Ascent of sap is the upward movement of water and minerals from the roots towards the leaves and aerial parts of the plant. This movement of the water and minerals occurs through the tracheary elements of xylem. The Ascent of sap occurs through the xylem has been demonstrated by the stain experiment.
Path of Ascent of sap:- Cut a shoot of herbaceous plant which is with leaves and flowers. Cut the shoot at the lower end under water. Now keep this lower end of the shoot in a stained water. After sometime the leaves become coloured. Now cut the transverse section of this shoot and observe under electron microscope. It can be easily observed that only xylem is coloured. This proves that water is conducted by xylem.
Theories of ascent of sap:- Various theories has been put forward to explain the mechanism of ascent of sap by different scientists at the different times. Three main theories are :-
i) Vital Force Theory
ii) Root Pressure Theory
iii) Transpiration Pull Theory
i) Vital Force Theory:- This theory was given by J.C Bose. It is also called pulsation theory. According to this theory the innermost cortical cells absorb water from outside the plant and pump this water towards the xylem cells. Living cells do not require for this process as in spite of cutting the roots and kills the living cells of the stem the water remain show the continuous upward movement.
ii) Root Pressure Theory:- This theory was put forward by Priestley in 1916. According to this theory the root pressure in the xylem cells helps in the upward movement of water in plants. Root pressure is observed in the certain seasons and maximum in the rainy season. It is 1-2 bars in the maximum water availability. This theory was not accepted because of some points like
a) Root pressure has not been found in all plants e.g in gymnosperms the root pressure is absent but these are the tallest trees in which water moves upwards and reaches upto almost 100 meter height.
b) Root pressure is absent in the unfavorable conditions but ascent of sap continues.
c) Water continues to move upward even in the absence of roots.
iii) Transpiration pull Theory and Cohesion Tension Theory:- This is the most accepted theory which was given by Dixon and Joly in 1894. It was further improved by Dixon therefore also called Dixon's Theory of ascent of sap. The main steps of this theory are:-
a) Continuous water column:- There are continuous column of water from the roots through the stem to the leaves. these water column forms the continuous system through the unthickened areas of the cells called pits.
b) Cohesion or Tensile strength:- Water molecules remain attached with one another by cohesive forces which is due to hydrogen bonding between adjacent molecules. Therefore, the cohesion force is also called tensile strength. Water molecules will also continue because of adhesive forces between water molecules and walls of tracheary elements.
c) Development of Transpiration Pull:- There is a loss of water from the mesophyll cells of the leaf during transpiration. Because of loss of water these mesophyll cells due to water deficit tends to absorb water from the adjacent cells. Likewise these adjacent cells after losing the water to mesophyll cells become water deficit and these cells will absorb water from the tracheids and vessels ( tracheary elements) of the plant. The grater the rate of transpiration the greater will be suction force. With the loss of water from the surface of leaves a pull or tension for is created for water and is known as Transpiration pull.
d) The conducting elements of the plant form a cotinuous system. The transpiration is exerted by living cells of the leaf at the top, throws the water in the xylem in a state of tension. Transpiration pull overcomes the - force of gravity, force of resistance offered by living cells in the roots and resistance offered by xylem channels.
Path of Water Through the plant
Evidences of transpiration pull :-
i) The rate of water absorption and ascent of sap are almost closely follow the rate of transpiration.
ii) Evaporation of water from a porous pot can produce a tension in a water column.
iii) Dendrography indicates that xylem vessels contract during day time and relax during night. it is due to the transpiration pull.
No comments:
Post a Comment