List of top two experiments on upward translocation of water by plants:- 1. Simple Demonstration of ‘Ascent of Sap’ 2. Determination of Suction Pressure Developed Due to Transpiration Pull.
Experiment # 1. Simple Demonstration of ‘Ascent of Sap’:
Absorption and subsequent translocation of water through conductive tissues is a vital process in plant life. By far the largest proportion of absorbed water is lost as vapour through the process of transpiration from the aerial parts. Much smaller quantities are utilized for growth and for various other metabolic processes of plants.
Water, therefore, continuously move up through conductive tissues, which is called ascent of sap. By simple experiments both ascent of sap (liquid containing water with organic and inorganic solutions) and the effect of transpiration on the rise of liquid can be shown.
Materials and Equipments:
1. A leafy twig with two branches.
2. Beakers, Stand with clamp, Rubber tube, Glass tube etc.
3. Water, Mercury etc.
1. Take a leafy twig of suitable size with two branches and keep its cut end dipped in water.
2. Remove the leaves from one of the side branches and decapitate this branch for fitting a narrow glass tube (with the help of rubber tube).
3. Then immerse the glass tube in a beaker containing mercury after filling the glass tube with water and the other branch of the twig in a beaker containing water by a stand with clamp (Fig. 3.10A).
4. Make all connections air-tight and place the whole set-up in bright sunlight for 2 to 3 hrs. Observation
After the desired time mercury is found to rise in the narrow glass tube.
As water molecules are lost by transpiration from the leaves, an upward pull develops causing mercury and water to move up into the glass tube due to cohesive forces.
Experiment # 2. Determination of Suction Pressure Developed Due to Transpiration Pull:
The upward translocation of water through the xylem, from roots to leaves, is called the transpiration stream, because its ascent depends on transpiration.
Transpiration stream is initiated by vaporization of water at the outermost surface of moist cell walls of mesophyll cells which line sub-stomatal spaces. The proportion of water which moves into roots by osmosis as compared to ‘transpiration pull’ depends on the rate of transpiration.
The lifting power of transpiration can be demonstrated readily with very little equipment. A fresh leafy twig is directly fitted with water column linked with mercury and allowed to transpire for a considerable period. Then the rise of mercury through water column will be noticed due to transpiration pull, which is created as a result of transpiration by the leafy twig.
Materials and Equipments:
1. A freshly collected leafy twig (preferably twig of Catharanthus or Gardenia or Ixora sp.).
2. Graduated glass tube, rubber tube, thread, sealing wax, knife, etc.
3. Beaker, mercury, water, stand with clamps, etc.
1. Take a suitable leafy twig (cut under water) and insert the twig into a graduated tube with the help of a rubber tubing.
2. Fill the tube with water, close the lower end with the thumb and make it air-tight by sealing wax (lac) after tying with thread.
3. The glass tube — which is now air-tight and completely filed with water — is carefully held with a finger and immersed in mercury contained in a beaker. The set-up is kept upright with stand and clamp (Fig. 3.10B).
4. Keep the whole set-up in bright sunlight for 1-2 hrs.
5. The rise of mercury column and the total transpiring surface is to be recorded for determination of suction pressure.
The rise of mercury through the graduated glass tube is recorded after desirable period is over. This rise of mercury within the glass tube indicates the suction or the transpiration pull exerted on the water in the xylem vessels which is transmitted to the water column in the glass tube and, ultimately, to the mercury level in the beaker causing the mercury to rise in the tube.
The suction force due to transpiration or the weight of the transpiration pull is calculated by the formula:
SP = πr2 × h × d × g dynes
where, π r2 = area of the bore of the glass tube
r = radius of the glass tube
h = the height of mercury column in the glass tube
d = density of mercury (13.6)
g = acceleration due to gravity (981 dynes per sec2)
SP = suction force or pressure