Here are 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 following reasons:
I. The magnitude of root pressure is M itch lower. .1–hus this pressure is
insufficient to raise water to much height. It rarely exceeds 3 atm. 13tu a pressure of about 20 atm. is required to raise the sap to the tops of tall trees.
- Root pressure shows seasonal fluctuations. New Ica% es are coming out and transpiration is slow in spring. Sc) it is highest in spring. But transpiration is very rapid in summer. So it has lower value in summer. .1 herefore, it does not play any role in ascent of sap in summer.
- Water continues to rise up in the absence of root pressure. ‘Vitus when a cut shoot is placed in water, it does not wilt
- . Root pressure has not been obsened in g.,mnosperms.
A glass tube with a namns 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. It occurs due to property of adhesion of water. Adhesion is the attraction of molecules of water v1/4 ith the glass. The phenomenon is called capillarity. The height of water in a capillary tube is ‘aversely proportional to diameter of the bore. It means smaller the capillary bore. more water is raised. X.) lem ducts acts as narrow capillary tubes. But capillarity. does not play much role in the ascent of sap due to following reasons:
I. .1.11c capillary bore of xy lem )essels and trachcids is about 40m in diameter. Its capillary force can raise ,yater on Iv up to 40 cm.
2. Capillarity works only if one of the open ends of the capillary tube is dipped in water. But this situation does not exist in nature. The xy lcm ducts of roots are not in contact with liquid water of the soil.
(c) VITAL THEORIES
rhere were IWO iews about vital theory:
I. Vital theory was first proposed by Godiewski. According this thpory the movement of water takes place due to the pumping activity of the cells of wood parenchyma and Medullary rays. ‘File periodic change in their osmotic pressure causes pumping action. There is an increase of osmotic pressure in these cells. It withdraws water from the bordering vessels. Then their osmotic pressure is decreased. It pumps 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) act ii. it of the cortical cells of the slew outside the
Strasburgher disapproyed this theory. I le showed that water continued to rise 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 lbllow ing factors play role in the ascent of sap.
I. Transpiration pull: Water is evaporated from the stomata by
transpiration. It produces negative pressure or suction pressure in the leaf. This pressure pulls the water upward.
- Cohesion: The force of :attraction among the water molecules is called cohesion. The cohesion holds water together. It forms a solid chain like column within the xylem tubes. .[here is hydrogen bonding between the molecules of water.
- Tension: The pulling ofw liter upward produces tension in the xylem tubes. The transpiration provides the necessary energy or three. The hydrogen bonds between. the water molecules produce this tension. the xylem Ysater tension is much stronger. It can pull the ‘Yater up to 200 meters (more than NO feet) in plants.
- Adhesion: The attraction between the water molecules and the cell wall of the xylem cells is called adhesion. The water molecules remains adhere to the xylem tissues. So the column 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.
- Strong xylem wall: The xylem walls have high tensile strength. So they do not buckle inwards. :[he lignin and cellulose provide strength to the cell wall of the xylem tissues.
Mechanism of transpiration pull
I. Mesophyll cells: Water eyaporates from the intercellular spaces of the leaves into the air. It w ithdraws water from the mesophyll cells. The water vapours pass out through the stomata. More water evaporates from the saturated walls of the mesophyll cells. The cell walls withdraw water from the vacuoles of cells. Hence the osmotic pressure of inesoph>II celk is increased. and their turgor pressure falls. It increases their suction pressure. Therefore. these cells ithdraw water from the deeper inesoph) II cell. In this a gradient of suction pressure is established.
- Xylems: This suction pressure reaches the parent)) ‘flaunts cells. ‘Mese cells draw out ksziter from the xylem vessels. The greater the %%quer 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.
- Root: “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 root. As a result the whole column of water is lifted up.
- 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 atm. 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.ObjectionsCohesion- tension theor% is the most accepted theory. But there are a fen objections to cohesion-tension theory .
- The water column in a glass tube would break on shaking. 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 % e very narrow bore. Therefore. there is greater adhesion between water and the xy lem nails. It does not al 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 x)lem. But there is high tension in the x% lent. This tension dissolves am n air bubble