Main engines of water current
The absorption of water by the root system occurs due to the operation of two end motors of water current: upper the end motor, or suction force of evaporation (transpiration), and the lower end motor, or root motor. The main force causing the flow and movement of water in a plant is the suction force of transpiration, which results in a gradient of water potential. Water potential is a measure of the energy used by water to move. Water potential and suction force are equal in absolute value, but opposite in sign. The less water saturation of a given system, the less (more negative) its water potential. When a plant loses water during the process of transpiration, the leaf cells are unsaturated with water, and as a result, suction force arises (the water potential drops). the flow of water goes in the direction of greater suction force, or less water potential.
Thus, the upper terminal motor of water flow in a plant is the suction force of leaf transpiration, and its work has little connection with the vital activity of the root system. Indeed, experiments have shown that water can enter the shoots through the dead root system, and in this case the absorption of water is even accelerated.
In addition to the upper end motor of water current, there is a lower end motor in plants. This is well proven by the example of such phenomena as Guttation.
The leaves of plants whose cells are saturated with water, under conditions of high air humidity, which prevents evaporation, secrete droplet-liquid water with a small amount of dissolved substances - guttation. Liquid is released through special water stomata - hydrators. The liquid released is gutta. Thus, the process of guttation is the result of a one-way flow of water occurring in the absence of transpiration, and is therefore caused by some other cause.
The same conclusion can be reached when considering the phenomenon cry plants. If you cut the shoots of a plant and attach a glass tube to the cut end, liquid will rise through it. Analysis shows that this is water with dissolved substances - sap. In some cases, especially in spring, crying is also observed when plant branches are cut. Determinations have shown that the volume of released liquid (sap) is many times greater than the volume of the root system. Thus, crying is not simply the leakage of fluid as a result of a cut. All of the above leads to the conclusion that crying, like guttation, is associated with the presence of a one-way flow of water through the root systems, independent of transpiration. The force that causes a one-way flow of water through vessels with dissolved substances, independent of the process of transpiration, is called root pressure. The presence of root pressure allows us to speak about the lower end motor of the water current. Root pressure can be measured by attaching a pressure gauge to the end left after cutting off the above-ground parts of the plant, or by placing the root system in a series of solutions of varying concentrations and choosing one that stops weeping. It turned out that root pressure is approximately 0.1 - 0.15 MPa (D.A. Sabinin). Determinations carried out by Soviet researchers L.V. Mozhaeva and V.N. Zholkevich showed that the concentration of the external solution that stops crying is significantly higher than the concentration of pasok. This allowed us to express the opinion that crying can go against the concentration gradient. It has also been shown that crying occurs only under conditions in which all cell life processes occur normally. Not only the killing of root cells, but also a decrease in the intensity of their vital activity, primarily the intensity of respiration, stops crying. In the absence of oxygen, under the influence of respiratory poisons, and when the temperature drops, crying stops. All of the above allowed D.A. Sabinin to give the following definition: crying plants- This is a lifetime one-way flow of water and nutrients, depending on the aerobic processing of assimilates. D.A. Sabinin proposed a diagram explaining the mechanism of one-way water flow in the root. According to this hypothesis, root cells are polarized in a certain direction. This is manifested in the fact that in different compartments of the same cell the metabolic processes are different. In one part of the cell, there are intensified processes of breakdown, in particular, of starch into sugars, as a result of which the concentration of cell sap increases. At the opposite end of the cell, synthesis processes predominate, due to which the concentration of solutes in this part of the cell decreases. It must be taken into account that all these mechanisms will only work if there is a sufficient amount of water in the environment and the metabolism is not impaired.
According to another hypothesis, the dependence of plant crying on the intensity of respiration is indirect. The energy of respiration is used to supply ions to the cells of the cortex, from where they are desorbed into the xylem vessels. As a result, the concentration of salts in the xylem vessels increases, which causes the flow of water.
