SUMMARY Feature of xylem and phloem and their role. Distribution of xylem and phloem (vascular bundles) in stem and root of dicotyledonous plants. Transport of water from the root to the atmosphere through a plant. Potometer. Experiment to prove which surface of leaf loses more water. Factors affecting transpiration rate. Adaptations of xerophytes to prevent water loss. Opening and closing of stomata. Importance of water in plants. Aphid experiment. Translocation experiment using radioactive isotopes. 1
Features of xylem and phloem and their role. Xylem: Role of Xylem vessels: 1. They conduct water, with its dissolved mineral salts from the roots to the stem and leaves. 2. They provide mechanical support within the plant. Adaptation of xylem to transport water and mineral salts: 1. They have a continuous lumen without any partition wall or protoplasm within to hinder the passage of water and mineral salts. 2. Their walls are lignified to prevent collapse of the vessel. (The vessels collectively provide mechanical support to the plant). Structure of xylem vessels: Phloem: Role of phloem: The phloem conducts manufactured food from green parts of the plant, especially the leaves to all parts of the plant. Adaptations of phloem: 1. Phloem vessels contain sieve tube cells which contain a thin layer of cytoplasm. The transport of manufactured food occurs through the cytoplasm of the sieve tube cells by diffusion and active transport. 2
2. Each sieve tube cell has a companion cell has a companion cell beside it to keep it alive and provide energy for active transport. 3
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Distribution of vascular bundles in stem and root of dicotyledonous plants. Root: The xylem vessel is star shaped. Xylem and phloem are not bundled together The radii of xylem and phloem are not same. 5
Stem: Xylem are inward. Phloem are outside. They are separated by cambium. The centre is known as Pith. The vascular bundles are arranged in a ring shape. 6
Transport of water from the root to the atmosphere through the plant. Water is absorbed into the root hair cells by osmosis. The Root Hair Cells contain cell sap which decreases its water potential and water moves into them. When water gets into the RHCs, their water potential becomes greater than the nearby cells and as a result, water moves into them through osmosis, and this goes on until water reaches the xylem vessels and from there, water is moved up the plants by many processes. Water is moved up the plant by: Root Pressure: The living cells around the xylem vessels in the root use active transport to pump ions into the vessel. This lowers the water potential in the xylem vessels. Water, therefore, passes from the living cells into the xylem vessels by osmosis and flows upwards. This is called root pressure. Capillarity: Water tends to move up inside fine capillary action. Since the xylem vessels in the plant are very narrow capillary tubes, water sticks to their surface and moves up. When water moves into the leaves, water is lost to the atmosphere. Water is lost constantly from the leaves by evaporation through the stomata. This is known as transpiration. As the water molecule evaporates, a low pressure is created. Hence, the nearby water molecule rushes to take its place. Therefore, a suction force is created in the leaves, which pulls the water through the xylem from the soil. This is known as transpiration pull, which is the main process by which water moves up a plant. This is how water moves through the plant from the soil to the atmosphere. 7
Potometer: leafy shoot reservoir tap closed graduated air 50 capillary tube bubble 0 mm A potometer is used to measure the rate of absorption of water by a plant and not the rate of transpiration. In an experiment with a potometer, we shall assume that, in a steady state, the rate of absorption of water is proportional to the rate of transpiration. The Potometer is a setup/apparatus to find out how quickly water is absorbed by a plant. Transpiration is the loss of water from a plant through the stomata in the leaf. Therefore, the faster the water is lost from a plant, the faster water will be absorbed. Hence, the Potometer can be used to determine the rate of transpiration of water. To measure the rate of absorption, the shoot is inserted through the hole on the cork of the Potometer. The cork is smeared with Vaseline which passes through the cork to make the apparatus airtight. The tap of the reservoir is opened to fill the graduated capillary tube with water. The tap is closed when the tube is filled. As the shoot transpires, it absorbs water from the Potometer to replace that which is lost during transpiration. This causes the water column in the capillary tube to move from one end to the other. The rate of movement of the water column gives the rate of water absorption by the shoot. The end of the water column can be pushed back to the other end by opening the tap of the reservoir, which causes a little water to flow down. But, before any Potometer experiment is conducted, the plant used should be allowed to reach a steady state. The plant is immersed in water and allowed to stand for some time. Later, the root is cut, immersed in water and again allowed to stand for two hours. This ensures that, the stem adjusts with the condition without the root. This is known as steady state. 8
