Negative pressure in a tree trunk. The movement of water through the plant. The effect of atmospheric pressure on plants

Why water rises up the vessels of the stem, you will learn them in this article.

Why does water continuously rise up the stem?

The movement of water is carried out through the dead and living vessels of the xylem, which makes up the stem. Water absorbed by the hairs of the root system travels a short distance of only a few millimeters through living cells, and then enters the xylem vessels themselves. The movement of water through living cells is carried out due to differences in the sucking power of the plant, which increases significantly from the root hair to the vessels. This is due to the evaporation of water from the leaves. The same situation is observed in the process of water movement through the living cells of the leaf from the vessels. That is, it turns out that in the leaves, in place of the water that has evaporated, a new one is constantly supplied, continuously rising up.

What is a plant stem?

Stem- this is the axial part of the shoot of the plant, which bears the buds, leaves, fruits and flowers. The main functions of the stem are:

support,

Conductive,

Reserve.

Its additional functions are: the stem is an organ of vegetative reproduction, as well as an organ of photosynthesis.

The stem has two main types: herbaceous and woody. The herbaceous stalk mainly exists for only one growing season, and during its existence it either thickens slightly or does not thicken at all. Such plants include nettle, quinoa. The woody stem is usually a perennial organ. It thickens indefinitely and is formed due to lignified tissues. Such plants include birch, grapes and currants.

Main water current engines
The absorption of water by the root system is due to the operation of two end motors of the water current: top end engine, or the suction force of evaporation (transpiration), and the lower end engine, or root engine. The main force causing the flow and movement of water in the plant is the suction force of transpiration, which results in a water potential gradient. Water potential is a measure of the energy used by water to move. The water potential and the suction force are the same in absolute value, but opposite in sign. The lower the water saturation of a given system, the lower (more negative) its water potential. When the plant loses water during transpiration, the leaf cells become unsaturated with water, as a result, a sucking force arises (the water potential drops). admission water is coming towards greater suction power, or less water potential.
Thus, the upper terminal motor of the water current in the plant is the suction force of leaf transpiration, and its work is little related to the vital activity of the root system. Indeed, experiments have shown that water can also enter the shoots through a dead root system, and in this case the absorption of water is even accelerated.
In addition to the upper terminal motor of the water current, plants have a lower terminal motor. This is well illustrated by examples such as guttation.
The leaves of plants whose cells are saturated with water, in conditions of high air humidity that prevents evaporation, secrete drop-liquid water with a small amount of dissolved substances - guttation. The secretion of fluid goes through special water stomata - hydators. The escaping fluid 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 off the shoots of a plant and attach a glass tube to the cut end, then liquid will rise through it. Analysis shows that this is water with dissolved substances - sap. In some cases, especially in the spring, weeping is also observed when the branches of plants are cut. The definitions showed that the volume of the released liquid (sap) is many times greater than the volume of the root system. Thus, crying is not just 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, which is independent of transpiration. The force that causes a one-way flow of water through vessels with dissolved substances, independent of the transpiration process, is called root pressure. The presence of root pressure allows us to speak about the lower terminal motor of the water current. Root pressure can be measured by attaching a manometer to the end left after cutting off the aboveground organs of the plant, or by placing the root system in a series of solutions of various concentrations and choosing one that stops crying. It turned out that the root pressure is approximately 0.1 - 0.15 MPa (D.A. Sabinin). The determinations carried out by Soviet researchers L.V.Mozhaeva, V.N.Zholkevich showed that the concentration of the external solution that stops crying is much higher than the concentration of sap. This led to the opinion that crying can go against the concentration gradient. It has also been shown that crying occurs only under conditions in which all vital processes of cells proceed 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, with a decrease in temperature, crying stops. All of the above allowed D.A. Sabinin to give the following definition: weeping plants is the lifetime one-way flow of water and nutrients, which depends on the aerobic processing of assimelates. 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, metabolic processes are different. In one part of the cell, there are enhanced processes of decomposition, in particular, 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 dissolved substances in this part of the cell decreases. It should be borne in mind that all these mechanisms will work only with a sufficient amount of water in the environment and an undisturbed metabolism.
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 vessels of the xylem. As a result, the concentration of salts in the vessels of the xylem increases, which causes the flow of water.

