Tag: transport of water in plants

The mechanism of the ascent of sap in plants occurs due to the activity of the living cells. The factors responsible are
root pressure-  plant roots absorb the excess of water by an active process and builds up a hydrostatic pressure within the root system, called root pressure.  This activity pushes the water upwards all along the length of the stem.
Transpiration -The pulling force that operates on xylem water columns is called Transpiration Pull or Suction Pressure.
Capillary action along with the root pressure, the ascent of sap is mainly due to passive forces that develop within the plant, due to certain environmental factors that act upon the plant.  tracheids and vessels of the xylem behave as capillary tubes. Ascent of sap is partly due to the capillarity of these tissues. Thus, the correct answer is option D.

When a Balsam plant is provided with a dilute solution of safranine, the same is absorbed by the root system and transported upwards which can be easily traced because of the red color of the safranine that binds and stains the cells responsible for ascent of sap.

Gravitational water percolates the soil layers and reaches the water table and not available by soil for plants. The amount of water retained by soil and available for plants is called field capacity. Thus, the correct option is C.

The classical vital theories attempted to explain mechanism of ascent of sap. The vital force theory was a theory presented by Berzelius. The theory stated that for the ascent of sap vital capacity of a living cell is necessary. Many scientists supported the theory by demonstrating their experiments. Some of such theories are Godlewski's relay pump theory and Bose's pulsatory movement theory. These classical theories have not been accepted much and rejected by the German scientist Wohler because of some reasons. For example, according to these theories ( Godlewski's relay pump theory), the force necessary for the ascent of sap is provided by the living cells of plants. He also assumed that a pumping action was generated due to the rythmic change in the water potential of the living cell to raise water upward against gravity. Further, it was found by Strasberger that translocation of water was continued even after the death of living cells. Many scientists like Dixon, Shull, MacDougal etc. proposed that pulsatory movement and the rate of water translocation is not related to each other. The normal rate of ascent of sap at which sap flows through pulsatory cells was observed 230 to 400 pulsatory cells per second. Whereas, earlier it was observed (by Bose) 14 seconds, for one pulsation. The remarkable difference was observed between them and the theory was rejected. Therefore, the correct option is C.

The capillary force theory was proposed by Christian Wolf. The theory was proposed in the year 1873. Water moves in the upward direction partially due to the capillary of xylem element and partially due to atmospheric pressure. This capillary acts as the tube which is responsible for the movement of water. 

Most acceptable theory for the ascent of sap is the 'Physical force theory', called as 'Cohesion-Transpiration pull theory'. This theory was put forward by Dixon and Joly. According to this theory, the ascent of sap in plants is a physical process, the lifting force for which is provided by the pulling force generated by transpiration. The cohesion forces between the water molecules help in maintaining a continuous water column and the adhesion forces between water molecules and the tracheary elements help in sticking to water to the xylary elements. Both of these forces help in maintaining a continuous water column in the xylary elements.

The ascent of sap is the mechanism by which water moves from the roots to the top of the trees. Studies on the process of an ascent of sap show that the strong cohesive force between the water molecules (cohesion), the attraction between water molecules and the wall of the xylem elements (adhesion) and the transpiration pull creates a tension on the water column, which ensures continuity of water column in plants. The water lost from the leaves during transpiration creates a high solute potential and reduces water potential. The reduced water potential creates a suction pull and allows ascent of sap. 

The transpiration pull theory is the most accepted theory for the ascent of sap. Dixon and Jolly gave transpiration pull theory. According to this theory, the ascent of sap in plants is a passive physical process, which occurs under the influence of transpiration pull generated due to loss of water from aerial parts. The water lost from the leaves creates a high solute potential and reduces water potential. The reduced water potential creates a suction pull and allows ascent of sap. Other than the pull, a continuous column of water is also essential for the rise of water in xylem elements. The continuous column of water is maintained by cohesion forces between water molecules and adhesive forces between water molecules and vascular elements of the plant.