Factors affecting transpiration rate. Humidity of the air: The intercellular spaces in the leaf are normally saturated with water vapor. If the air outside is dry, water vapor will diffuse more rapidly out of the leaf, i.e. the rate of transpiration will increase. On the other hand, if the air is humid, it hinders evaporation. Therefore, the more humid the air, the slower the rate of transpiration. Temperature of the air: Assuming the other factors remain constant, a rise in the temperature of the surrounding increase the rate of evaporation, thus the rate of transpiration is greater. Strong wind: In still air, the water vapor that diffuses out of the leaf makes the air around the leaf moist. This decreases the rate of movement of water vapor out of the leaf. If there is a wind, it blows away the water vapor around the leaf, making the air less damp. This increases the rate of transpiration. The stronger the wind, the higher is the rate of transpiration. However, if the wind is very strong, the stomata may close because the guard cells lose too much water. Light: Light affects the size of the stomata on the leaf. It will, therefore, affect the rate of transpiration. In sunlight the stomata open and become wider. This increases the rate of transpiration. In darkness, the stomata close. Experiment to prove which surface of the plant loses more water. Place an intact leaf between two pieces of dry cobalt chloride paper, then sandwich the leaf between two dry glass slides held in position by rubber bands. The glass slides prevent the cobalt chloride from coming in contact with the water vapor in the air. [Note: Cobalt chloride paper is blue when dry and turns pink when it comes in contact with water vapour.] RESULT: Both the cobalt chloride papers change colours from blue to pink, thus proving that transpiration does occur in the leaves. However, the cobalt chloride paper on the lower epidermis changes colour from more quickly than that on the upper epidermis, thus proving that more water is lost through the lower surface of the plant. [Note: The experiment can only be performed on a warm dry day. If the atmosphere is damp, e.g early in the morning, the experiment cannot be carried out successfully since the cobalt chloride papers will turn pink the moment they come in contact with damp air.] 9
Adaptations of xerophytes to prevent water loss. Xerophytes are plants which can live in conditions of prolonged draught in their habitat. They reduce the rate of transpiration by shedding their young leaves. Their stems become fleshy, storing up much water which is made available to the plants in times of drought. The stems are green and take over the function of photosynthesis from the leaves. In xerophytes called Casuarinas, the leaves are reduced to tiny sheaths at the nodes and food manufacture is carried out by the long, green stems whose stomata lie in grooves protected by minute hairs. All these features help to reduce water loss. In marram grass, the leaves have sunken stomata that lie in grooves in the upper surface. These grooves or crypts bear tiny hairs that trap water vapor diffusing out of the stomata. This increases the humidity around the stomata and so, reduces the rate of transpiration. Importance of Transpiration Draws water and mineral salts from the roots to the stems and the leaves. Evaporation of water from the cells in the leaves removes latent heat of vaporisation, so the plant is cooled. Water transported to the leaves is used for photosynthesis and maintaining the turgidity of the leaf cells. Opening and closing of the stomata: Guard cells are the only epidermal cells which can make sugar. According to one theory, in sunlight the concentration of potassium ions increases in the guard cells. This, together with the sugars formed, lowers the water potential in the guard cells. As a result, water from other cells enters the guard cells by osmosis, so they swell and become turgid, and because the guard cells have a thicker cellulose wall on one side of the cell, i.e. the side around the stomata pore, the swollen guard cells become more curved and pulls the stomata open. At night, the sugar is used up and water leaves the guard cells, so they become flaccid and the stomata pore closes. In this way, they reduce the amount of water vapor escaping from the leaf. 10
Importance of water in plants: The turgor pressure in the mesophyll cells in the leaf helps to support the leaf and keep the leaf firm and widely spread out to absorb maximum sunlight for photosynthesis. in strong sunlight, excessive transpiration causes the cells to lose their turgor. They become flaccid and the plant wilts. Advantages of wilting: Rate of transpiration is reduced because the leaf folds up reducing the surface that is exposed to sunlight. This causes the guard cells to become flaccid and the stomata close. Disadvantages of wilting: Rate of photosynthesis is reduced because water becomes a limiting factor. Also, as the stomata are closed, the amount of carbon dioxide entering the leaf is also reduced. 11
