water transfer. This can be explained by the fact that water passes from one vessel to another or bypasses the air plug, moving along neighboring parenchyma cells and their walls. In addition, according to calculations, to maintain the observed flow rate, it is quite sufficient that at least a small part of the vessels function at any given time. In some trees and shrubs, water moves only along the youngest outer layer of wood, which is called sapwood. In oak and ash, for example, water moves mainly through the vessels of the current year, and the rest of the sapwood acts as a water reserve. During the growing season, more and more new vessels are added all the time, but most of them are formed at the beginning of the season, when the flow rate is much higher.

The second force that is involved in the movement of water through the xylem is root pressure. It can be detected and measured at the moment when the crown is cut off, and the trunk with roots continues to secrete juice from the xylem vessels. This exudation process is inhibited by cyanide and other respiration inhibitors and is terminated by a lack of oxygen or a decrease in temperature. To operate such a mechanism, apparently, active secretion into the xylem sap of salts and other water-soluble substances that reduce the water potential is required. Then water enters the xylem by osmosis from neighboring root cells.

A positive hydrostatic pressure of about 100-200 kPa (in exceptional cases up to 800 kPa) generated by root pressure alone is usually not enough to ensure the movement of water up the xylem, but its contribution in many plants is undoubted. In slowly transpiring herbaceous forms, however, this pressure is quite sufficient to induce guttation. guttation- this is the removal of water in the form of liquid droplets on the surface of the plant (whereas during transpiration, water comes out in the form of vapor). All conditions that reduce transpiration, i.e., low light, high humidity, etc., promote guttation. It is quite common in many tropical rainforest plants and is often seen on the leaf tips of young seedlings.

14.21. List the properties of xylem, due to which it provides the transport of water and substances dissolved in it over long distances.

Sequoias growing in California are among the tallest trees in the world. They reach a height of 110 meters. Some trees are 2000-3000 years old! It is difficult to convey the indelible impression that a walk among these giants leaves. The truth of creation is powerfully revealed here. The cells of a tree are organized to make up roots, trunk, bark, water columns, branches, and leaves. The tree resembles a giant chemical factory. Extremely complex chemical processes take place in it in perfect order.

The amazing thing is that this huge tree grows from a small seed weighing about 5 grams. Just think: all the information about the development and organization of these giants is embedded in their DNA, in a tiny round seed. The seed follows all the "instructions" that are in its DNA, and turns into a giant structure, incomparable in appearance and size, containing 2500 tons of wood. Amazing, isn't it?

Giant sequoia "General Sherman".
Its height is 83.8 m, and the perimeter of the trunk at the base is 34.9 m. The age of the tree is 2500 years. This tree is considered the largest living organism on Earth. Its weight, together with the root system, is 2,500 tons. The volume of a tree is 17,000 cubic meters, which is 10 times more than the volume of a blue whale.

Scripture says: “God is exalted in His might, and who is a teacher like Him? …Remember to exalt His works that people see. All people can see them; a man can see them from afar” (Job 36:22,24-25). Indeed, all people can see the works of God.

A sequoia produces up to 600 liters of water per day through its leaves, so it constantly raises water from roots to branches, overcoming the force of gravity. How is it possible for a tree that does not have mechanical pumps? 100 meters is a really impressive height, comparable to two 14-story buildings. It turns out that inside the trunk of a sequoia there is a special system of narrow interconnected tubules called xylem. This complex internal tissue of the tree serves to conduct water from the roots to the leaves. Xylem tubes form cells located one above the other. Together they form an incredibly long column, extending from the roots through the trunk to the leaves. To "pump" water, the sequoia must form a continuous column of water in this pipe.

