i. The upward movement of water through stem is called ascent of sap (water with dissolved minerals).
ii. Goldlewski proposed relay pump theory and J.C. Bose proposed pulsation theory to explain the vitalistic view of ascent of sap.
iii. The three most prominent theories are-
(1) Root pressure theory:
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Stocking (1956) defined the root pressure “a pressure developed in the tracheary elements of xylem as a result of metabolic activities of the root.
i. The development of root pressure is an active process which depends upon active secretion of salts or other solutes into the xylem sap, thereby lowering its osmotic potential.
ii. This theory cannot account for water movement up the xylem in tall trees.
(2) Capillarity:
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In capillarity, water moves upward in narrow tubes due to the force of surface tension in small sized plants.
Boehm (1809) proposed that xylem vessels are narrow and ascent of sap occurs through capillarity further supported by atmospheric pressure.
According to this theory, water is first taken in due to the force of adhesion between water and the wall of thin xylem vessels. As the water flows upwards along the wall, strong cohesive forces between water molecules come into play to pull the water upward. This continues until the forces of adhesion and cohesion are balanced by the downward force of gravity.
(3) Cohesion theory:
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Given by Henry Dixon in 1914. This depends upon the force of cohesion between water molecules. The important points are:
(i) Cohesion force:
Water molecules are held together by strong cohesion force (due to hydrogen bonds). Another force of adhesion holds water to the walls of xylem channels.
(ii) Continuous water column:
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Present in the xylem channels of plant.
(iii) Transpiration pulls:
Transpiration from aerial parts causes a suction pressure in the water column of the plant. It is known as transpiration pull which lifts the water upwardly.
i. Water column is continuous and cannot be pulled away from xylem walls due to cohesive and adhesive properties.
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ii. The value of cohesive force for plant sap has been calculated to be 47 – 207 atm. by Dixon and Jolly.
iii. Cohesive force is also called as tensile strength.
The loss of water in the form of vapours from the living tissues of aerial parts of the plant is called transpiration.
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About 98% of water absorbed by land plant is transpired.
Types of transpiration
(1) Cuticular:
Cuticle provides a relatively impermeable covering. If cutilcle is thin and green, about 20% of the total transpiration takes place through it, but if its thickness is increased, the extent of water vapour loss is significantly reduced.
(2) Lenticular:
Lenticels are small regions on bark etc. and possess small loosely arranged cells called as complementary cells. According to Huber, this loss through lenticels is only 1.0 per cent of the total transpiration.
(3) Stomatal:
Stomata are tiny pores present on the epidermal surface of leaves, young stems and in certain truits. Nearly 97 per cent of the total transpiration takes place through these stomata.
Stomata :
Help in aseous exchange at the time of respiration and photosynthesis.
i. There are two kidney shaped guard cells which bound a minute elliptical pore in a stoma. Guard cells are modified epidermal cells. The wall of guard cells near the pore is thick. The outer wall is thin, elastic and semipermeable.
ii. Loftfield classified stomata on the basis of their daily movement into four main types-
(a) Alfalfa type:
Open throughout the day and closed all night e.g., radish, mustard, turnip, apple, pea.
(b) Potato type:
Open throughout the day and night except few hours following sunset e.g., cabbage, pumpkin, onion.
(c) Barley type:
Open only for a few hours in day e.g., barley, maize, oat, wheat.
(d) Equisetum type:
Always open throughout day and night e.g., Equisetum.
Distribution of stomata on the leaves
1. Apple or mulberry type:
On the under surface of leaves only e.g. apple.
2. Potato type:
More on under surface than upperside. e.g. Potato, brinjal.
3. Oat type:
Equally distributed on both the surfaces, e.g. rice, wheat.
4. Water lily type:
Only on the upper surface e.g. water lily.
5. Potamogeton type:
Either absent or if present, they functionless, present mostly in submerged aquatic plants, e.g. Potamogeton.
Significance of transpiration:
i. Removal of water in the form of vapours has a cooling effect on the leaves. So it does not allow the leaf temperature to rise to detrimental levels.
ii. It helps in movement of xylem sap which simultaneously increases the absorption of mineral nutrients by the roots from the soil. It enhances extensive development of root system that supports the heavy aerial growth against wind.
iii. Curtis (1926) called transpiration as necessary evil.