Movement of water through the plant
Water absorbed by root cells, under the influence of the difference in water potentials that arise due to transpiration, as well as the force of root pressure, moves to the xylem pathways. According to modern concepts, water in the root system moves not only through living cells. Back in 1932, the German physiologist Munch developed the idea of the existence in the root system of two relatively independent volumes through which water moves - the apoplast and the symplast. Apoplast – This is the free space of the root, which includes intercellular spaces, cell membranes, and xylem vessels. Simplast – This is a collection of protoplasts of all cells, delimited by a semi-permeable membrane. Thanks to the numerous plasmodesmata connecting the protoplast of individual cells, the symplast represents a single system. The apoplast is apparently not continuous, but is divided into two volumes. The first part of the apoplast is located in the root cortex before the endodermal cells, the second is on the other side of the endodermal cells, and includes xylem vessels. Endoderm cells, thanks to Casparian belts, represent a barrier to the movement of water through free space (intercellular spaces and cell membranes). In order to enter the xylem vessels, water must pass through a semi-permeable membrane and mainly through the apoplast and only partially through the symplast. However, in endodermal cells, the movement of water apparently occurs along the symplast. Next, water enters the xylem vessels. Then the movement of water occurs through the vascular system of the root, stem and leaf.
From the vessels of the stem, water moves through the petiole or leaf sheath into the leaf. In the leaf blade, water-conducting vessels are located in the veins. The veins gradually branch out and become smaller. The denser the network of veins, the less resistance water encounters when moving to the mesophyll cells of the leaf. Sometimes there are so many small branches of leaf veins that they supply water to almost every cell. All water in the cell is in an equilibrium state. 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. Water moves from cell to cell due to a gradient of suction force.
All the water in the plant represents a single interconnected system. Since between water molecules there are adhesion forces(cohesion), water rises to a height significantly greater than 10 m. The adhesion force increases, since water molecules have a greater affinity for each other. Cohesive forces also exist between water and the walls of vessels.
The degree of tension of water threads in 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.
In the event that air enters individual segments of the vessels, they are apparently switched off from the general current of water conduction. This is the path of water movement through the plant (Fig. 1).
Rice. 1. The path of water in the plant.
The speed at which water moves through the plant changes throughout the day. During the daytime it is much larger. At the same time, different types of plants differ in the speed of water movement. Temperature changes and the introduction of metabolic inhibitors do not affect the movement of water. At the same time, this process, as one would expect, very much depends on the rate of transpiration and on the diameter of water-conducting vessels. In wider vessels, water encounters less resistance. However, it must be taken into account that air bubbles may get into wider vessels or some other disturbance in the flow of water may occur.
Video: Movement of water and organic matter along the stem.
The xylem of flowering plants consists of two types of structures that transport water, tracheids and vessels. In Sect. 8.2.1 we have already talked about how the corresponding cells look in a light microscope, as well as in micrographs obtained using a scanning electron microscope (Fig. 8.11). We will consider the structure of secondary xylem (wood) in section. 21.6.6.
Xylem, together with phloem, forms the conductive tissue of higher plants. This fabric consists of so-called conductive bundles, which consist of special tubular structures. In Fig. Figure 14.15 shows how vascular bundles are arranged and how they are located in the primary stem of dicotyledonous and monocotyledonous plants.
14.19. Summarize in tabular form the differences in the structure of the primary stem in dicotyledonous and monocotyledonous plants.
14.20. What is the three-dimensional shape of the following tissue components: a) epidermis; b) xylem; c) dicotyledon pericycle and d) pith?
That water can move up the xylem can be demonstrated very easily by immersing the lower end of a cut stem in a dilute solution of a dye such as eosin. The dye rises through the xylem and spreads throughout the network of leaf veins. If thin sections are taken and viewed under a light microscope, the dye will be found in the xylem.
The fact that xylem conducts water is best demonstrated by experiments with “ringing”. Such experiments were carried out long before radioactive isotopes began to be used, making it very easy to trace the path of a substance in a living organism. In one version of the experiment, a ring of bark with phloem is cut out. If the experiment is not very long, such “ringing” does not affect the rise of water along the stem. However, if you peel off a piece of bark and cut out the xylem without damaging the piece of bark, 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 approximately 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 lift water through such tubes to the top of a tall tree, a pressure of about 4000 kPa is required. The tallest trees - Californian 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, due to these forces alone, even through the thinnest xylem vessels, 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 leaf cells. As we already said in section. 14.3, evaporation leads to a decrease in the water potential of cells adjacent to the xylem. Therefore, water enters these cells from xylem sap, which has a higher water potential; in doing so, 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 xylem vessels 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 (Section 5.1.2). In addition, they tend to stick to the walls of blood vessels under the influence of forces adhesion. The high cohesion of water molecules means that a relatively large tensile force is required to break a column of water; in other words, the water column has high tensile strength. The tension in the xylem vessels reaches such a force that it can pull the entire column of water upward, 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 xylem vessels are under the influence of a large tensile force was obtained by measuring daily changes in trunk diameter in trees using an instrument called a dendrometer. The minimum values were recorded during the daytime, when the transpiration rate is maximum. The tiny compression of individual xylem vessels added up and gave a completely measurable decrease in the diameter of the entire trunk.