Aphid experiment: Insects like aphids feed on plant juices. The mouthparts (proboscis) of the aphid penetrate the leaf or stem. This aphid can be anaesthetized with carbon dioxide while it is feeding. The body of the animal is cut off, leaving only the proboscis. An analysis of this liquid shows that it contains sucrose and amino acids. Sectioning of the stem and examining it under the microscope shows that the proboscis is inserted into the phloem sieve tube. This shows that the translocation of sugar and amino acids occurs in the phloem. Translocation experiment using radioactive isotopes: 14 C is a radioactive isotope. Its presence can be detected by X-ray photographic film. A leaf is fed with carbon dioxide containing the radioactive carbon, 14 C. When photosynthesis takes place, the sugars formed will contain radioactive carbon. The stem is layer cut and a section of it is exposed onto an X-ray photographic film. It is found that the radioactive substances are present in the phloem. If 14 CO 2 is supplied to the plant, it will be fixed in the glucose upon photosynthesis: 14 C 6 H 12 O 6 When the stem is cut and placed on a X-ray film, only the phloem contains radioactivity Thus the outer region shows black colour. Ringing Experiment Cut off a ring of bark, including the phloem, but leaving the xylem, and immerse in water and observe. Swelling is observed above the cut due to accumulation of organic solutes that came from higher up the tree and could no longer continue downward because of the disruption of the phloem. Later, the bark below the girdle died because it no longer received sugars from the leaves. Eventually the roots, and then the entire tree, died. 12
Experiment to show that transpiration occurs largely through the stomata: PROCEDURE: 1. Leaves(Organism used) in which stomata are mainly confined to the lower surface of the leaves are to be chosen. 2. Three leaves, of about the same size and surface area(standardised) are to be taken and the petioles are to be coated with Vaseline to prevent loss of water by evaporation. 3. The leaves are to be treated in the following ways: Leaf A: The upper surface is to be covered with a layer of Vaseline. Leaf B: The lower surface is to be covered with a layer of Vaseline. Leaf C: Both surfaces are to be covered with Vaseline(Control). 4. The initial mass of each leaf is to be recorded(measurement). 5. The three leaves are to be hung up near a window where they can get sunlight. 6. The condition of the three leaves are to be noted down after a few hours. 7. The final masses of the three leaves are then to be recorded. RESULT: Leaf A: In leaf A, only cuticular respiration is prohibited but transpiration occurs theough the stomata thus the leaf dries up.moreover, after calculation, it will be seen that leaf A looses most weight thus proving that most transpiration occurs through the stomata. Leaf B:In leaf B, the lower surface is covered with Vaseline.As a result maximum transpiration is prevented and thus the leaf does not dry much and also does not loose too much weight. Leaf C:In leaf C, no transpiration occurs since both stomata and upper surface are covered thus the condition and weight of the leaf remains unchanged. 13
Experiment using a potometer to compare the rates of transpiration under different conditions: PROCEDURE: [Note: A shoot that is to be used in a potometer must be cut under water and the cut end kept immersed under water for a few hours before use.this is to allow the shoot to adjust to the conditions in the potometer(new environment) i.e to reach a steady state.] 1. To measure the rate of water absorption of a shoot, a shoot(organism used) is to be prepared as said in the note above. 2. The shoot is to be inserted through a hole in the cork of the potometer.vaseline is to be smeared round the region of the shoot which passes through the cork to make the apparatus airtight.the tap of the reservoir is to be opened to fill the graduated capillary tube with water.the tap is to be closed when the tube is full. 3. The reading of the water column at B is to be noted.the time taken for the end of the water column to move from B to A is to be noted(measurement). 4. The experiment is to be repeated with the apparatus setup in different conditions such as blowing it with a fan,putting it in a cupboard or putting it in a refrigerator(repetition). EXPLANATION: As the shoot transpires, it absorbs water from the potometer to replace that which is lost during transpiration.this causes the water column in the capillary tube to move from B to A.The rate of movement of the water column gives the rate of water absorption by the shoot. CALCULATION: Volume of water column from B to A = a cm3 Time taken = b minutes Rate of transpiration = a/b cm3/minute(expt. Continued next page) [Note: The end of the water column can be pushed back to B by opening the tap of the reservoir to allow a little water to flow down. In this way several readings can be taken and the average rate of transpiration calculated.] 14
Experiment with an aphid to show that phloem is translocating food substances downwards. PROCEDURE: The mouthparts (proboscis) of the aphid penetrate the leaf or stem. The aphid can be anaesthesized with carbon dioxide while it is feeding. The body of the animal is to be cut off, leaving the proboscis in the plant tissues. A liquid will exude from the cut end of the proboscis. RESULT: An analysis of the liquid (test for sucrose) shows that the liquid contains sucrose and amino acids. Sectioning of the stem and examining it under the microscope shows that the proboscis is inserted into the phloem sieve tube. This shows that the translocation of sugars and amino acids occurs in the phloem. Experiment with radioactivity to demonstrate that phloem transports organic substances PROCEDURE: A leaf is to be fed with carbon dioxide containing radioactive carbon. It is then to be placed in bright sunlight so that it can photosynthesise. The stem is later to be cut and a section of it is to be exposed onto an X-ray photographic film. RESULT: The presence of the radioactive carbon can be detected by X-ray photographic film. When photosynthesis takes place, the sugars formed will contain radioactive carbon. On cutting the stem and exposing a section of it onto an X-ray photographic film, it is found that the radioactive substances are present in the phloem. 15