A tree maintains water throughout its life. Remember how a strong wind bends a tree and branches. However, due to the fact that the conductive tube is made up of millions of small pieces, butted together, the flow of water is constantly kept. One solid tube would not have done the job. Since water usually doesn't flow upwards, how does a tree manage to pump it to such a height? The roots "pull" the water up, and the action of capillarity (the ability of water to rise slightly along the walls of the tube) adds pressure. However, this force provides the tree with a rise in water only by 2-3 meters. The main driving force is evaporation and attraction between water molecules. Molecules have positively and negatively charged particles, due to which they adhere to each other with tremendous force, which, according to experimental measurements, is 25-30 atmospheres (1 atmosphere is equal to normal atmospheric pressure at sea level). That's enough to push through a World War II submarine 350 meters underwater. Sequoia easily maintains a pressure of 14 atmospheres at the top of the water column. Water, evaporating from the leaves, generates suction power. The water molecule evaporates from the leaf and, due to the force of molecular attraction, pulls other molecules around it with it, which creates a slight suction in the water column and pulls water from neighboring cells of the leaf. These molecules, in turn, attract the surrounding molecules. The chain of motion continues all the way to the ground and moves the water from the roots to the top of the tree, much like a pump lifts water from a well to the surface.

We understand that the tree itself could not come up with such a complex system that wisely uses the physics of water and the energy of the Sun. We give all Glory to God, the Creator of heaven and earth. Giant trees testify to the historicity of the book of Genesis, which reveals to us their true origin: “And God said, Let the earth bring forth grass, grass yielding seed, fruitful tree yielding fruit after its kind, in which is its seed, on the earth. And it was so” (Genesis 1:11).

D. Kurovsky

The amazing thing is that this huge tree grows from a small seed weighing 58 grams. Just think: all the information about the development and organization of these giants is in their DNA, in a tiny, round seed. The seed fulfills all the "instructions" that are in its DNA and turns into a giant structure, incomparable to anything in appearance and size. Amazing, isn't it?

Giant sequoia "General Sherman". Its height is 83.8 m, and the perimeter of the trunk at the base is 34.9 m. The age of the tree is 2500 years. This tree is considered the largest living organism on Earth. Its weight together with the root system is 2500 tons. The volume of the tree is 17000 cubic meters, which is 10 times more than the volume of the blue whale.

Scripture says: “God is exalted in His might, and who is a teacher like Him? Remember to exalt His works that people see. All people can see them; a person can see them from afar". (Job 36:22-25) Indeed, all people can see His deeds.

Raising water to the height of a 30-story building

Through your leaves sequoia emits up to 600 liters of water per day, so it constantly raises water from roots to branches, overcoming the force of gravity. How is it possible for a tree that does not have mechanical pumps? 100 meters is a really impressive height, comparable to two 14-story buildings. It turns out that inside the trunk redwoods there is a special system of narrow interconnected tubules called xylem. This complex internal tissue of the tree serves to conduct water from the roots to the leaves. Xylem tubes form cells located one above the other. Together they form an incredibly long column, extending from the roots through the trunk to the leaves. To "pump" water, sequoia should form a continuous column of water in this pipe.

A tree maintains water throughout its life. Remember how a strong wind bends a tree and branches. However, due to the fact that the conductive tube is made up of millions of small pieces, butted together, the flow of water is constantly kept. One solid tube would not have done the job. Since water usually doesn't flow upwards, how does a tree manage to pump it to such a height? The roots "pull" the water up, and the action of capillarity (the ability of water to rise slightly along the walls of the tube) adds pressure. However, this force provides sequoias with a rise in water of only 2-3 meters. The underlying driving force is evaporation and attraction between water molecules. Molecules have positively and negatively charged particles, due to which they adhere to each other with tremendous force, which, according to experimental measurements, is 25-30 atmospheres (1 atmosphere is equal to normal atmospheric pressure at sea level).

Distribution system shown in cross section. Transfer tubes are made up of cells and are designed to carry substances: water and minerals to the leaves through various channels. One important feature of this system in plants is the constant renewal of the xylem and phloem tubes.

