Physical theories which consider the involvement of physical pressure exerted inside the dead cells of the plant during the ascent of sap are known as physical theories. Here, living cells do not take part in the translocation of water. Hence, the physical nature of internal tissue is more important than its living activities.
According to physical theories, the passage of sap takes place through the lumen of xylem elements. Some of the important physical theories are as follows:
- Boehm’s capillarity theory (1809): It is a well known fact that capillaries exist in plant body and finer the capillaries greater the raise of liquid through them. This is the basis of capillarity theory proposed by Boehm. However, the calculations of various workers have clearly revealed that even very fine capillary cannot lift water for long distance. A capillary of 0.03 mm diameter carries water up to 4 feet. The diameter of thinnest plant is 0.63 micron. Hence, this theory is discarded. Moreover, in gymnosperm, vessels are absent and tracheids have numerous septa which offer resistance to capillary movement. Capillarity also requires free surface and direct contact with soil water.
- Sach’s imbibition theory: This theory was put forward by Sachs in 1878 to explain the ascent of sap by imbibitional activity of cell wall of the xylem elements. Water moves upwards due to imbibitional force between the cell wall of the xylem and the water column. But experimentally, it has been found out that water rises up through lumen of xylem and not along their walls. Hence, this theory is also discarded.
- Jamin’s chain theory: Jamin believed that air and water were present in regular alternate layers in xylem. The water was pushed upwards when air expanded. The theory could not be accepted because it does not account for the rapid unidirectional flow of water.
- Atmospheric pressure theory: Active transpiration creates vacuum in the xylem elements and to fill up this vacuum, water moves upward from below. Atmospheric pressure in this way can bring about the rise of water up to 30 feet or above it. Moreover, operation of atmospheric pressure depends on the free surface at the base, which does not exist in the plant.
- Transpiration pull and cohesion of water theory or cohesion tension theory: This theory was proposed by Dixon and Jolly in 1894 and is based on the following features:
- Cohesive and adhesive properties of water molecules to form an unbroken continuous water column in the xylem: The water molecules have a strong mutual attraction, i.e., they tend to stick to each other. This is called cohesion. They also tend to stick to the wall of the xylem elements; this is called adhesion. A high cohesion of water molecules means that a relatively large tension is required to break a column of water.
- Continuity of water column: Water in the tracheids and vessels of leaf veinlets is continuous from the leaves to the roots. The cohesive and adhesive forces are very great and do not allow the water column to break or pull away from the walls of the xylem. The cohesive force or the tensile strength of sap in xylem vessels and tracheids may withstand tensions greater than 300 atm. Since 1 atm. pressure can maintain a column of approximately 9 meters, hence about 13 atm. pressure should be sufficient for raising water to around 120 meters.
- Transpiration pull or tension exerted on the water column.:
Xylem vessels are tubular structures which extent from root to the apex of plant cells. They are placed one above the other, with their end walls perforated forming a continuous tube. These are further supported by tracheids which are characterized by having force in their wall. One end of the xylem tube is connected with root hairs via pericycle, endodermis and cortex. The other end is connected with sub stomatal cavities of leaves via mesophyll cells. These tubes remain always filled with water.
Water is filled inside the xylem capillaries and due to cohesion and adhesion properties of water, it forms a continuous water column. The water column cannot be broken or pulled away because of cohesion and adhesion. The water column is subjected to various forces which try to break it, these forces are weight of column itself and resistance put it due to translocation. All these forces combined together have been found to be 50 atm. but the magnitude of cohesive force is much high, i.e., 350 atm. Hence, the column of water is not broken by other forces.
The water vapour evaporates from mesophyll cells to the intercellular spaces as a result of active transpiration. The water vapors are transpired through the stomatal force. The cell sap of mesophyll cells by using water becomes concentrated. Its osmotic pressure is increased. As a result, diffusion pressure deficit also increases. With the increase in diffusion pressure deficit absorbs water from the adjoining mesophyll cells and ultimately, the water is absorbed from the vascular bundles. Vascular bundles in turn absorb water from xylem elements of the stem. Since the xylem elements are filled with unbroken continuous water column, a tension or pull is generated at the top of this column. This is called transpiration pull or tension. This tension is transmitted downwards from petiole, stem and finally reaches to the roots. This results in the upward movement of water.
EVIDENCES IN SUPPORT OF COHESION TENSION THEORY
Scholander et al provided evidences in favor of continuous free movement of sap column.
- All the forces combined together have been found to be 50 atm. in the tallest tree which creates obstacles, but cohesive force of water is up to 350 atm. which prevents breaking of water column.
- A leafy twig cut under water and the cut end of the twig sealed to the top of the mercury manometer has been shown to pull the mercury above barometric level.
- If water is under tension, the strain in the vessel should cause the diameter to decrease. A decrease in diameter has been observed when transpiration is high.