In this article we will discuss about the meaning and theories of ascent of sap.
Meaning of Ascent of Sap:
Plants lose enormous amounts of water by transpiration. Water is absorbed from the soil and is conducted upwards through roots and stems ultimately to reach the leaves. This becomes possible due to the presence of continuous conducting channels in the organs. Vessels and tracheids of the xylem are the main paths for ascent of water with dissolved matters, what is termed sap.
The fact can be easily demonstrated by placing a twig in water coloured with a harmless dye like eosin. Within a short time the veins become coloured, and sections also show colour in the xylem vessels and tracheids. A simple experiment of this nature with an inflorescence of lovely tube-rose is really very interesting.
The inflorescence stalk is put in water coloured with eosin. It is noticed after a few hours that the beautiful white flowers are turning reddish in colour. The standard ‘ringing experiment’ has been cited as evidence that ascent of sap takes place through xylem elements.
It is this in a plant the soft outer portions up to cambium are removed in the form of a ring (girdled) but the xylem is left intact. Leaves above the ring do not show sign of wilting. Girdling, of course, turns fatal in the long run, because removal of phloem interrupts downward movement of food.
The factors responsible for the ascent of sap, particularly up to the top of all trees like Eucalyptus, pines, etc., have been subjects of keen interest and extensive investigations since a long time; but no entirely satisfactory explanation has been offered.
Theories Governing the Ascent of Sap:
However, the probable theories governing the ascent of sap are given below:
i. Root Pressure:
The root pressure forces water up to the xylem vessels becomes evident from characteristic ‘bleeding’ from decapitated stumps of small plants. But this force is not sufficient for lifting water in the tall trees.
Moreover, root pressure definitely decreases when the rate of transpiration is vigorous. Even negative pressure is registered in the stems. Root pressure might be all right for small herbs and shrubs in the tropical countries; but it has never been found to be of sufficient magnitude in the European countries.
ii. Capillarity:
As the vessels and tracheids are capillary tubes, water is expected to rise by capillarity. The smaller the diameters of the capillary tubes, the higher the rise of water—is the general rule. So for ascent of sap it would be expected that the tall trees should have narrow tubes. But just the reverse is true.
Particularly in spring when the rate of assimilation is vigorous and there is great need of water, the vessels formed have wide diameters (spring wood). So capillarity as a force responsible for the ascent of sap, cannot be accepted.
iii. Imbibition Force:
It was thought to be the cause of movement of water through xylem by German physiologist, Sachs. The forces of imbibition are certainly great and may seem adequate for carrying water to required distance, but the imbibition rates are extremely slow. And for imbibition to be effective, the cell wall must carry water. But it has been clearly demonstrated that water moves up through the lumen of the xylem vessels.
Indian physiologist, Acharyya Jagadish Chandra Bose, believed that the living cells of the cortex exert osmotic forces, as a result of which sap moves upwards by a sort of pumping. Bose claimed to have proved conclusively that pulsatory activities brought about by alternate contraction and expansion of the living cells, thought to be the cells of the innermost layer of cortex, maintain the ascent of sap in plants.
According to Bose, xylem vessels merely act as reservoirs of water. The pulsatory activities have been compared to heart beats of animals by which water is pumped up to next higher cells.
Though the experimental evidence as yet available does not support Bose’s conclusions, still the possibility that the living cells adjoining the tracheids and vessels may influence the ascent of sap through the stem cannot be altogether dismissed.
iv. Cohesion Theory:
Due to evaporation from the leaf surface a tremendous suction force or pull is produced which works on the top of water column present in the xylem. Water particles cohere in the xylem vessels and tracheids, thus forming a continuous column from roots to the leaves. This property makes it possible for it to be pulled to the top of even the tallest tree by the force resulting from transpiration.
The xylem vessels actually function merely as tubes through which water is drawn. The column of water resists breaking because of cohesion between water molecules. Water is pulled up from the roots to the leaves very much as a rope is pulled through a hard rigid tube.
As a result of the pull, sap is lifted upwards even in tall trees. This theory, which explains the ascent of sap purely on a physical basis, is the most widely accepted one, in spite of its certain weaknesses. Undoubtedly this is the only mechanism adequate for the actual water conduction rates through the plants.