Estimates of tensile strength for a xylem sap column range from about 3000 to 30,000 kPa, with lower values obtained more recently. The leaves have a water potential of about -4000 kPa, and the strength of the xylem sap column is probably sufficient to withstand the tension created. It is possible, of course, that a column of water may sometimes rupture, especially in vessels of large diameter.
Critics of this theory point out that any disruption of the continuity of the column of juice should immediately stop the entire flow, since the vessel should be filled with air and water vapor (the phenomenon cavitation). Cavitation can be caused by strong shaking, bending of the trunk 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. It is used in the timber industry because the wood has better buoyancy. However, the rupture of the water column in some 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 enough for at least a small part of the vessels to 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 serves 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 suppressed by cyanide and other respiration inhibitors and stops when there is a lack of oxygen or a decrease in temperature. For this mechanism to work, it appears that active secretion into the xylem sap of salts and other water-soluble substances that reduce water potential is required. Water then enters the xylem by osmosis from neighboring root cells.
Positive hydrostatic pressure alone of about 100-200 kPa (in exceptional cases up to 800 kPa), created by root pressure, 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, this pressure, however, is quite sufficient to cause guttation. Guttation- this is the removal of water in the form of drops of liquid on the surface of the plant (while during transpiration, water comes out in the form of steam). All conditions that reduce transpiration, i.e. low light, high humidity, etc., promote guttation. It is quite common in many plants of tropical rainforests and is often observed on the tips of the leaves of young seedlings.
14.21. List the properties of xylem due to which it ensures the transport of water and substances dissolved in it over long distances.
Xylem of flowering plants contains two types of water-conducting structures - tracheids and vessels. In the article we have already talked about how these structures look in a light microscope, as well as in micrographs obtained using a scanning electron microscope. The structure of secondary xylem (wood) is discussed in the article. Xylem and phloem form the conducting tissue of higher, or vascular, plants. This tissue consists of so-called vascular bundles, the structure and distribution of which in the stems of dicotyledonous plants with a primary structure is shown in the figure.
What water rises through the xylem, can be easily demonstrated by immersing the cut end of the shoot in a dilute aqueous solution of a dye, such as eosin. The colored liquid, spreading up the stem, fills the network of veins running through the leaves. If you then take thin sections and examine them under a light microscope, you will find that the dye is in the xylem.
More effective proof water rising through the xylem give experiments with “ringing”. Such experiments were carried out long before radioactive isotopes began to be used, making it very easy to trace the path of substances in a living organism. In one version of the experiment, a narrow ring of bark is removed from a woody stem along with the phloem, i.e., phloem. For quite a long time after this, the shoots located above the cut ring continue to grow normally: therefore, such ringing does not affect the rise of water along the stem. However, if you lift a piece of bark and cut out a segment of wood, i.e., xylem, from under it, the plant will quickly wither. Thus, water moves into the shoots from the soil precisely along this conductive tissue.
Any theory that explains transport of water through xylem, cannot ignore the following observations.
1. Anatomical elements of xylem- thin dead tubes, the diameter of which varies from 0.01 mm in “summer” wood to 0.2 mm in “spring” wood.
2. Large quantities water moves through the xylem at a relatively high speed: for tall trees it is up to 8 m/h, and for other plants it is about 1 m/h.
3. To lift water through such tubes to the top of a tall tree a pressure of about 4000 kPa is required. The tallest trees - redwoods in California and eucalyptus in Australia - reach a height of more than 100 m. Water is able to rise through thin wettable tubes due to its high surface tension (this phenomenon is called capillarity), but only due to these forces even through the thinnest xylem vessels the water does not rise above 3 m.
A satisfactory explanation for this The theory of linkage gives facts(cohesion), or tension theory. According to this theory, the rise of water from the roots is due to its evaporation by leaf cells. As we already said in the article, evaporation reduces the water potential of mesophyll cells adjacent to the xylem, and water enters these cells from xylem sap, whose water potential is higher; in doing so, it passes through the moist cell walls at the ends of the veins, as shown in the figure.
