- Here are the following theories which explain the ascent of sap in plants:
(a) Root pressure
(c) Vital theory and
(d) Cohesion – tension theory.
(a) ROOT PRESSURE
- The hydrostatic pressure generated in the root which forces the water upward in the stem is called root pressure. Root pressure forces the water up from below. But it is believed that root pressure does not play much role in the ascent of sap due to the following reasons:
- The magnitude of root pressure is Mitch lower. . 1 – hus this pressure is insufficient to raise, Water to much height. It rarely exceeds Jam. 13tuapressure of about 20atm. is required to raise the sap to the tops of tall trees.
- Root pressure shows sensonal fluctuations. New lca% es are coming out and transpiration isslowinspring. Sctitishighest in spring. But transpiration is very rapid in summer. Soithas. lower value in summer. Therefore, it does not play any role in ascentlsap in summer,
- Water continues torise up in the absence of root pressure. ‘Vitus when acutshoot is placed in water, it does not wilt.
- Root pressure has not been obsened in g. mnosperms.
- A glass tube withanamns bore is called a capillary tube. If one end of the capillary tube is dipped in water. the level of water rises in the tube ltoccurs due to property ofadhesion of water. Adhesion is the airection of molecules of water vl / 4 ith the glass, the phenomenon is called capillarity. The height of water in a capillary tube is’ aveniely proportional to diameter of the bore. It means smaller the capillary bore. more water is mised. x. ) lem ducts acts as narrow capillary tubes. But capillarity. does not play much role in the ascent of sap due to the following reasons:
- 1.11c capillary bore ofxy lem essels and trachcids is about 40m in diameter. lts capillary force can raise yater on iv up to 40 cm,
- Capillarity worlessonlyifone of the open endsofthe capillary tube is dipped in water. But this situation does not exist in nature. The xy iem ducts of roots are not in contact with liquid water of the soil.
(c) VITAL THEORIES
- There were IWO iews about vital theory:
- Vital theory was first proposed by Godiewski. According thisthpory the movement of water Iakes place due to the pumping activitvofthe cellsofwood parenchyma and Medullary. rava File periodic chanee intheirasmatic nnessure causes pumping action. There isan increarn otosmotic pressure in these cells. It withdraws water from the bordering vessels, then their Osmotic pressure is decreased. Itpumps water into the above essel. In this way water moved up step by step. . 2. According to IC. Bose (1923) the ascent of sap is due to the pulsator) actii. it of the cortical cells of theslew outside the endoderm is.
- Strasburgher disapproyed this theory. lleshowed that water continued torise in plants after death of their cells.
(d) COHESION TENSION THEORY
- His theory %s as enunciated by Dixon (1910). This theory states that the negative pressure or pull or suction is created in the leaves as a result of transpiration pulls the water upward. According to this theory lollowing factors play role in the ascent of sap.
1. Transpiration pull:
- Water is evaporated from the stomala by transpiration. It produces negative pressure or suction pressure in the leaf. This pressure pulls the water upward.
- The force of: attraction among the water molecules is called cohesion. The cohesion holds water together. It formsasolid chain like column within the xylem tubes. – [here ishydrogen bonding between the molecules of water.
- The pullingofwliter upward produces tension in the xylem tubes. The transpiration provides the necessary energy or three. The hydrogen bonds between the water molecules 200 meters (more than NO feet) in plants. . produce this tension. the xylemi Yater tension is much stronger. t. can pull the ‘Yater up to S.
- The attraction between the water molecules and the cell wall of the xylem cells iscalled adhesion. The water moleculesremainsadhere to the xylem tissues. So the columin of water in the xylem tissues does not break. The composition of the cell wall provides this necessary adhesion for the water molecules. It helps the water molecules to creep tip. The cellulose of the cell wall has special affinity with water. Cellulose can imbibe water.
4. Strongxylem wall:
- The xylem walls have high tensile strength. So they do not buckleinwards. Thelignin and cellulose provide strength to the cell wall of the xylem tissues.
Mechanism of transpiration pull:
1. Msophyll cells:
- Water eyaporates from the intercellular spaces of the leaves into the air, Twithdraws water from the mesophyll celk. The water vapours pass out through the stomata. More water evaporates from the satunted walls of the mesophyll cells. The cell walls withdraw water from the vacuoles of cells. Hence the osmotic pressure of inesophall cell. is increased. and their turgor pressure falli, it increases their suction pressure. Therefore. these cells ithdraw water from the deeper inesoph) ll cell. In thisagradient of suction pressure is established.
- This suction pressure reaches the parent)) ‘flaunts cells. ‘Mese cells draw out ksziter from the xylem vessels. The greater the%% queer loss by transpiration the greater will be the magnitude of the force. This force then pulls up the water in the xylem vessels of the stem from above.
- Transpiration pull produces tension in the xylem. This tension is transmitted down the column of water in the xylem elements through the stem. It reaches the absorbing region of the rooLAs a result the whole column of water is lifted up.
4. Water column:
- Cohesive forces are present between the water molecules. These forces maintain the continuity of% voter column in the xylem. The magnitudes of this cohesive force are very large. It may be 350 am. This means that a force greater than 350 atm. is required to break a column of water. Nese enormous. cohesive holds water molecules together like a cord or wire. The rising column of water is called transpiration stream. Objections Cohesion – tension theorlk is the most accepted theory. But there are a fen objections tocohesion – tension theory.
- The water column in a glass tube would breakonshaking. The plants are bent severely in a strong w Md. It can break the column of water. This does not happen. The xylem ducts ha% every narrow bore. Therefore there is greater adhesion between water and the xy lem mails. n does notal low the water column to break,
- Dissolved gases may appear and form babbles inside the xylem ducts due to change in temperature. These bubbles can break the continuity of water column in the xilem. But there ishigh tension in thexsilent. This tension dissolves am nair bubble