That's enough to push through a World War II-era submarine 350 meters underwater. Sequoia it easily maintains a pressure of 14 atmospheres at the top of the water column. Water, evaporating from the leaves, generates suction power. The water molecule evaporates from the leaf and, due to the force of molecular attraction, pulls other molecules around it with it. This creates a slight suction in the water column and draws water away from neighboring leaf cells. These molecules, in turn, attract the surrounding molecules. The chain of motion continues all the way to the ground and moves the water from the roots to the top of the tree, much like a pump lifts water from a well to the surface.

We understand that tree could not have come up with such a complex system on its own, having learned to use the physics of water and the energy of the Sun so wisely. We give all Glory to God, the Creator of heaven and earth. Giant trees testify to the historicity of the book of Genesis, which reveals to us their true origin: “And God said, Let the earth bring forth grass, herb yielding seed, fruitful tree yielding fruit after its kind, in which is its seed, on the earth. And so it became". (Gen. 1:11-12)

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Water that has entered the root cells, under the influence of the difference in water potentials that arise due to transpiration and root pressure, moves to the conductive elements of the xylem. According to modern concepts, water in the root system moves not only through living cells. Back in 1932. The German physiologist Münch developed the concept of the existence in the root system of two relatively independent volumes along which water moves - the apoplast and the symplast.

The apoplast is the free space of the root, which includes intercellular spaces, cell membranes, and xylem vessels. A symplast is a collection of protoplasts of all cells delimited by a semipermeable membrane. Due to the numerous plasmodesmata connecting the protoplast of individual cells, the symplast is a single system. The apoplast is not continuous, but is divided into two volumes. The first part of the apoplast is located in the root cortex up to the endoderm cells, the second part is on the other side of the endoderm cells and includes xylem vessels. Endoderm cells due to belts. Caspars are like a barrier to the movement of water in free space (intercellular spaces and cell membranes). The movement of water along the root cortex proceeds mainly along the apoplast, where it encounters less resistance, and only partially along the symplast.

However, in order to enter the xylem vessels, water must pass through the semi-permeable membrane of the endoderm cells. Thus, we are dealing, as it were, with an osmometer, in which a semipermeable membrane is located in the cells of the endoderm. Water rushes through this membrane towards a smaller (more negative) water potential. The water then enters the xylem vessels. As already mentioned, there are various opinions on the issue of the causes that cause the secretion of water into the vessels of the xylem. According to the Crafts hypothesis, this is a consequence of the release of salts into the xylem vessels, as a result of which an increased concentration of salts is created there, and the water potential becomes more negative. It is assumed that as a result of active (with the expenditure of energy) salt intake accumulates in the root cells. However, the intensity of respiration in the cells surrounding the vessels of the xylem (pericycle) is very low, and they do not retain salts, which are thereby desorbed into the vessels. Further movement of water goes through the vascular system of the root, stem and leaf. The conducting elements of the xylem consist of vessels and tracheids.

Banding experiments showed that the ascending current of water through the plant moves mainly along the xylem. In the conductive elements of the xylem, water encounters little resistance, which naturally facilitates the movement of water over long distances. True, a certain amount of water also moves outside the vascular system. However, compared with xylem, the resistance to water movement of other tissues is much greater (by at least three orders of magnitude). This leads to the fact that only 1 to 10% of the total water flow moves outside the xylem. From the vessels of the stem, water enters the vessels of the leaf. Water moves from the stem through the petiole or leaf sheath into the leaf. In the leaf blade, water-carrying vessels are located in the veins. Veins, gradually branching, become smaller and smaller. The denser the network of veins, the less resistance the water encounters when moving to the cells of the leaf mesophyll. That is why the density of leaf venation is considered one of the most important signs of a xeromorphic structure - a hallmark of drought-resistant plants.

Sometimes there are so many small branches of leaf veins that they bring water to almost every cell. All water in the cell is in equilibrium. In other words, in the sense of saturation with water, there is an equilibrium between the vacuole, cytoplasm and cell membrane, their water potentials are equal. In this regard, as soon as the cell walls of parenchymal cells become unsaturated with water due to the process of transpiration, it is immediately transferred inside the cell, the water potential of which falls. Water moves from cell to cell due to the water potential gradient. Apparently, the movement of water from cell to cell in the leaf parenchyma does not proceed along the symplast, but mainly along the cell walls, where the resistance is much less.