Xylem vessels fills a continuous column of water; as the water leaves the vessels, tension is created in this column; it is transmitted down the stem to the root due to the adhesion (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. In addition, they are attracted to the walls of xylem vessels, i.e., they adhere to them. The strong cohesion of water molecules means that its column is difficult to break - it has a high tensile strength. Tensile stress in xylem cells generates a force capable of moving the entire water column upward through a volumetric flow mechanism. From below, water enters the xylem from neighboring root cells. It is very important that the walls of the xylem elements are rigid and do not collapse when the pressure inside drops, as happens when you suck a cocktail through a soft straw. The rigidity of the walls is provided by lignin. Evidence that the fluid inside the xylem vessels is highly stressed (stretched) is provided by daily fluctuations in the diameter of tree trunks, measured by an instrument called a dendrograph.
The minimum diameter is marked on the day when the transpiration rate is highest. Post tension water in xylem vessel pulls its walls a little inside (due to adhesion), and the combination of these microscopic compressions gives the overall “shrinkage” of the barrel, recorded by the device.
Strength estimates for rupture of the xylem sap column varied from 3000 to 3000 kPa, with lower values obtained later. The leaves have a water potential of about -4000 kPa, and the strength of the xylem sap column is probably sufficient to withstand the tension created. It is possible, of course, that a column of water may sometimes rupture, especially in vessels of large diameter.
Critics of the stated theory emphasize that any violation of the continuity of the juice column should immediately stop the entire flow, since the vessel will be filled with air and steam (the phenomenon of cavitation). Cavitation can be caused by strong shaking, bending of the trunk, and 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. Loggers take advantage of this because such trees are easier to float. However, the rupture of the water column in some vessels has little effect on the overall velocity of the volumetric flow. Perhaps the fact is that water flows into parallel vessels or bypasses the air plug, moving along neighboring parenchyma cells and along the walls. In addition, according to calculations, to maintain the observed flow rate, it is sufficient for at least a small proportion of xylem elements to function at any given time. In some trees and shrubs, water moves only through the younger outer wood, called sapwood. In oak and ash, for example, the conductive function is performed mainly by the vessels of the current year, and the rest of the sapwood plays the role of a water reserve. New xylem vessels are formed throughout the growing season, but mainly at the beginning, when the speed of water flow is maximum.
Second force ensuring the movement of water through the xylem, - 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 for some time. This process is suppressed by respiration inhibitors, such as cyanide, and stops when there is a lack of oxygen and a decrease in temperature. The operation of this mechanism is apparently due to the active secretion of salts and other water-soluble substances into xylem sap. As a result, its water potential drops and water enters the xylem from neighboring root cells by osmosis.
This mechanism creates hydrostatic pressure of the order of 100-200 kPa (in exceptional cases 800 kPa); one for him water rising through the xylem usually not enough, but in many plants it undoubtedly contributes to the maintenance of xylem flow. In slowly transpiring herbaceous forms, this pressure is quite enough to cause mutation in them. This is the name given to the release of water on the surface of a plant1 in the form of liquid droplets rather than steam. All conditions that inhibit transpiration, such as low light and high humidity, promote guttation. It is common in many tropical rainforest species and is often observed on the leaf tips of grass seedlings.
The redwood trees found 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 here in impeccable order.
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 embedded in their DNA, in a tiny, round seed. The seed fulfills all the “instructions” found in its DNA and turns into a gigantic structure, incomparable 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 tree is 2500 years old. 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 17,000 cubic meters, which is 10 times more than the volume of a blue whale.
The Scripture says: “God is exalted in His power, and who is a teacher like Him? Remember to extol 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 works.
Raising water to the height of a 30-story building
Through your leaves sequoia releases up to 600 liters of water per day, so it constantly lifts water from the roots to the branches, overcoming the force of gravity. How does a tree that does not have mechanical pumps do this? 100 meters is a truly impressive height, comparable to two 14-story buildings. It turns out that inside the trunk redwoods there is a special system of narrow interconnected tubes called xylem. This complex internal tree tissue 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.
The 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 sections joined together, the flow of water is constantly contained. One solid tube would not accomplish this task. Since water doesn't normally flow upward, how does a tree manage to pump it that high? 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 ensures that the sequoia rises water 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 enormous force, which, according to experimental measurements, is 25-30 atmospheres (1 atmosphere is equal to normal atmospheric pressure at sea level).
The distribution system shown in cross section. Transmission pipes are made up of cells and are designed to transport 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.
