The water is absorbed mainly by the roots of the plants, but is necessary by the entire plant. So the absorbed water is transported to different parts of the plant via the stem. Thus the upward conduction of water from the roots to different parts of the plant body is called ascent of sap. The Ringing experiment has proved that the path of ascent of sap is xylem.
Mechanism: In small trees and herbaceous plants the ascent of sap can be explained easily but in tall trees like Eucalyptus and some conifers which reach a height of about hundred meters, the ascent of sap, in fact becomes a problem.
Although the mechanism of ascent of sap is not well understood, a number of theories have been put forward to explain it. These are divided into the following main categories.
A. Vital force theories
B. Root pressure theory
C. Physical force theories.
Vital force theories:
The proponents of vital force theories believe that the ascent of sap is under the control of vital activities of the stem. Two theories are put under this category.
i. Rolay pump theory :
Godlewski (1884) proposed this theory. According him the rhythmic changes in the osmotic pressure of living cells of xylem parenchyme are responsible for upward movement of water.
This theory seemed only hypothetical and was discarded because ascent of sap continues in the stem in which the living cells are killed by poison.
ii. Pulsation theory :
According to sir J. C. Bose (1923) ascent of sap takes place due to pulsatory mechanism of living cortical cells present nearer to the vessels and tracheids of xylem. By means of electric probe needle he demonstrated the pulsatory mechanism of living cells.
This theory was also rejected because many workers could not repeat the experiment and there was no co relation between pulsatory activity and the ascent of sap.
Root pressure theory:
Root pressure is defined as "A pressure developed in the treachery elements of xylem as a result of metabolic activities of roots".
If a well watered tomato plant is cut near its base, the xylem sap is seen to flow out through the cut end with a pressure. This phenomenon is quite common in many herbaceous plants. The pressure of exudation can be demonstrated by placing a vertical tube to the cut end of the stem, a column of sap is seen to rise in it. This pressure is actually the hydrostatic pressure developed in the root system called root pressure. It is an active process.
It is believed that root pressure may be a factor of some significance in the ascent of sap. But the theories discarded because of the following points.
i. Ascent of sap is observed in the plants in which roots are removed.
ii. The magnitude of root pressure is hardly one to two atoms, while a pressure of about 20 atoms is needed to raise water to the tops of tall trees.
iii. In gymnosperms root pressure is rarely observed.
Physical force theories:
All those theories according to which dead cells are responsible for ascent of sap are called physical force theories. Under this category there are several theories that are briefly discussed below.
i. Imbibitions theory :
It was proposed (1868) and supported by Sachs (1878). According to this theory the ascent of sap occurs due to imbinitonal activity of the cell walls of the xylem elements.
This theory is discarded as it has been experimentally found that water rises up through the lumen of the xylem and not through the walls.
ii. Capillary force theory :
This was proposed by bochm (1809). According to this theory the tracheids and vessels of the xylem behave as capillary tubes. Ascent of sap is partly due to the capillarity of these tissues.
There are many objections to this theory:
i. For capillarity a free surface is required.
ii. The magnitude of capillary force is low.
iii. The taller plants should have narrow elements while small plants have broader elements. But anatomically it is not found to be correct.
Atmospheric pressure theory:
It was proposed that atmospheric pressure must be responsible for the ascent of sap. The water transpires from the leaf which reduces the pressure in the xylem cells and this gap is filled by the water just below if due to atmospheric pressure.
There are serious objection to this theory:
i. The atmospheric pressure can raise water only upto 34 feet and not beyond it.
ii. Pressure of green surface at the lower end is required for the atmospheric pressure to upper end.
Transpiration pull and cohesion of water theory:
This theory was originally proposed by Dixon and holy (1894) and was supported by Renner (1911), Curtis and Clark (1951), Bonner and Galston (1952), Karmer and Kozlowski (1960) and many others.
The theories based on the following three features:
a. Strong cohesive force of water molecules.
b. Continuity of the water column in the plant and
c. Tension on the water column due to transpiration pull.
The above system inside the xylem elements of the plant satisfies all the above conditions, and thereby, offers a strong support for the validity of the theory.
Although H-bond is very weak, when they are present in enormous numbers as in case of water; they develop a very strong mutual force of attraction known as cohesion. The magnitude of this force is as high as 350 atoms. Therefore, they can not break easily. Moreover, there is an attraction between molecules and the wall of the xylem elements known as adhesion. The magnitude of this force is around 50 atoms. Due to the cohesive and adhesive force a continuous water column is formed in the xylem. Because of its strong cohesive force the column in not broken by other force. Further air bubble does not block the system because the continuity is maintained through the pits present in the walls of tracheids and vessels.
When transpiration take place from the mesophlly cells of leaves, it results in diffusion pressure deficit inside them. The mesophlly cells than absorb water from the nearby xylem elements. Loss of water from xylem elements results a similar diffusion pressure deficit in them and ultimately a tension is created. This tension acts as a pull which is called transpiration pull. As a result of this pull the tension transmitted downloads upto roots through the stem and upward movement of water takes place which is analogous to pulling a bucket of water from a well.
The transpiration pull theory inspite of some objections of widely accepted.
i. There is correlation between the rate of transpiration and ascent of sap.
ii. In a branch cut from a rapidly transpiring plant water snaps away from the roots.
iii. With the help of depend graph Mac Dougal observed that tree trunks show diurnal contraction.