The upward movement of water from the metaxylem of roots upto the substomatal cavities of leaves via xylem of the stem and veins of leaves is called “Ascent of sap” or conduction of water in an upward direction. It has been experimentally verified that xylem is the main water conducting tissue.
Various theories have been forwarded to explain the mechanism involved in ascent of sap. Some scientists are of the opinion that living cells are actively involved in pumping water in an upward direction while others explain the mechanism of ascent of sap is independent of life activity. The various theories can be broadly dealt fewer than three headings.
1. Vital theories
2. Root pressure theory
3. Physical theories
1. Vital Theories
All those theories which consider living cells to be responsible to effect ascent of sap are dealt under vital theories. Westermeir, Godlewski, Janse stated that the living cells play an essential role in ascent of sap.
Godlewski proposed relay pump theory to explain ascent of sap. According to him there was a rhythmic change in the osmotic pressure of the living cells of xylem parenchyma and medullary rays which brought about a pumping action of water in an upward direction.
The tracheids and vessels acted as water reservoirs. Janse supported Godlewski and showed that if the lower portion of branch ws killed, the leaves above were affected.
Sir J.C. Bose (1923) advocated pulsation theory to explain ascent of sap. Lie experimented to prove that the living cells of innermost layer of cortex lying just outside the endodermis were in a state of rhythmic pulsations which caused the pumping of water from cell to cell in an upward direction.
2. Root Pressure Theory
If the stem of a plant is cut near its base or incisions are made in the stem, xylem sap is seen to flow out through them. This phenomenon is called “exudation” or “bleeding”. Pristley explained that this process of upward movement of water was due to a hydrostatic pressure developed in the root system Dr. J.C. Bose’s pulsation theory known as root pressure. If a vertical explained by this device tube is attached to a bleeding stamp, a column of sap will rise in it.
Since the living roots are essential for the development of root pressure, it seems most reasonable to think that though root pressure is purely a matter of diffusion pressure gradient, it is however, maintained by the activity of – living cells. Than it is an active process.
Objections to root pressure theory
Kramer, Steward, Dixon and Joly have objected very strongly the involvement of root pressure in causing ascent of sap. It is true that root pressure is a dynamic process, but in itself it is not sufficient to drive water in case of tall trees ranging in height from 200 to 400 feet. Further root pressure has not been observed in all plants. Xo or little pressure is found in gymnosperms. Besides, root pressure has been found to be lowest. Demonstration of during summer when the rate of transpiration is very rapid where as it is root pressure highest in spring when the rate of transpiration is quite slow.
3. Physical Theories
All those theories which consider the dead cells of the plant to be responsible for the ascent of sap are called physical theories.
(i) Boehm’s theory
Boehm is of the view that water ascends in the xylem vessels because of capillary action. If a capillary tube is kept vertically in the water, the water rises up in the tube automatically to a certain height.
This happens due to capillary force and high surface tension of water. It is known that the highest rise of water column attained by capillary forces in narrow circular tubes having diametor 0.03 mm is less than 4 feet. Capillarity also implies that both the ends of the tube must have free sufaccs. In case of plants the xylem duct is not in dircct contact with soil water to effect ascent of sap. Then this theory fails to account ascent of sap in tall trees.
(ii) Sach’s imbibition theory
Sach believed that ascent of sap is affected due to imbibitional activity of the cell walls of xylem.
(iii) Jamin’s chain theory Jamin believed that air and water were alternatively arranged inside the xylem duct. Lie explained that when air gets expanded, it moved up carrying along with it the water column present above it. The theory is unconvincing as it fails to explain the rapid unidirectional flow of sap.