This is enough to push through a World War II submarine floating at a depth of 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 force. A water molecule evaporates from the leaf and, thanks 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 from neighboring leaf cells. These molecules, in turn, attract molecules around them. The chain of motion continues down to the ground and moves water from the roots to the top of the tree, just as a pump lifts water from a standpipe to the surface.
We understand that tree It itself could not have come up with such a complex system, 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 produce green grass, grass yielding seed, fruitful trees, bearing fruit according to its kind, in which is its seed on the earth. And it became so". (Gen. 1:11-12)
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A higher plant is divided into organs that perform different functions, but have many common properties, including the need for nutrients, substances and water for physiological processes to occur. Since water is not absorbed by all organs, but mainly by the root system, there is a need for its movement throughout the plant. This process constitutes the so-called ascending current. It should be noted that this name does not reflect the direction, but the nature of movement and its localization in the plant. It passes mainly through dead tissues of the stem or petiole - vessels or trachea in angiosperms and tracheids in gymnosperms. However, this localization is not absolute: water can also move through other anatomical elements, for example, through the phloem system.
Water with minerals and substances dissolved in it rises through the vessels of the wood.
If we take into account the entire length of the path of the ascending current, then it can be divided into two sections of unequal length.
1. Dead histological elements in the middle of the conducting path of vessels or tracheids. The length of this section is significant, but water passes through it relatively easily, since it moves passively along dead elements without experiencing significant resistance from them.
2. Living cells of the root and leaf, located at the beginning and end of the movement path. This path is spatially short, but it is overcome with great difficulty, since cell membranes prevent the movement of water.
The movement of water in an upward current is important in the life of a plant. This current supplies all organs and tissues with water, bringing them into a state of turgor. The upward flow of water captures mineral ions absorbed by the root, transports them and thereby facilitates distribution (but not absorption!) throughout the plant.
In order for water to move through the plant (and not just move, but rise up), a certain amount of energy is required, the application points of which are located at the ends of the current, as a result of which they are called end motors.
Bottom end motor, or root pressure. Its role is manifested mainly during active absorption - injection of water. With the participation of contractile proteins, it not only supplies water to the root system, but also pushes it further into the vessels of the root and up the stem. Water injection
An active energy-dependent process that is most pronounced in the root cortex. The force developed by the end motor is small (about 0.15 MPa); it can ensure lifting of water to a height of no more than one meter, i.e., sufficient for herbaceous plants and small shrubs.
Symplast is a system of interconnected plant protoplasts. Protoplasts of neighboring cells are connected to each other by plasmodesmata - cytoplasmic strands passing through pores in the cell walls. Water with any substances dissolved in it, having entered the protoplast of one cell, can move further along the symplast without crossing any membranes. This movement is sometimes facilitated by the ordered flow of cytoplasm.
An apoplast is a system of contiguous cell walls that forms a continuous network throughout the plant. Up to 50% of such a cellulose frame is a kind of “free space” that can be occupied by water. When it evaporates into the intercellular spaces from the surface of the mesophyll cells, tension arises in the continuous apoplastic layer of water, and the whole of it, according to the mechanism of volumetric flow, is pulled to the place of decrease due to the cohesion (“adhesion”) of water molecules. The apoplast receives water from the xylem.
Upper end motor, or suction force of transpiration. With the constant evaporation of water in the leaves of plants, the suction force (1 - 1.5 MPa) is broken, sucking water from the nearest cells and transmitted to subsequent cells through which water moves, up to the vessels. There is no cytoplasm in the vessels, therefore there is no osmotic pressure, and the absorption of liquid occurs with the participation of the entire magnitude of the suction force. It allows you to raise water several meters, acting like a hydraulic pump. This force is sufficient to provide water to shrubs and relatively small trees.
Rising water up a tree trunk
End motors can raise water to a height of 10 m. But many woody plants have a much longer trunk, and then both end motors cannot provide water lifting. In such plants, adhesion forces between water molecules come to the rescue, which are very large and can reach 30 - 35 MPa. This force is enough to raise water 1 - 2 km, which is significantly higher than the height of any tree.
The adhesive forces of water molecules act only under certain conditions: water streams in vessels must flow continuously, without air bubbles. If air gets into them, which is possible if they are injured or cut, the movement of water is interrupted. This explains the withering of shoots of woody plants with leaves and flowers (for example, lilac), when they are not immediately placed in water after cutting, but after some time.