Water moves through the vessels due to the water potential gradient created due to transpiration, the free energy gradient (from a system with greater freedom of energy to a system with less). We can give an approximate distribution of water potentials, which causes the movement of water: water potential of the soil (-0.5 bar), root (-2 bar), stem (-5 bar), leaves (-15 bar), air at a relative humidity of 50 % (-1000 bar).

However, no suction pump can lift water to a height of more than 10m. Meanwhile, there are trees whose water rises to a height of more than 100m. The explanation for this is provided by the clutch theory put forward by the Russian scientist E. F. Votchal and the English physiologist E. Dixon. For a better understanding, consider the following experiment. A tube filled with water is placed in a cup with mercury, which ends with a funnel made of porous porcelain. The whole system is devoid of air bubbles. As the water evaporates, the mercury rises up the tube. At the same time, the height of the rise of mercury exceeds 760 mm. This is due to the presence of cohesive forces between water and mercury molecules, which are fully manifested in the absence of air. A similar position, only more pronounced, is found in the vessels of plants.

All water in a plant is a single interconnected system. Since there are adhesion forces (cohesion) between water molecules, water rises to a height much greater than 10m. Calculations have shown that due to the presence of affinity between water molecules, the cohesive forces reach a value of - 30 bar. This is such a force that allows you to raise water to a height of 120m without breaking the water threads, which is approximately the maximum height of trees. 120m, without breaking the water threads, which is approximately the maximum height of the trees. Cohesive forces also exist between water and vessel walls (adhesion). The walls of the conducting elements of the xylem are elastic. Due to these two circumstances, even with a lack of water, the connection between water molecules and vessel walls is not broken. This is confirmed by studies on changes in the stem thickness of herbaceous plants. The determinations showed that in the midday hours the thickness of the plant stem decreases. If you cut the stem, the vessels immediately expand and air rushes into them. From this experience it can be seen that with strong evaporation, the vessels narrow and this leads to the appearance of negative pressure. Thereby

W in. vessel \u003d - W osm. + (- W pressure.).

The degree of tension of the water threads in the vessels depends on the ratio of the processes of absorption and evaporation of water. All this allows the plant organism to maintain a single water system and not necessarily replenish every drop of evaporated water. Thus, with a normal water supply, water continuity is created in the soil, plant and atmosphere. In the event that air enters the individual segments of the vessels, they are apparently switched off from the general current of water conduction. This is the path of water through the plant and its main driving forces. Modern research methods make it possible to determine the speed of movement of water through the plant. The speed of water movement is determined by the difference in water potentials at the beginning and end of the path, as well as the resistance that it meets. According to the data obtained, the speed of water movement during the day changes. During the daytime it is much larger. At the same time, different types of plants differ in the speed of movement of water. If the speed of movement in conifers is usually 0.5-1.2 m/h, then in hardwoods it is much higher. In oak, for example, the speed of movement is 27 - 40 m / h. The speed of water movement depends little on the intensity of metabolism. Temperature changes, the introduction of metabolic inhibitors do not affect the movement of water. At the same time, this process, as one would expect, depends very much on the rate of transpiration and on the diameter of the water-conducting vessels. In larger vessels, water encounters less resistance. However, it must be borne in mind that air bubbles or any other disturbances in the flow of water can more likely enter into wider vessels.

Water enters the plant from the soil through the root hairs and is carried through the vessels throughout its aerial part. Various substances are dissolved in the vacuoles of plant cells. Particles of these substances put pressure on the protoplasm, which passes water well, but prevents the passage of particles dissolved in water through it. The pressure of solutes on the protoplasm is called osmotic pressure. Water absorbed by dissolved substances stretches the elastic membrane of the cell to a certain limit. As soon as there are fewer dissolved substances in the solution, the water content decreases, the shell contracts and takes on a minimum size. Osmotic pressure constantly maintains the plant tissue in a tense state, and only with a large loss of water, during wilting, this tension - turgor - stops in the plant.