The movement of water through the plant
Water absorbed by the 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. As early as 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. Apoplast - this is the free space of the root, which includes intercellular spaces, cell membranes, and xylem vessels. Simplast - it is a set of protoplasts of all cells delimited by a semipermeable membrane. Due to the numerous plasmodesmata connecting the protoplast of individual cells, the symplast is single system. The apoplast, apparently, 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. The cells of the endoderm, thanks to the Casparian bands, are like a barrier to the movement of water through the free space (intercellular spaces and cell membranes). In order to enter the xylem vessels, water must pass through a semipermeable membrane and mainly through the apoplast and only partially through the symplast. However, in the cells of the endoderm, the movement of water apparently proceeds along the symplast. The water then enters the xylem vessels. Then the movement of water goes through the vascular system of the root, stem and leaf.
From the stem vessels, water moves 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. The denser the network of veins, the less resistance the water encounters when moving to the cells of the leaf mesophyll. 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. Water moves from cell to cell due to the gradient of the suction force.
All water in a plant is a single interconnected system. Since there are adhesion force(cohesion), water rises to a height much 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 vessel walls.
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 plant organism maintain a single water system and not necessarily replenish every drop of evaporated water.
In the event that air enters the individual segments of the vessels, they, apparently, are switched off from the general current of water conduction. This is the way water moves through the plant (Fig. 1).

Rice. 1. The path of water in a plant.

The speed of movement of water through the plant during the day changes. During the daytime, it is much larger. Wherein different types plants differ in the speed of movement of water. 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 should be borne in mind that air bubbles or any other disturbances in the flow of water may occur in wider vessels.

Video: The movement of water and organic matter along the stem.

Redwoods growing in California are among the most tall 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 perfect 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 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 to anything in its appearance and sizes. 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 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. 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 wood 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 - hallmark drought tolerant 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 into 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, 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 stem thickness. 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 methods studies allow you 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.

In humans and animals, blood circulates through the body driven by a powerful pump, which is the heart. Thus, each cell of the body receives all the substances necessary for its vital activity. Each part of the tree is also washed from the inside with a solution of nutrients in water - the sap of the plant. However, no tree has a heart. How, then, does the sap rise up the tree?

Science still cannot give an exact answer to this question. None of the theories that exist today offers a complete and final explanation of this phenomenon. Therefore, scientists are inclined to think that the movement of sap along the tree is carried out under the action of several forces acting simultaneously.

The most widely accepted theory is osmotic pressure. The fact is that in all living organisms, a solution of nutrients enters the cells through thin membranes. This is because the concentration of dissolved substances on different sides of the membranes is different, and therefore, according to the laws of physics, it tends to equalize. Such a phenomenon (occurring, by the way, not only in wildlife) is called osmosis, and the difference in the concentrations of a substance on different sides of the membrane, which is the driving force of the process, is called osmotic pressure. Thus, the greater this concentration difference, the greater the amount of liquid transferred through the membrane.

The water and mineral salts necessary for plants to sustain life are found in the soil. Since their content is higher there than in the roots of trees, osmotic pressure arises, forcing moisture with salts dissolved in it to penetrate into the plant. Due to the same effect, the juice rises up the root into the trunk and further to the rest of the tree. Mineral salts remain in the cells of the tree as the solution passes through them, and excess water evaporates from the leaves.

There is another hypothesis in this regard. According to her, the movement of juice occurs due, firstly, to the evaporation of water from the leaves, and secondly, to the presence of "cohesion" of water. Cohesion is a force that causes a kind of "sticking" of one small particle of matter to another.

According to this theory, when moisture evaporates from leaves, a vacuum occurs in their cells, and as a result, they begin to attract water from neighboring cells. The same thing happens there, and so on, until it reaches the roots that absorb moisture (and with it, nutrients) from the soil. As far as cohesion is concerned, it holds the water particles together as they move up the shaft, which keeps this flow uninterrupted.