When the osmotic pressure is balanced by the stretched membrane, no water can enter the cell. But as soon as the cell loses some of the water, the shell contracts, the cell sap in the cell becomes more concentrated, and water begins to flow into the cell until the shell stretches again and balances the osmotic pressure. The more water the plant has lost, the more water enters the cells with more force. The osmotic pressure in plant cells is quite high, and it is measured, like the pressure in steam boilers, with atmospheres. The force with which a plant sucks in water - the sucking force - is also expressed in atmospheres. The suction force in plants often reaches 15 atmospheres and above.

The plant continuously evaporates water through the stomata in the leaves. The stomata can open and close, forming either a wide or a narrow gap. In the light, the stomata open, and in the dark and with too much water loss, they close. Depending on this, the evaporation of water goes either intensively or almost completely stops.
If you cut the plant at the root, juice begins to ooze from the hemp. This shows that the root itself pumps water into the stem. Therefore, the water supply to the plant depends not only on the evaporation of water through the leaves, but also on the root pressure. It distills water from the living cells of the root into the hollow tubes of dead blood vessels. Since there is no living protoplasm in the cells of these vessels, water moves freely along them to the leaves, where it evaporates through the stomata.

Evaporation is very important for a plant. With moving water, minerals absorbed by the root are carried throughout the plant.
Evaporation lowers the body temperature of the plant and thus prevents it from overheating. The plant absorbs only 2-3 parts of the water it absorbs from the soil, the remaining 997-998 parts evaporate into the atmosphere. To form one gram of dry matter, a plant in our climate evaporates from 300 g to a kilogram of water.

The apoplast is the free space of the root, which includes intercellular spaces, cell membranes, and xylem vessels. A symplast is a collection of protoplasts of all cells delimited by a semipermeable membrane. Due to the numerous plasmodesmata connecting the protoplast of individual cells, the symplast is a single system. The apoplast is not continuous, but is divided into two volumes. The first part of the apoplast is located in the root cortex up to the endoderm cells, the second part is located on the other side of the endoderm cells and includes xylem vessels. Endoderm cells due to belts. Caspars are like a barrier to the movement of water in free space (intercellular spaces and cell membranes). The movement of water along the root cortex proceeds mainly along the apoplast, where it encounters less resistance, and only partially along the symplast.

Water moves through the vessels due to the water potential gradient created due to transpiration, the free energy gradient (from a system with greater freedom of energy to a system with less). We can give an approximate distribution of water potentials, which causes the movement of water: water potential of the soil (0.5 bar), root (2 bar), stem (5 bar), leaves (15 bar), air at a relative humidity of 50% (1000 bar ).

All water in a plant is a single interconnected system. Since there are adhesion forces (cohesion) between water molecules, water rises to a height much greater than 10m. Calculations showed that due to the presence of affinity between water molecules, the cohesive forces reach a value of - 30 bar. This is such a force that allows you to raise water to a height of 120m without breaking the water threads, which is approximately the maximum height of trees. 120m, without breaking the water threads, which is approximately the maximum height of the trees. Cohesive forces also exist between water and vessel walls (adhesion). The walls of the conducting elements of the xylem are elastic. Due to these two circumstances, even with a lack of water, the connection between water molecules and vessel walls is not broken. This is confirmed by studies on changes in the stem thickness of herbaceous plants. The determinations showed that in the midday hours the thickness of the plant stem decreases. If you cut the stem, the vessels immediately expand and air rushes into them. From this experience it can be seen that with strong evaporation, the vessels narrow and this leads to the appearance of negative pressure. Thereby

Ψ in. vessel \u003d Ψ osm. + Ψ pressure.